summaryrefslogtreecommitdiffstats
path: root/Documentation/virtual/virtio-spec.txt
blob: da094737e2f8660519d7fefbfde2c11f6acd00fd (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
[Generated file: see http://ozlabs.org/~rusty/virtio-spec/]
Virtio PCI Card Specification
v0.9.1 DRAFT
-

Rusty Russell <rusty@rustcorp.com.au>IBM Corporation (Editor)

2011 August 1.

Purpose and Description

This document describes the specifications of the “virtio” family
of PCI[LaTeX Command: nomenclature] devices. These are devices
are found in virtual environments[LaTeX Command: nomenclature],
yet by design they are not all that different from physical PCI
devices, and this document treats them as such. This allows the
guest to use standard PCI drivers and discovery mechanisms.

The purpose of virtio and this specification is that virtual
environments and guests should have a straightforward, efficient,
standard and extensible mechanism for virtual devices, rather
than boutique per-environment or per-OS mechanisms.

  Straightforward: Virtio PCI devices use normal PCI mechanisms
  of interrupts and DMA which should be familiar to any device
  driver author. There is no exotic page-flipping or COW
  mechanism: it's just a PCI device.[footnote:
This lack of page-sharing implies that the implementation of the
device (e.g. the hypervisor or host) needs full access to the
guest memory. Communication with untrusted parties (i.e.
inter-guest communication) requires copying.
]

  Efficient: Virtio PCI devices consist of rings of descriptors
  for input and output, which are neatly separated to avoid cache
  effects from both guest and device writing to the same cache
  lines.

  Standard: Virtio PCI makes no assumptions about the environment
  in which it operates, beyond supporting PCI. In fact the virtio
  devices specified in the appendices do not require PCI at all:
  they have been implemented on non-PCI buses.[footnote:
The Linux implementation further separates the PCI virtio code
from the specific virtio drivers: these drivers are shared with
the non-PCI implementations (currently lguest and S/390).
]

  Extensible: Virtio PCI devices contain feature bits which are
  acknowledged by the guest operating system during device setup.
  This allows forwards and backwards compatibility: the device
  offers all the features it knows about, and the driver
  acknowledges those it understands and wishes to use.

  Virtqueues

The mechanism for bulk data transport on virtio PCI devices is
pretentiously called a virtqueue. Each device can have zero or
more virtqueues: for example, the network device has one for
transmit and one for receive.

Each virtqueue occupies two or more physically-contiguous pages
(defined, for the purposes of this specification, as 4096 bytes),
and consists of three parts:


+-------------------+-----------------------------------+-----------+
| Descriptor Table  |   Available Ring     (padding)    | Used Ring |
+-------------------+-----------------------------------+-----------+


When the driver wants to send buffers to the device, it puts them
in one or more slots in the descriptor table, and writes the
descriptor indices into the available ring. It then notifies the
device. When the device has finished with the buffers, it writes
the descriptors into the used ring, and sends an interrupt.

Specification

  PCI Discovery

Any PCI device with Vendor ID 0x1AF4, and Device ID 0x1000
through 0x103F inclusive is a virtio device[footnote:
The actual value within this range is ignored
]. The device must also have a Revision ID of 0 to match this
specification.

The Subsystem Device ID indicates which virtio device is
supported by the device. The Subsystem Vendor ID should reflect
the PCI Vendor ID of the environment (it's currently only used
for informational purposes by the guest).


+----------------------+--------------------+---------------+
| Subsystem Device ID  |   Virtio Device    | Specification |
+----------------------+--------------------+---------------+
+----------------------+--------------------+---------------+
|          1           |   network card     |  Appendix C   |
+----------------------+--------------------+---------------+
|          2           |   block device     |  Appendix D   |
+----------------------+--------------------+---------------+
|          3           |      console       |  Appendix E   |
+----------------------+--------------------+---------------+
|          4           |  entropy source    |  Appendix F   |
+----------------------+--------------------+---------------+
|          5           | memory ballooning  |  Appendix G   |
+----------------------+--------------------+---------------+
|          6           |     ioMemory       |       -       |
+----------------------+--------------------+---------------+
|          9           |   9P transport     |       -       |
+----------------------+--------------------+---------------+


  Device Configuration

To configure the device, we use the first I/O region of the PCI
device. This contains a virtio header followed by a
device-specific region.

There may be different widths of accesses to the I/O region; the “
natural” access method for each field in the virtio header must
be used (i.e. 32-bit accesses for 32-bit fields, etc), but the
device-specific region can be accessed using any width accesses,
and should obtain the same results.

Note that this is possible because while the virtio header is PCI
(i.e. little) endian, the device-specific region is encoded in
the native endian of the guest (where such distinction is
applicable).

  Device Initialization Sequence

We start with an overview of device initialization, then expand
on the details of the device and how each step is preformed.

  Reset the device. This is not required on initial start up.

  The ACKNOWLEDGE status bit is set: we have noticed the device.

  The DRIVER status bit is set: we know how to drive the device.

  Device-specific setup, including reading the Device Feature
  Bits, discovery of virtqueues for the device, optional MSI-X
  setup, and reading and possibly writing the virtio
  configuration space.

  The subset of Device Feature Bits understood by the driver is
  written to the device.

  The DRIVER_OK status bit is set.

  The device can now be used (ie. buffers added to the
  virtqueues)[footnote:
Historically, drivers have used the device before steps 5 and 6.
This is only allowed if the driver does not use any features
which would alter this early use of the device.
]

If any of these steps go irrecoverably wrong, the guest should
set the FAILED status bit to indicate that it has given up on the
device (it can reset the device later to restart if desired).

We now cover the fields required for general setup in detail.

  Virtio Header

The virtio header looks as follows:


+------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+
| Bits       || 32                  | 32                  | 32       | 16     | 16      | 16      | 8       | 8      |
+------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+
| Read/Write || R                   | R+W                 | R+W      | R      | R+W     | R+W     | R+W     | R      |
+------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+
| Purpose    || Device              | Guest               | Queue    | Queue  | Queue   | Queue   | Device  | ISR    |
|            || Features bits 0:31  | Features bits 0:31  | Address  | Size   | Select  | Notify  | Status  | Status |
+------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+


If MSI-X is enabled for the device, two additional fields
immediately follow this header:


+------------++----------------+--------+
| Bits       || 16             | 16     |
              +----------------+--------+
+------------++----------------+--------+
| Read/Write || R+W            | R+W    |
+------------++----------------+--------+
| Purpose    || Configuration  | Queue  |
| (MSI-X)    || Vector         | Vector |
+------------++----------------+--------+


Finally, if feature bits (VIRTIO_F_FEATURES_HI) this is
immediately followed by two additional fields:


+------------++----------------------+----------------------
| Bits       || 32                   | 32
+------------++----------------------+----------------------
| Read/Write || R                    | R+W
+------------++----------------------+----------------------
| Purpose    || Device               | Guest
|            || Features bits 32:63  | Features bits 32:63
+------------++----------------------+----------------------


Immediately following these general headers, there may be
device-specific headers:


+------------++--------------------+
| Bits       || Device Specific    |
              +--------------------+
+------------++--------------------+
| Read/Write || Device Specific    |
+------------++--------------------+
| Purpose    || Device Specific... |
|            ||                    |
+------------++--------------------+


  Device Status

The Device Status field is updated by the guest to indicate its
progress. This provides a simple low-level diagnostic: it's most
useful to imagine them hooked up to traffic lights on the console
indicating the status of each device.

The device can be reset by writing a 0 to this field, otherwise
at least one bit should be set:

  ACKNOWLEDGE (1) Indicates that the guest OS has found the
  device and recognized it as a valid virtio device.

  DRIVER (2) Indicates that the guest OS knows how to drive the
  device. Under Linux, drivers can be loadable modules so there
  may be a significant (or infinite) delay before setting this
  bit.

  DRIVER_OK (3) Indicates that the driver is set up and ready to
  drive the device.

  FAILED (8) Indicates that something went wrong in the guest,
  and it has given up on the device. This could be an internal
  error, or the driver didn't like the device for some reason, or
  even a fatal error during device operation. The device must be
  reset before attempting to re-initialize.

  Feature Bits

The least significant 31 bits of the first configuration field
indicates the features that the device supports (the high bit is
reserved, and will be used to indicate the presence of future
feature bits elsewhere). If more than 31 feature bits are
supported, the device indicates so by setting feature bit 31 (see
[cha:Reserved-Feature-Bits]). The bits are allocated as follows:

  0 to 23 Feature bits for the specific device type

  24 to 40 Feature bits reserved for extensions to the queue and
  feature negotiation mechanisms

  41 to 63 Feature bits reserved for future extensions

For example, feature bit 0 for a network device (i.e. Subsystem
Device ID 1) indicates that the device supports checksumming of
packets.

The feature bits are negotiated: the device lists all the
features it understands in the Device Features field, and the
guest writes the subset that it understands into the Guest
Features field. The only way to renegotiate is to reset the
device.

In particular, new fields in the device configuration header are
indicated by offering a feature bit, so the guest can check
before accessing that part of the configuration space.

This allows for forwards and backwards compatibility: if the
device is enhanced with a new feature bit, older guests will not
write that feature bit back to the Guest Features field and it
can go into backwards compatibility mode. Similarly, if a guest
is enhanced with a feature that the device doesn't support, it
will not see that feature bit in the Device Features field and
can go into backwards compatibility mode (or, for poor
implementations, set the FAILED Device Status bit).

Access to feature bits 32 to 63 is enabled by Guest by setting
feature bit 31. If this bit is unset, Device must assume that all
feature bits > 31 are unset.

  Configuration/Queue Vectors

When MSI-X capability is present and enabled in the device
(through standard PCI configuration space) 4 bytes at byte offset
20 are used to map configuration change and queue interrupts to
MSI-X vectors. In this case, the ISR Status field is unused, and
device specific configuration starts at byte offset 24 in virtio
header structure. When MSI-X capability is not enabled, device
specific configuration starts at byte offset 20 in virtio header.

Writing a valid MSI-X Table entry number, 0 to 0x7FF, to one of
Configuration/Queue Vector registers, maps interrupts triggered
by the configuration change/selected queue events respectively to
the corresponding MSI-X vector. To disable interrupts for a
specific event type, unmap it by writing a special NO_VECTOR
value:

/* Vector value used to disable MSI for queue */

#define VIRTIO_MSI_NO_VECTOR            0xffff

Reading these registers returns vector mapped to a given event,
or NO_VECTOR if unmapped. All queue and configuration change
events are unmapped by default.

Note that mapping an event to vector might require allocating
internal device resources, and might fail. Devices report such
failures by returning the NO_VECTOR value when the relevant
Vector field is read. After mapping an event to vector, the
driver must verify success by reading the Vector field value: on
success, the previously written value is returned, and on
failure, NO_VECTOR is returned. If a mapping failure is detected,
the driver can retry mapping with fewervectors, or disable MSI-X.

  Virtqueue Configuration

As a device can have zero or more virtqueues for bulk data
transport (for example, the network driver has two), the driver
needs to configure them as part of the device-specific
configuration.

This is done as follows, for each virtqueue a device has:

  Write the virtqueue index (first queue is 0) to the Queue
  Select field.

  Read the virtqueue size from the Queue Size field, which is
  always a power of 2. This controls how big the virtqueue is
  (see below). If this field is 0, the virtqueue does not exist.

  Allocate and zero virtqueue in contiguous physical memory, on a
  4096 byte alignment. Write the physical address, divided by
  4096 to the Queue Address field.[footnote:
The 4096 is based on the x86 page size, but it's also large
enough to ensure that the separate parts of the virtqueue are on
separate cache lines.
]

  Optionally, if MSI-X capability is present and enabled on the
  device, select a vector to use to request interrupts triggered
  by virtqueue events. Write the MSI-X Table entry number
  corresponding to this vector in Queue Vector field. Read the
  Queue Vector field: on success, previously written value is
  returned; on failure, NO_VECTOR value is returned.

The Queue Size field controls the total number of bytes required
for the virtqueue according to the following formula:

#define ALIGN(x) (((x) + 4095) & ~4095)

static inline unsigned vring_size(unsigned int qsz)

{

     return ALIGN(sizeof(struct vring_desc)*qsz + sizeof(u16)*(2
+ qsz))

          + ALIGN(sizeof(struct vring_used_elem)*qsz);

}

This currently wastes some space with padding, but also allows
future extensions. The virtqueue layout structure looks like this
(qsz is the Queue Size field, which is a variable, so this code
won't compile):

struct vring {

    /* The actual descriptors (16 bytes each) */

    struct vring_desc desc[qsz];



    /* A ring of available descriptor heads with free-running
index. */

    struct vring_avail avail;



    // Padding to the next 4096 boundary.

    char pad[];



    // A ring of used descriptor heads with free-running index.

    struct vring_used used;

};

  A Note on Virtqueue Endianness

Note that the endian of these fields and everything else in the
virtqueue is the native endian of the guest, not little-endian as
PCI normally is. This makes for simpler guest code, and it is
assumed that the host already has to be deeply aware of the guest
endian so such an “endian-aware” device is not a significant
issue.

  Descriptor Table

The descriptor table refers to the buffers the guest is using for
the device. The addresses are physical addresses, and the buffers
can be chained via the next field. Each descriptor describes a
buffer which is read-only or write-only, but a chain of
descriptors can contain both read-only and write-only buffers.

No descriptor chain may be more than 2^32 bytes long in total.struct vring_desc {

    /* Address (guest-physical). */

    u64 addr;

    /* Length. */

    u32 len;

/* This marks a buffer as continuing via the next field. */

#define VRING_DESC_F_NEXT   1

/* This marks a buffer as write-only (otherwise read-only). */

#define VRING_DESC_F_WRITE     2

/* This means the buffer contains a list of buffer descriptors.
*/

#define VRING_DESC_F_INDIRECT   4

    /* The flags as indicated above. */

    u16 flags;

    /* Next field if flags & NEXT */

    u16 next;

};

The number of descriptors in the table is specified by the Queue
Size field for this virtqueue.

  <sub:Indirect-Descriptors>Indirect Descriptors

Some devices benefit by concurrently dispatching a large number
of large requests. The VIRTIO_RING_F_INDIRECT_DESC feature can be
used to allow this (see [cha:Reserved-Feature-Bits]). To increase
ring capacity it is possible to store a table of indirect
descriptors anywhere in memory, and insert a descriptor in main
virtqueue (with flags&INDIRECT on) that refers to memory buffer
containing this indirect descriptor table; fields addr and len
refer to the indirect table address and length in bytes,
respectively. The indirect table layout structure looks like this
(len is the length of the descriptor that refers to this table,
which is a variable, so this code won't compile):

struct indirect_descriptor_table {

    /* The actual descriptors (16 bytes each) */

    struct vring_desc desc[len / 16];

};

The first indirect descriptor is located at start of the indirect
descriptor table (index 0), additional indirect descriptors are
chained by next field. An indirect descriptor without next field
(with flags&NEXT off) signals the end of the indirect descriptor
table, and transfers control back to the main virtqueue. An
indirect descriptor can not refer to another indirect descriptor
table (flags&INDIRECT must be off). A single indirect descriptor
table can include both read-only and write-only descriptors;
write-only flag (flags&WRITE) in the descriptor that refers to it
is ignored.

  Available Ring

The available ring refers to what descriptors we are offering the
device: it refers to the head of a descriptor chain. The “flags”
field is currently 0 or 1: 1 indicating that we do not need an
interrupt when the device consumes a descriptor from the
available ring. Alternatively, the guest can ask the device to
delay interrupts until an entry with an index specified by the “
used_event” field is written in the used ring (equivalently,
until the idx field in the used ring will reach the value
used_event + 1). The method employed by the device is controlled
by the VIRTIO_RING_F_EVENT_IDX feature bit (see [cha:Reserved-Feature-Bits]
). This interrupt suppression is merely an optimization; it may
not suppress interrupts entirely.

The “idx” field indicates where we would put the next descriptor
entry (modulo the ring size). This starts at 0, and increases.

struct vring_avail {

#define VRING_AVAIL_F_NO_INTERRUPT      1

   u16 flags;

   u16 idx;

   u16 ring[qsz]; /* qsz is the Queue Size field read from device
*/

   u16 used_event;

};

  Used Ring

The used ring is where the device returns buffers once it is done
with them. The flags field can be used by the device to hint that
no notification is necessary when the guest adds to the available
ring. Alternatively, the “avail_event” field can be used by the
device to hint that no notification is necessary until an entry
with an index specified by the “avail_event” is written in the
available ring (equivalently, until the idx field in the
available ring will reach the value avail_event + 1). The method
employed by the device is controlled by the guest through the
VIRTIO_RING_F_EVENT_IDX feature bit (see [cha:Reserved-Feature-Bits]
). [footnote:
These fields are kept here because this is the only part of the
virtqueue written by the device
].

Each entry in the ring is a pair: the head entry of the
descriptor chain describing the buffer (this matches an entry
placed in the available ring by the guest earlier), and the total
of bytes written into the buffer. The latter is extremely useful
for guests using untrusted buffers: if you do not know exactly
how much has been written by the device, you usually have to zero
the buffer to ensure no data leakage occurs.

/* u32 is used here for ids for padding reasons. */

struct vring_used_elem {

    /* Index of start of used descriptor chain. */

    u32 id;

    /* Total length of the descriptor chain which was used
(written to) */

    u32 len;

};



struct vring_used {

#define VRING_USED_F_NO_NOTIFY  1

    u16 flags;

    u16 idx;

    struct vring_used_elem ring[qsz];

    u16 avail_event;

};

  Helpers for Managing Virtqueues

The Linux Kernel Source code contains the definitions above and
helper routines in a more usable form, in
include/linux/virtio_ring.h. This was explicitly licensed by IBM
and Red Hat under the (3-clause) BSD license so that it can be
freely used by all other projects, and is reproduced (with slight
variation to remove Linux assumptions) in Appendix A.

  Device Operation

There are two parts to device operation: supplying new buffers to
the device, and processing used buffers from the device. As an
example, the virtio network device has two virtqueues: the
transmit virtqueue and the receive virtqueue. The driver adds
outgoing (read-only) packets to the transmit virtqueue, and then
frees them after they are used. Similarly, incoming (write-only)
buffers are added to the receive virtqueue, and processed after
they are used.

  Supplying Buffers to The Device

Actual transfer of buffers from the guest OS to the device
operates as follows:

  Place the buffer(s) into free descriptor(s).

  If there are no free descriptors, the guest may choose to
    notify the device even if notifications are suppressed (to
    reduce latency).[footnote:
The Linux drivers do this only for read-only buffers: for
write-only buffers, it is assumed that the driver is merely
trying to keep the receive buffer ring full, and no notification
of this expected condition is necessary.
]

  Place the id of the buffer in the next ring entry of the
  available ring.

  The steps (1) and (2) may be performed repeatedly if batching
  is possible.

  A memory barrier should be executed to ensure the device sees
  the updated descriptor table and available ring before the next
  step.

  The available “idx” field should be increased by the number of
  entries added to the available ring.

  A memory barrier should be executed to ensure that we update
  the idx field before checking for notification suppression.

  If notifications are not suppressed, the device should be
  notified of the new buffers.

Note that the above code does not take precautions against the
available ring buffer wrapping around: this is not possible since
the ring buffer is the same size as the descriptor table, so step
(1) will prevent such a condition.

In addition, the maximum queue size is 32768 (it must be a power
of 2 which fits in 16 bits), so the 16-bit “idx” value can always
distinguish between a full and empty buffer.

Here is a description of each stage in more detail.

  Placing Buffers Into The Descriptor Table

A buffer consists of zero or more read-only physically-contiguous
elements followed by zero or more physically-contiguous
write-only elements (it must have at least one element). This
algorithm maps it into the descriptor table:

  for each buffer element, b:

  Get the next free descriptor table entry, d

  Set d.addr to the physical address of the start of b

  Set d.len to the length of b.

  If b is write-only, set d.flags to VRING_DESC_F_WRITE,
    otherwise 0.

  If there is a buffer element after this:

    Set d.next to the index of the next free descriptor element.

    Set the VRING_DESC_F_NEXT bit in d.flags.

In practice, the d.next fields are usually used to chain free
descriptors, and a separate count kept to check there are enough
free descriptors before beginning the mappings.

  Updating The Available Ring

The head of the buffer we mapped is the first d in the algorithm
above. A naive implementation would do the following:

avail->ring[avail->idx % qsz] = head;

However, in general we can add many descriptors before we update
the “idx” field (at which point they become visible to the
device), so we keep a counter of how many we've added:

avail->ring[(avail->idx + added++) % qsz] = head;

  Updating The Index Field

Once the idx field of the virtqueue is updated, the device will
be able to access the descriptor entries we've created and the
memory they refer to. This is why a memory barrier is generally
used before the idx update, to ensure it sees the most up-to-date
copy.

The idx field always increments, and we let it wrap naturally at
65536:

avail->idx += added;

  <sub:Notifying-The-Device>Notifying The Device

Device notification occurs by writing the 16-bit virtqueue index
of this virtqueue to the Queue Notify field of the virtio header
in the first I/O region of the PCI device. This can be expensive,
however, so the device can suppress such notifications if it
doesn't need them. We have to be careful to expose the new idx
value before checking the suppression flag: it's OK to notify
gratuitously, but not to omit a required notification. So again,
we use a memory barrier here before reading the flags or the
avail_event field.

If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated, and if
the VRING_USED_F_NOTIFY flag is not set, we go ahead and write to
the PCI configuration space.

If the VIRTIO_F_RING_EVENT_IDX feature is negotiated, we read the
avail_event field in the available ring structure. If the
available index crossed_the avail_event field value since the
last notification, we go ahead and write to the PCI configuration
space. The avail_event field wraps naturally at 65536 as well:

(u16)(new_idx - avail_event - 1) < (u16)(new_idx - old_idx)

  <sub:Receiving-Used-Buffers>Receiving Used Buffers From The
  Device

Once the device has used a buffer (read from or written to it, or
parts of both, depending on the nature of the virtqueue and the
device), it sends an interrupt, following an algorithm very
similar to the algorithm used for the driver to send the device a
buffer:

  Write the head descriptor number to the next field in the used
  ring.

  Update the used ring idx.

  Determine whether an interrupt is necessary:

  If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated: check
    if f the VRING_AVAIL_F_NO_INTERRUPT flag is not set in avail-
    >flags

  If the VIRTIO_F_RING_EVENT_IDX feature is negotiated: check
    whether the used index crossed the used_event field value
    since the last update. The used_event field wraps naturally
    at 65536 as well:(u16)(new_idx - used_event - 1) < (u16)(new_idx - old_idx)

  If an interrupt is necessary:

  If MSI-X capability is disabled:

    Set the lower bit of the ISR Status field for the device.

    Send the appropriate PCI interrupt for the device.

  If MSI-X capability is enabled:

    Request the appropriate MSI-X interrupt message for the
      device, Queue Vector field sets the MSI-X Table entry
      number.

    If Queue Vector field value is NO_VECTOR, no interrupt
      message is requested for this event.

The guest interrupt handler should:

  If MSI-X capability is disabled: read the ISR Status field,
  which will reset it to zero. If the lower bit is zero, the
  interrupt was not for this device. Otherwise, the guest driver
  should look through the used rings of each virtqueue for the
  device, to see if any progress has been made by the device
  which requires servicing.

  If MSI-X capability is enabled: look through the used rings of
  each virtqueue mapped to the specific MSI-X vector for the
  device, to see if any progress has been made by the device
  which requires servicing.

For each ring, guest should then disable interrupts by writing
VRING_AVAIL_F_NO_INTERRUPT flag in avail structure, if required.
It can then process used ring entries finally enabling interrupts
by clearing the VRING_AVAIL_F_NO_INTERRUPT flag or updating the
EVENT_IDX field in the available structure, Guest should then
execute a memory barrier, and then recheck the ring empty
condition. This is necessary to handle the case where, after the
last check and before enabling interrupts, an interrupt has been
suppressed by the device:

vring_disable_interrupts(vq);

for (;;) {

    if (vq->last_seen_used != vring->used.idx) {

		vring_enable_interrupts(vq);

		mb();

		if (vq->last_seen_used != vring->used.idx)

			break;

    }

    struct vring_used_elem *e =
vring.used->ring[vq->last_seen_used%vsz];

    process_buffer(e);

    vq->last_seen_used++;

}

  Dealing With Configuration Changes

Some virtio PCI devices can change the device configuration
state, as reflected in the virtio header in the PCI configuration
space. In this case:

  If MSI-X capability is disabled: an interrupt is delivered and
  the second highest bit is set in the ISR Status field to
  indicate that the driver should re-examine the configuration
  space.Note that a single interrupt can indicate both that one
  or more virtqueue has been used and that the configuration
  space has changed: even if the config bit is set, virtqueues
  must be scanned.

  If MSI-X capability is enabled: an interrupt message is
  requested. The Configuration Vector field sets the MSI-X Table
  entry number to use. If Configuration Vector field value is
  NO_VECTOR, no interrupt message is requested for this event.

Creating New Device Types

Various considerations are necessary when creating a new device
type:

  How Many Virtqueues?

It is possible that a very simple device will operate entirely
through its configuration space, but most will need at least one
virtqueue in which it will place requests. A device with both
input and output (eg. console and network devices described here)
need two queues: one which the driver fills with buffers to
receive input, and one which the driver places buffers to
transmit output.

  What Configuration Space Layout?

Configuration space is generally used for rarely-changing or
initialization-time parameters. But it is a limited resource, so
it might be better to use a virtqueue to update configuration
information (the network device does this for filtering,
otherwise the table in the config space could potentially be very
large).

Note that this space is generally the guest's native endian,
rather than PCI's little-endian.

  What Device Number?

Currently device numbers are assigned quite freely: a simple
request mail to the author of this document or the Linux
virtualization mailing list[footnote:

https://lists.linux-foundation.org/mailman/listinfo/virtualization
] will be sufficient to secure a unique one.

Meanwhile for experimental drivers, use 65535 and work backwards.

  How many MSI-X vectors?

Using the optional MSI-X capability devices can speed up
interrupt processing by removing the need to read ISR Status
register by guest driver (which might be an expensive operation),
reducing interrupt sharing between devices and queues within the
device, and handling interrupts from multiple CPUs. However, some
systems impose a limit (which might be as low as 256) on the
total number of MSI-X vectors that can be allocated to all
devices. Devices and/or device drivers should take this into
account, limiting the number of vectors used unless the device is
expected to cause a high volume of interrupts. Devices can
control the number of vectors used by limiting the MSI-X Table
Size or not presenting MSI-X capability in PCI configuration
space. Drivers can control this by mapping events to as small
number of vectors as possible, or disabling MSI-X capability
altogether.

  Message Framing

The descriptors used for a buffer should not effect the semantics
of the message, except for the total length of the buffer. For
example, a network buffer consists of a 10 byte header followed
by the network packet. Whether this is presented in the ring
descriptor chain as (say) a 10 byte buffer and a 1514 byte
buffer, or a single 1524 byte buffer, or even three buffers,
should have no effect.

In particular, no implementation should use the descriptor
boundaries to determine the size of any header in a request.[footnote:
The current qemu device implementations mistakenly insist that
the first descriptor cover the header in these cases exactly, so
a cautious driver should arrange it so.
]

  Device Improvements

Any change to configuration space, or new virtqueues, or
behavioural changes, should be indicated by negotiation of a new
feature bit. This establishes clarity[footnote:
Even if it does mean documenting design or implementation
mistakes!
] and avoids future expansion problems.

Clusters of functionality which are always implemented together
can use a single bit, but if one feature makes sense without the
others they should not be gratuitously grouped together to
conserve feature bits. We can always extend the spec when the
first person needs more than 24 feature bits for their device.

[LaTeX Command: printnomenclature]

Appendix A: virtio_ring.h

#ifndef VIRTIO_RING_H

#define VIRTIO_RING_H

/* An interface for efficient virtio implementation.

 *

 * This header is BSD licensed so anyone can use the definitions

 * to implement compatible drivers/servers.

 *

 * Copyright 2007, 2009, IBM Corporation

 * Copyright 2011, Red Hat, Inc

 * All rights reserved.

 *

 * Redistribution and use in source and binary forms, with or
without

 * modification, are permitted provided that the following
conditions

 * are met:

 * 1. Redistributions of source code must retain the above
copyright

 *    notice, this list of conditions and the following
disclaimer.

 * 2. Redistributions in binary form must reproduce the above
copyright

 *    notice, this list of conditions and the following
disclaimer in the

 *    documentation and/or other materials provided with the
distribution.

 * 3. Neither the name of IBM nor the names of its contributors

 *    may be used to endorse or promote products derived from
this software

 *    without specific prior written permission.

 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
CONTRIBUTORS ``AS IS'' AND

 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
TO, THE

 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
PARTICULAR PURPOSE

 * ARE DISCLAIMED.  IN NO EVENT SHALL IBM OR CONTRIBUTORS BE
LIABLE

 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
CONSEQUENTIAL

 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
SUBSTITUTE GOODS

 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
INTERRUPTION)

 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
CONTRACT, STRICT

 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
IN ANY WAY

 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
POSSIBILITY OF

 * SUCH DAMAGE.

 */



/* This marks a buffer as continuing via the next field. */

#define VRING_DESC_F_NEXT       1

/* This marks a buffer as write-only (otherwise read-only). */

#define VRING_DESC_F_WRITE      2



/* The Host uses this in used->flags to advise the Guest: don't
kick me

 * when you add a buffer.  It's unreliable, so it's simply an

 * optimization.  Guest will still kick if it's out of buffers.
*/

#define VRING_USED_F_NO_NOTIFY  1

/* The Guest uses this in avail->flags to advise the Host: don't

 * interrupt me when you consume a buffer.  It's unreliable, so
it's

 * simply an optimization.  */

#define VRING_AVAIL_F_NO_INTERRUPT      1



/* Virtio ring descriptors: 16 bytes.

 * These can chain together via "next". */

struct vring_desc {

        /* Address (guest-physical). */

        uint64_t addr;

        /* Length. */

        uint32_t len;

        /* The flags as indicated above. */

        uint16_t flags;

        /* We chain unused descriptors via this, too */

        uint16_t next;

};



struct vring_avail {

        uint16_t flags;

        uint16_t idx;

        uint16_t ring[];

        uint16_t used_event;

};



/* u32 is used here for ids for padding reasons. */

struct vring_used_elem {

        /* Index of start of used descriptor chain. */

        uint32_t id;

        /* Total length of the descriptor chain which was written
to. */

        uint32_t len;

};



struct vring_used {

        uint16_t flags;

        uint16_t idx;

        struct vring_used_elem ring[];

        uint16_t avail_event;

};



struct vring {

        unsigned int num;



        struct vring_desc *desc;

        struct vring_avail *avail;

        struct vring_used *used;

};



/* The standard layout for the ring is a continuous chunk of
memory which

 * looks like this.  We assume num is a power of 2.

 *

 * struct vring {

 *      // The actual descriptors (16 bytes each)

 *      struct vring_desc desc[num];

 *

 *      // A ring of available descriptor heads with free-running
index.

 *      __u16 avail_flags;

 *      __u16 avail_idx;

 *      __u16 available[num];

 *

 *      // Padding to the next align boundary.

 *      char pad[];

 *

 *      // A ring of used descriptor heads with free-running
index.

 *      __u16 used_flags;

 *      __u16 EVENT_IDX;

 *      struct vring_used_elem used[num];

 * };

 * Note: for virtio PCI, align is 4096.

 */

static inline void vring_init(struct vring *vr, unsigned int num,
void *p,

                              unsigned long align)

{

        vr->num = num;

        vr->desc = p;

        vr->avail = p + num*sizeof(struct vring_desc);

        vr->used = (void *)(((unsigned long)&vr->avail->ring[num]

                              + align-1)

                            & ~(align - 1));

}



static inline unsigned vring_size(unsigned int num, unsigned long
align)

{

        return ((sizeof(struct vring_desc)*num +
sizeof(uint16_t)*(2+num)

                 + align - 1) & ~(align - 1))

                + sizeof(uint16_t)*3 + sizeof(struct
vring_used_elem)*num;

}



static inline int vring_need_event(uint16_t event_idx, uint16_t
new_idx, uint16_t old_idx)

{

         return (uint16_t)(new_idx - event_idx - 1) <
(uint16_t)(new_idx - old_idx);

}

#endif /* VIRTIO_RING_H */

<cha:Reserved-Feature-Bits>Appendix B: Reserved Feature Bits

Currently there are five device-independent feature bits defined:

  VIRTIO_F_NOTIFY_ON_EMPTY (24) Negotiating this feature
  indicates that the driver wants an interrupt if the device runs
  out of available descriptors on a virtqueue, even though
  interrupts are suppressed using the VRING_AVAIL_F_NO_INTERRUPT
  flag or the used_event field. An example of this is the
  networking driver: it doesn't need to know every time a packet
  is transmitted, but it does need to free the transmitted
  packets a finite time after they are transmitted. It can avoid
  using a timer if the device interrupts it when all the packets
  are transmitted.

  VIRTIO_F_RING_INDIRECT_DESC (28) Negotiating this feature
  indicates that the driver can use descriptors with the
  VRING_DESC_F_INDIRECT flag set, as described in [sub:Indirect-Descriptors]
  .

  VIRTIO_F_RING_EVENT_IDX(29) This feature enables the used_event
  and the avail_event fields. If set, it indicates that the
  device should ignore the flags field in the available ring
  structure. Instead, the used_event field in this structure is
  used by guest to suppress device interrupts. Further, the
  driver should ignore the flags field in the used ring
  structure. Instead, the avail_event field in this structure is
  used by the device to suppress notifications. If unset, the
  driver should ignore the used_event field; the device should
  ignore the avail_event field; the flags field is used

  VIRTIO_F_BAD_FEATURE(30) This feature should never be
  negotiated by the guest; doing so is an indication that the
  guest is faulty[footnote:
An experimental virtio PCI driver contained in Linux version
2.6.25 had this problem, and this feature bit can be used to
detect it.
]

  VIRTIO_F_FEATURES_HIGH(31) This feature indicates that the
  device supports feature bits 32:63. If unset, feature bits
  32:63 are unset.

Appendix C: Network Device

The virtio network device is a virtual ethernet card, and is the
most complex of the devices supported so far by virtio. It has
enhanced rapidly and demonstrates clearly how support for new
features should be added to an existing device. Empty buffers are
placed in one virtqueue for receiving packets, and outgoing
packets are enqueued into another for transmission in that order.
A third command queue is used to control advanced filtering
features.

  Configuration

  Subsystem Device ID 1

  Virtqueues 0:receiveq. 1:transmitq. 2:controlq[footnote:
Only if VIRTIO_NET_F_CTRL_VQ set
]

  Feature bits

  VIRTIO_NET_F_CSUM (0) Device handles packets with partial
    checksum

  VIRTIO_NET_F_GUEST_CSUM (1) Guest handles packets with partial
    checksum

  VIRTIO_NET_F_MAC (5) Device has given MAC address.

  VIRTIO_NET_F_GSO (6) (Deprecated) device handles packets with
    any GSO type.[footnote:
It was supposed to indicate segmentation offload support, but
upon further investigation it became clear that multiple bits
were required.
]

  VIRTIO_NET_F_GUEST_TSO4 (7) Guest can receive TSOv4.

  VIRTIO_NET_F_GUEST_TSO6 (8) Guest can receive TSOv6.

  VIRTIO_NET_F_GUEST_ECN (9) Guest can receive TSO with ECN.

  VIRTIO_NET_F_GUEST_UFO (10) Guest can receive UFO.

  VIRTIO_NET_F_HOST_TSO4 (11) Device can receive TSOv4.

  VIRTIO_NET_F_HOST_TSO6 (12) Device can receive TSOv6.

  VIRTIO_NET_F_HOST_ECN (13) Device can receive TSO with ECN.

  VIRTIO_NET_F_HOST_UFO (14) Device can receive UFO.

  VIRTIO_NET_F_MRG_RXBUF (15) Guest can merge receive buffers.

  VIRTIO_NET_F_STATUS (16) Configuration status field is
    available.

  VIRTIO_NET_F_CTRL_VQ (17) Control channel is available.

  VIRTIO_NET_F_CTRL_RX (18) Control channel RX mode support.

  VIRTIO_NET_F_CTRL_VLAN (19) Control channel VLAN filtering.

  Device configuration layout Two configuration fields are
  currently defined. The mac address field always exists (though
  is only valid if VIRTIO_NET_F_MAC is set), and the status field
  only exists if VIRTIO_NET_F_STATUS is set. Only one bit is
  currently defined for the status field: VIRTIO_NET_S_LINK_UP. #define VIRTIO_NET_S_LINK_UP	1



struct virtio_net_config {

    u8 mac[6];

    u16 status;

};

  Device Initialization

  The initialization routine should identify the receive and
  transmission virtqueues.

  If the VIRTIO_NET_F_MAC feature bit is set, the configuration
  space “mac” entry indicates the “physical” address of the the
  network card, otherwise a private MAC address should be
  assigned. All guests are expected to negotiate this feature if
  it is set.

  If the VIRTIO_NET_F_CTRL_VQ feature bit is negotiated, identify
  the control virtqueue.

  If the VIRTIO_NET_F_STATUS feature bit is negotiated, the link
  status can be read from the bottom bit of the “status” config
  field. Otherwise, the link should be assumed active.

  The receive virtqueue should be filled with receive buffers.
  This is described in detail below in “Setting Up Receive
  Buffers”.

  A driver can indicate that it will generate checksumless
  packets by negotating the VIRTIO_NET_F_CSUM feature. This “
  checksum offload” is a common feature on modern network cards.

  If that feature is negotiated, a driver can use TCP or UDP
  segmentation offload by negotiating the VIRTIO_NET_F_HOST_TSO4
  (IPv4 TCP), VIRTIO_NET_F_HOST_TSO6 (IPv6 TCP) and
  VIRTIO_NET_F_HOST_UFO (UDP fragmentation) features. It should
  not send TCP packets requiring segmentation offload which have
  the Explicit Congestion Notification bit set, unless the
  VIRTIO_NET_F_HOST_ECN feature is negotiated.[footnote:
This is a common restriction in real, older network cards.
]

  The converse features are also available: a driver can save the
  virtual device some work by negotiating these features.[footnote:
For example, a network packet transported between two guests on
the same system may not require checksumming at all, nor
segmentation, if both guests are amenable.
] The VIRTIO_NET_F_GUEST_CSUM feature indicates that partially
  checksummed packets can be received, and if it can do that then
  the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6,
  VIRTIO_NET_F_GUEST_UFO and VIRTIO_NET_F_GUEST_ECN are the input
  equivalents of the features described above. See “Receiving
  Packets” below.

  Device Operation

Packets are transmitted by placing them in the transmitq, and
buffers for incoming packets are placed in the receiveq. In each
case, the packet itself is preceded by a header:

struct virtio_net_hdr {

#define VIRTIO_NET_HDR_F_NEEDS_CSUM    1

	u8 flags;

#define VIRTIO_NET_HDR_GSO_NONE        0

#define VIRTIO_NET_HDR_GSO_TCPV4       1

#define VIRTIO_NET_HDR_GSO_UDP		 3

#define VIRTIO_NET_HDR_GSO_TCPV6       4

#define VIRTIO_NET_HDR_GSO_ECN      0x80

	u8 gso_type;

	u16 hdr_len;

	u16 gso_size;

	u16 csum_start;

	u16 csum_offset;

/* Only if VIRTIO_NET_F_MRG_RXBUF: */

	u16 num_buffers

};

The controlq is used to control device features such as
filtering.

  Packet Transmission

Transmitting a single packet is simple, but varies depending on
the different features the driver negotiated.

  If the driver negotiated VIRTIO_NET_F_CSUM, and the packet has
  not been fully checksummed, then the virtio_net_hdr's fields
  are set as follows. Otherwise, the packet must be fully
  checksummed, and flags is zero.

  flags has the VIRTIO_NET_HDR_F_NEEDS_CSUM set,

  <ite:csum_start-is-set>csum_start is set to the offset within
    the packet to begin checksumming, and

  csum_offset indicates how many bytes after the csum_start the
    new (16 bit ones' complement) checksum should be placed.[footnote:
For example, consider a partially checksummed TCP (IPv4) packet.
It will have a 14 byte ethernet header and 20 byte IP header
followed by the TCP header (with the TCP checksum field 16 bytes
into that header). csum_start will be 14+20 = 34 (the TCP
checksum includes the header), and csum_offset will be 16. The
value in the TCP checksum field will be the sum of the TCP pseudo
header, so that replacing it by the ones' complement checksum of
the TCP header and body will give the correct result.
]

  <enu:If-the-driver>If the driver negotiated
  VIRTIO_NET_F_HOST_TSO4, TSO6 or UFO, and the packet requires
  TCP segmentation or UDP fragmentation, then the “gso_type”
  field is set to VIRTIO_NET_HDR_GSO_TCPV4, TCPV6 or UDP.
  (Otherwise, it is set to VIRTIO_NET_HDR_GSO_NONE). In this
  case, packets larger than 1514 bytes can be transmitted: the
  metadata indicates how to replicate the packet header to cut it
  into smaller packets. The other gso fields are set:

  hdr_len is a hint to the device as to how much of the header
    needs to be kept to copy into each packet, usually set to the
    length of the headers, including the transport header.[footnote:
Due to various bugs in implementations, this field is not useful
as a guarantee of the transport header size.
]

  gso_size is the size of the packet beyond that header (ie.
    MSS).

  If the driver negotiated the VIRTIO_NET_F_HOST_ECN feature, the
    VIRTIO_NET_HDR_GSO_ECN bit may be set in “gso_type” as well,
    indicating that the TCP packet has the ECN bit set.[footnote:
This case is not handled by some older hardware, so is called out
specifically in the protocol.
]

  If the driver negotiated the VIRTIO_NET_F_MRG_RXBUF feature,
  the num_buffers field is set to zero.

  The header and packet are added as one output buffer to the
  transmitq, and the device is notified of the new entry (see [sub:Notifying-The-Device]
  ).[footnote:
Note that the header will be two bytes longer for the
VIRTIO_NET_F_MRG_RXBUF case.
]

  Packet Transmission Interrupt

Often a driver will suppress transmission interrupts using the
VRING_AVAIL_F_NO_INTERRUPT flag (see [sub:Receiving-Used-Buffers]
) and check for used packets in the transmit path of following
packets. However, it will still receive interrupts if the
VIRTIO_F_NOTIFY_ON_EMPTY feature is negotiated, indicating that
the transmission queue is completely emptied.

The normal behavior in this interrupt handler is to retrieve and
new descriptors from the used ring and free the corresponding
headers and packets.

  Setting Up Receive Buffers

It is generally a good idea to keep the receive virtqueue as
fully populated as possible: if it runs out, network performance
will suffer.

If the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6 or
VIRTIO_NET_F_GUEST_UFO features are used, the Guest will need to
accept packets of up to 65550 bytes long (the maximum size of a
TCP or UDP packet, plus the 14 byte ethernet header), otherwise
1514 bytes. So unless VIRTIO_NET_F_MRG_RXBUF is negotiated, every
buffer in the receive queue needs to be at least this length [footnote:
Obviously each one can be split across multiple descriptor
elements.
].

If VIRTIO_NET_F_MRG_RXBUF is negotiated, each buffer must be at
least the size of the struct virtio_net_hdr.

  Packet Receive Interrupt

When a packet is copied into a buffer in the receiveq, the
optimal path is to disable further interrupts for the receiveq
(see [sub:Receiving-Used-Buffers]) and process packets until no
more are found, then re-enable them.

Processing packet involves:

  If the driver negotiated the VIRTIO_NET_F_MRG_RXBUF feature,
  then the “num_buffers” field indicates how many descriptors
  this packet is spread over (including this one). This allows
  receipt of large packets without having to allocate large
  buffers. In this case, there will be at least “num_buffers” in
  the used ring, and they should be chained together to form a
  single packet. The other buffers will not begin with a struct
  virtio_net_hdr.

  If the VIRTIO_NET_F_MRG_RXBUF feature was not negotiated, or
  the “num_buffers” field is one, then the entire packet will be
  contained within this buffer, immediately following the struct
  virtio_net_hdr.

  If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the
  VIRTIO_NET_HDR_F_NEEDS_CSUM bit in the “flags” field may be
  set: if so, the checksum on the packet is incomplete and the “
  csum_start” and “csum_offset” fields indicate how to calculate
  it (see [ite:csum_start-is-set]).

  If the VIRTIO_NET_F_GUEST_TSO4, TSO6 or UFO options were
  negotiated, then the “gso_type” may be something other than
  VIRTIO_NET_HDR_GSO_NONE, and the “gso_size” field indicates the
  desired MSS (see [enu:If-the-driver]).Control Virtqueue

The driver uses the control virtqueue (if VIRTIO_NET_F_VTRL_VQ is
negotiated) to send commands to manipulate various features of
the device which would not easily map into the configuration
space.

All commands are of the following form:

struct virtio_net_ctrl {

	u8 class;

	u8 command;

	u8 command-specific-data[];

	u8 ack;

};



/* ack values */

#define VIRTIO_NET_OK     0

#define VIRTIO_NET_ERR    1

The class, command and command-specific-data are set by the
driver, and the device sets the ack byte. There is little it can
do except issue a diagnostic if the ack byte is not
VIRTIO_NET_OK.

  Packet Receive Filtering

If the VIRTIO_NET_F_CTRL_RX feature is negotiated, the driver can
send control commands for promiscuous mode, multicast receiving,
and filtering of MAC addresses.

Note that in general, these commands are best-effort: unwanted
packets may still arrive.

  Setting Promiscuous Mode

#define VIRTIO_NET_CTRL_RX    0

 #define VIRTIO_NET_CTRL_RX_PROMISC      0

 #define VIRTIO_NET_CTRL_RX_ALLMULTI     1

The class VIRTIO_NET_CTRL_RX has two commands:
VIRTIO_NET_CTRL_RX_PROMISC turns promiscuous mode on and off, and
VIRTIO_NET_CTRL_RX_ALLMULTI turns all-multicast receive on and
off. The command-specific-data is one byte containing 0 (off) or
1 (on).

  Setting MAC Address Filtering

struct virtio_net_ctrl_mac {

	u32 entries;

	u8 macs[entries][ETH_ALEN];

};



#define VIRTIO_NET_CTRL_MAC    1

 #define VIRTIO_NET_CTRL_MAC_TABLE_SET        0

The device can filter incoming packets by any number of
destination MAC addresses.[footnote:
Since there are no guarantees, it can use a hash filter
orsilently switch to allmulti or promiscuous mode if it is given
too many addresses.
] This table is set using the class VIRTIO_NET_CTRL_MAC and the
command VIRTIO_NET_CTRL_MAC_TABLE_SET. The command-specific-data
is two variable length tables of 6-byte MAC addresses. The first
table contains unicast addresses, and the second contains
multicast addresses.

  VLAN Filtering

If the driver negotiates the VIRTION_NET_F_CTRL_VLAN feature, it
can control a VLAN filter table in the device.

#define VIRTIO_NET_CTRL_VLAN       2

 #define VIRTIO_NET_CTRL_VLAN_ADD             0

 #define VIRTIO_NET_CTRL_VLAN_DEL             1

Both the VIRTIO_NET_CTRL_VLAN_ADD and VIRTIO_NET_CTRL_VLAN_DEL
command take a 16-bit VLAN id as the command-specific-data.

Appendix D: Block Device

The virtio block device is a simple virtual block device (ie.
disk). Read and write requests (and other exotic requests) are
placed in the queue, and serviced (probably out of order) by the
device except where noted.

  Configuration

  Subsystem Device ID 2

  Virtqueues 0:requestq.

  Feature bits

  VIRTIO_BLK_F_BARRIER (0) Host supports request barriers.

  VIRTIO_BLK_F_SIZE_MAX (1) Maximum size of any single segment is
    in “size_max”.

  VIRTIO_BLK_F_SEG_MAX (2) Maximum number of segments in a
    request is in “seg_max”.

  VIRTIO_BLK_F_GEOMETRY (4) Disk-style geometry specified in “
    geometry”.

  VIRTIO_BLK_F_RO (5) Device is read-only.

  VIRTIO_BLK_F_BLK_SIZE (6) Block size of disk is in “blk_size”.

  VIRTIO_BLK_F_SCSI (7) Device supports scsi packet commands.

  VIRTIO_BLK_F_FLUSH (9) Cache flush command support.



  Device configuration layout The capacity of the device
  (expressed in 512-byte sectors) is always present. The
  availability of the others all depend on various feature bits
  as indicated above. struct virtio_blk_config {

	u64 capacity;

	u32 size_max;

	u32 seg_max;

	struct virtio_blk_geometry {

		u16 cylinders;

		u8 heads;

		u8 sectors;

	} geometry;

	u32 blk_size;



};

  Device Initialization

  The device size should be read from the “capacity”
  configuration field. No requests should be submitted which goes
  beyond this limit.

  If the VIRTIO_BLK_F_BLK_SIZE feature is negotiated, the
  blk_size field can be read to determine the optimal sector size
  for the driver to use. This does not effect the units used in
  the protocol (always 512 bytes), but awareness of the correct
  value can effect performance.

  If the VIRTIO_BLK_F_RO feature is set by the device, any write
  requests will fail.



  Device Operation

The driver queues requests to the virtqueue, and they are used by
the device (not necessarily in order). Each request is of form:

struct virtio_blk_req {



	u32 type;

	u32 ioprio;

	u64 sector;

	char data[][512];

	u8 status;

};

If the device has VIRTIO_BLK_F_SCSI feature, it can also support
scsi packet command requests, each of these requests is of form:struct virtio_scsi_pc_req {

	u32 type;

	u32 ioprio;

	u64 sector;

    char cmd[];

	char data[][512];

#define SCSI_SENSE_BUFFERSIZE   96

    u8 sense[SCSI_SENSE_BUFFERSIZE];

    u32 errors;

    u32 data_len;

    u32 sense_len;

    u32 residual;

	u8 status;

};

The type of the request is either a read (VIRTIO_BLK_T_IN), a
write (VIRTIO_BLK_T_OUT), a scsi packet command
(VIRTIO_BLK_T_SCSI_CMD or VIRTIO_BLK_T_SCSI_CMD_OUT[footnote:
the SCSI_CMD and SCSI_CMD_OUT types are equivalent, the device
does not distinguish between them
]) or a flush (VIRTIO_BLK_T_FLUSH or VIRTIO_BLK_T_FLUSH_OUT[footnote:
the FLUSH and FLUSH_OUT types are equivalent, the device does not
distinguish between them
]). If the device has VIRTIO_BLK_F_BARRIER feature the high bit
(VIRTIO_BLK_T_BARRIER) indicates that this request acts as a
barrier and that all preceding requests must be complete before
this one, and all following requests must not be started until
this is complete. Note that a barrier does not flush caches in
the underlying backend device in host, and thus does not serve as
data consistency guarantee. Driver must use FLUSH request to
flush the host cache.

#define VIRTIO_BLK_T_IN           0

#define VIRTIO_BLK_T_OUT          1

#define VIRTIO_BLK_T_SCSI_CMD     2

#define VIRTIO_BLK_T_SCSI_CMD_OUT 3

#define VIRTIO_BLK_T_FLUSH        4

#define VIRTIO_BLK_T_FLUSH_OUT    5

#define VIRTIO_BLK_T_BARRIER	 0x80000000

The ioprio field is a hint about the relative priorities of
requests to the device: higher numbers indicate more important
requests.

The sector number indicates the offset (multiplied by 512) where
the read or write is to occur. This field is unused and set to 0
for scsi packet commands and for flush commands.

The cmd field is only present for scsi packet command requests,
and indicates the command to perform. This field must reside in a
single, separate read-only buffer; command length can be derived
from the length of this buffer.

Note that these first three (four for scsi packet commands)
fields are always read-only: the data field is either read-only
or write-only, depending on the request. The size of the read or
write can be derived from the total size of the request buffers.

The sense field is only present for scsi packet command requests,
and indicates the buffer for scsi sense data.

The data_len field is only present for scsi packet command
requests, this field is deprecated, and should be ignored by the
driver. Historically, devices copied data length there.

The sense_len field is only present for scsi packet command
requests and indicates the number of bytes actually written to
the sense buffer.

The residual field is only present for scsi packet command
requests and indicates the residual size, calculated as data
length - number of bytes actually transferred.

The final status byte is written by the device: either
VIRTIO_BLK_S_OK for success, VIRTIO_BLK_S_IOERR for host or guest
error or VIRTIO_BLK_S_UNSUPP for a request unsupported by host:#define VIRTIO_BLK_S_OK        0

#define VIRTIO_BLK_S_IOERR     1

#define VIRTIO_BLK_S_UNSUPP    2

Historically, devices assumed that the fields type, ioprio and
sector reside in a single, separate read-only buffer; the fields
errors, data_len, sense_len and residual reside in a single,
separate write-only buffer; the sense field in a separate
write-only buffer of size 96 bytes, by itself; the fields errors,
data_len, sense_len and residual in a single write-only buffer;
and the status field is a separate read-only buffer of size 1
byte, by itself.

Appendix E: Console Device

The virtio console device is a simple device for data input and
output. A device may have one or more ports. Each port has a pair
of input and output virtqueues. Moreover, a device has a pair of
control IO virtqueues. The control virtqueues are used to
communicate information between the device and the driver about
ports being opened and closed on either side of the connection,
indication from the host about whether a particular port is a
console port, adding new ports, port hot-plug/unplug, etc., and
indication from the guest about whether a port or a device was
successfully added, port open/close, etc.. For data IO, one or
more empty buffers are placed in the receive queue for incoming
data and outgoing characters are placed in the transmit queue.

  Configuration

  Subsystem Device ID 3

  Virtqueues 0:receiveq(port0). 1:transmitq(port0), 2:control
  receiveq[footnote:
Ports 2 onwards only if VIRTIO_CONSOLE_F_MULTIPORT is set
], 3:control transmitq, 4:receiveq(port1), 5:transmitq(port1),
  ...

  Feature bits

  VIRTIO_CONSOLE_F_SIZE (0) Configuration cols and rows fields
    are valid.

  VIRTIO_CONSOLE_F_MULTIPORT(1) Device has support for multiple
    ports; configuration fields nr_ports and max_nr_ports are
    valid and control virtqueues will be used.

  Device configuration layout The size of the console is supplied
  in the configuration space if the VIRTIO_CONSOLE_F_SIZE feature
  is set. Furthermore, if the VIRTIO_CONSOLE_F_MULTIPORT feature
  is set, the maximum number of ports supported by the device can
  be fetched.struct virtio_console_config {

	u16 cols;

	u16 rows;



	u32 max_nr_ports;

};

  Device Initialization

  If the VIRTIO_CONSOLE_F_SIZE feature is negotiated, the driver
  can read the console dimensions from the configuration fields.

  If the VIRTIO_CONSOLE_F_MULTIPORT feature is negotiated, the
  driver can spawn multiple ports, not all of which may be
  attached to a console. Some could be generic ports. In this
  case, the control virtqueues are enabled and according to the
  max_nr_ports configuration-space value, the appropriate number
  of virtqueues are created. A control message indicating the
  driver is ready is sent to the host. The host can then send
  control messages for adding new ports to the device. After
  creating and initializing each port, a
  VIRTIO_CONSOLE_PORT_READY control message is sent to the host
  for that port so the host can let us know of any additional
  configuration options set for that port.

  The receiveq for each port is populated with one or more
  receive buffers.

  Device Operation

  For output, a buffer containing the characters is placed in the
  port's transmitq.[footnote:
Because this is high importance and low bandwidth, the current
Linux implementation polls for the buffer to be used, rather than
waiting for an interrupt, simplifying the implementation
significantly. However, for generic serial ports with the
O_NONBLOCK flag set, the polling limitation is relaxed and the
consumed buffers are freed upon the next write or poll call or
when a port is closed or hot-unplugged.
]

  When a buffer is used in the receiveq (signalled by an
  interrupt), the contents is the input to the port associated
  with the virtqueue for which the notification was received.

  If the driver negotiated the VIRTIO_CONSOLE_F_SIZE feature, a
  configuration change interrupt may occur. The updated size can
  be read from the configuration fields.

  If the driver negotiated the VIRTIO_CONSOLE_F_MULTIPORT
  feature, active ports are announced by the host using the
  VIRTIO_CONSOLE_PORT_ADD control message. The same message is
  used for port hot-plug as well.

  If the host specified a port `name', a sysfs attribute is
  created with the name filled in, so that udev rules can be
  written that can create a symlink from the port's name to the
  char device for port discovery by applications in the guest.

  Changes to ports' state are effected by control messages.
  Appropriate action is taken on the port indicated in the
  control message. The layout of the structure of the control
  buffer and the events associated are:struct virtio_console_control {

	uint32_t id;    /* Port number */

	uint16_t event; /* The kind of control event */

	uint16_t value; /* Extra information for the event */

};



/* Some events for the internal messages (control packets) */



#define VIRTIO_CONSOLE_DEVICE_READY     0

#define VIRTIO_CONSOLE_PORT_ADD         1

#define VIRTIO_CONSOLE_PORT_REMOVE      2

#define VIRTIO_CONSOLE_PORT_READY       3

#define VIRTIO_CONSOLE_CONSOLE_PORT     4

#define VIRTIO_CONSOLE_RESIZE           5

#define VIRTIO_CONSOLE_PORT_OPEN        6

#define VIRTIO_CONSOLE_PORT_NAME        7

Appendix F: Entropy Device

The virtio entropy device supplies high-quality randomness for
guest use.

  Configuration

  Subsystem Device ID 4

  Virtqueues 0:requestq.

  Feature bits None currently defined

  Device configuration layout None currently defined.

  Device Initialization

  The virtqueue is initialized

  Device Operation

When the driver requires random bytes, it places the descriptor
of one or more buffers in the queue. It will be completely filled
by random data by the device.

Appendix G: Memory Balloon Device

The virtio memory balloon device is a primitive device for
managing guest memory: the device asks for a certain amount of
memory, and the guest supplies it (or withdraws it, if the device
has more than it asks for). This allows the guest to adapt to
changes in allowance of underlying physical memory. If the
feature is negotiated, the device can also be used to communicate
guest memory statistics to the host.

  Configuration

  Subsystem Device ID 5

  Virtqueues 0:inflateq. 1:deflateq. 2:statsq.[footnote:
Only if VIRTIO_BALLON_F_STATS_VQ set
]

  Feature bits

  VIRTIO_BALLOON_F_MUST_TELL_HOST (0) Host must be told before
    pages from the balloon are used.

  VIRTIO_BALLOON_F_STATS_VQ (1) A virtqueue for reporting guest
    memory statistics is present.

  Device configuration layout Both fields of this configuration
  are always available. Note that they are little endian, despite
  convention that device fields are guest endian:struct virtio_balloon_config {

	u32 num_pages;

	u32 actual;

};

  Device Initialization

  The inflate and deflate virtqueues are identified.

  If the VIRTIO_BALLOON_F_STATS_VQ feature bit is negotiated:

  Identify the stats virtqueue.

  Add one empty buffer to the stats virtqueue and notify the
    host.

Device operation begins immediately.

  Device Operation

  Memory Ballooning The device is driven by the receipt of a
  configuration change interrupt.

  The “num_pages” configuration field is examined. If this is
  greater than the “actual” number of pages, memory must be given
  to the balloon. If it is less than the “actual” number of
  pages, memory may be taken back from the balloon for general
  use.

  To supply memory to the balloon (aka. inflate):

  The driver constructs an array of addresses of unused memory
    pages. These addresses are divided by 4096[footnote:
This is historical, and independent of the guest page size
] and the descriptor describing the resulting 32-bit array is
    added to the inflateq.

  To remove memory from the balloon (aka. deflate):

  The driver constructs an array of addresses of memory pages it
    has previously given to the balloon, as described above. This
    descriptor is added to the deflateq.

  If the VIRTIO_BALLOON_F_MUST_TELL_HOST feature is set, the
    guest may not use these requested pages until that descriptor
    in the deflateq has been used by the device.

  Otherwise, the guest may begin to re-use pages previously given
    to the balloon before the device has acknowledged their
    withdrawal. [footnote:
In this case, deflation advice is merely a courtesy
]

  In either case, once the device has completed the inflation or
  deflation, the “actual” field of the configuration should be
  updated to reflect the new number of pages in the balloon.[footnote:
As updates to configuration space are not atomic, this field
isn't particularly reliable, but can be used to diagnose buggy
guests.
]

  Memory Statistics

The stats virtqueue is atypical because communication is driven
by the device (not the driver). The channel becomes active at
driver initialization time when the driver adds an empty buffer
and notifies the device. A request for memory statistics proceeds
as follows:

  The device pushes the buffer onto the used ring and sends an
  interrupt.

  The driver pops the used buffer and discards it.

  The driver collects memory statistics and writes them into a
  new buffer.

  The driver adds the buffer to the virtqueue and notifies the
  device.

  The device pops the buffer (retaining it to initiate a
  subsequent request) and consumes the statistics.

  Memory Statistics Format Each statistic consists of a 16 bit
  tag and a 64 bit value. Both quantities are represented in the
  native endian of the guest. All statistics are optional and the
  driver may choose which ones to supply. To guarantee backwards
  compatibility, unsupported statistics should be omitted.

  struct virtio_balloon_stat {

#define VIRTIO_BALLOON_S_SWAP_IN  0

#define VIRTIO_BALLOON_S_SWAP_OUT 1

#define VIRTIO_BALLOON_S_MAJFLT   2

#define VIRTIO_BALLOON_S_MINFLT   3

#define VIRTIO_BALLOON_S_MEMFREE  4

#define VIRTIO_BALLOON_S_MEMTOT   5

	u16 tag;

	u64 val;

} __attribute__((packed));

  Tags

  VIRTIO_BALLOON_S_SWAP_IN The amount of memory that has been
  swapped in (in bytes).

  VIRTIO_BALLOON_S_SWAP_OUT The amount of memory that has been
  swapped out to disk (in bytes).

  VIRTIO_BALLOON_S_MAJFLT The number of major page faults that
  have occurred.

  VIRTIO_BALLOON_S_MINFLT The number of minor page faults that
  have occurred.

  VIRTIO_BALLOON_S_MEMFREE The amount of memory not being used
  for any purpose (in bytes).

  VIRTIO_BALLOON_S_MEMTOT The total amount of memory available
  (in bytes).

OpenPOWER on IntegriCloud