hyperv/hn: Use buf_ring for txdesc list

So one spinlock is avoided, which would be potentially dangerous for
virtual machine, if the spinlock holder was scheduled out by the host,
as noted by royger.

Old spinlock based txdesc list is still kept around, so we could have
a safe fallback.

No performance regression nor improvement is observed.

Reviewed by:	adrian
Approved by:	adrian (mentor)
MFC after:	1 week
Sponsored by:	Microsoft OSTC
Differential Revision:	https://reviews.freebsd.org/D5290

2 files changed, 66 insertions, 10 deletions

diff --git a/sys/dev/hyperv/netvsc/hv_net_vsc.h b/sys/dev/hyperv/netvsc/hv_net_vsc.h
index fa71af2..693c584 100644
--- a/sys/dev/hyperv/netvsc/hv_net_vsc.h
+++ b/sys/dev/hyperv/netvsc/hv_net_vsc.h
@@ -58,6 +58,8 @@
 
 #include <dev/hyperv/include/hyperv.h>
 
+#define HN_USE_TXDESC_BUFRING
+
 MALLOC_DECLARE(M_NETVSC);
 
 #define NVSP_INVALID_PROTOCOL_VERSION           (0xFFFFFFFF)
@@ -990,8 +992,12 @@ typedef struct {
 	hv_bool_uint8_t	link_state;
 } netvsc_device_info;
 
+#ifndef HN_USE_TXDESC_BUFRING
 struct hn_txdesc;
 SLIST_HEAD(hn_txdesc_list, hn_txdesc);
+#else
+struct buf_ring;
+#endif
 
 struct hn_rx_ring {
 	struct lro_ctrl	hn_lro;
@@ -1012,8 +1018,12 @@ struct hn_rx_ring {
 #define HN_TRUST_HCSUM_UDP	0x0004
 
 struct hn_tx_ring {
+#ifndef HN_USE_TXDESC_BUFRING
 	struct mtx	hn_txlist_spin;
 	struct hn_txdesc_list hn_txlist;
+#else
+	struct buf_ring	*hn_txdesc_br;
+#endif
 	int		hn_txdesc_cnt;
 	int		hn_txdesc_avail;
 	int		hn_txeof;
diff --git a/sys/dev/hyperv/netvsc/hv_netvsc_drv_freebsd.c b/sys/dev/hyperv/netvsc/hv_netvsc_drv_freebsd.c
index 23e43ce..9a2b2f7 100644
--- a/sys/dev/hyperv/netvsc/hv_netvsc_drv_freebsd.c
+++ b/sys/dev/hyperv/netvsc/hv_netvsc_drv_freebsd.c
@@ -70,6 +70,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/lock.h>
 #include <sys/sx.h>
 #include <sys/sysctl.h>
+#include <sys/buf_ring.h>
 
 #include <net/if.h>
 #include <net/if_arp.h>
@@ -151,7 +152,9 @@ __FBSDID("$FreeBSD$");
 #define HN_DIRECT_TX_SIZE_DEF		128
 
 struct hn_txdesc {
+#ifndef HN_USE_TXDESC_BUFRING
 	SLIST_ENTRY(hn_txdesc) link;
+#endif
 	struct mbuf	*m;
 	struct hn_tx_ring *txr;
 	int		refs;
@@ -258,6 +261,14 @@ SYSCTL_INT(_hw_hn, OID_AUTO, share_tx_taskq, CTLFLAG_RDTUN,
 
 static struct taskqueue	*hn_tx_taskq;
 
+#ifndef HN_USE_TXDESC_BUFRING
+static int hn_use_txdesc_bufring = 0;
+#else
+static int hn_use_txdesc_bufring = 1;
+#endif
+SYSCTL_INT(_hw_hn, OID_AUTO, use_txdesc_bufring, CTLFLAG_RD,
+    &hn_use_txdesc_bufring, 0, "Use buf_ring for TX descriptors");
+
 /*
  * Forward declarations
  */
@@ -570,6 +581,7 @@ hn_txdesc_put(struct hn_tx_ring *txr, struct hn_txdesc *txd)
 
 	txd->flags |= HN_TXD_FLAG_ONLIST;
 
+#ifndef HN_USE_TXDESC_BUFRING
 	mtx_lock_spin(&txr->hn_txlist_spin);
 	KASSERT(txr->hn_txdesc_avail >= 0 &&
 	    txr->hn_txdesc_avail < txr->hn_txdesc_cnt,
@@ -577,6 +589,10 @@ hn_txdesc_put(struct hn_tx_ring *txr, struct hn_txdesc *txd)
 	txr->hn_txdesc_avail++;
 	SLIST_INSERT_HEAD(&txr->hn_txlist, txd, link);
 	mtx_unlock_spin(&txr->hn_txlist_spin);
+#else
+	atomic_add_int(&txr->hn_txdesc_avail, 1);
+	buf_ring_enqueue(txr->hn_txdesc_br, txd);
+#endif
 
 	return 1;
 }
@@ -586,6 +602,7 @@ hn_txdesc_get(struct hn_tx_ring *txr)
 {
 	struct hn_txdesc *txd;
 
+#ifndef HN_USE_TXDESC_BUFRING
 	mtx_lock_spin(&txr->hn_txlist_spin);
 	txd = SLIST_FIRST(&txr->hn_txlist);
 	if (txd != NULL) {
@@ -595,8 +612,14 @@ hn_txdesc_get(struct hn_tx_ring *txr)
 		SLIST_REMOVE_HEAD(&txr->hn_txlist, link);
 	}
 	mtx_unlock_spin(&txr->hn_txlist_spin);
+#else
+	txd = buf_ring_dequeue_sc(txr->hn_txdesc_br);
+#endif
 
 	if (txd != NULL) {
+#ifdef HN_USE_TXDESC_BUFRING
+		atomic_subtract_int(&txr->hn_txdesc_avail, 1);
+#endif
 		KASSERT(txd->m == NULL && txd->refs == 0 &&
 		    (txd->flags & HN_TXD_FLAG_ONLIST), ("invalid txd"));
 		txd->flags &= ~HN_TXD_FLAG_ONLIST;
@@ -2048,13 +2071,20 @@ hn_create_tx_ring(struct hn_softc *sc, int id)
 
 	txr->hn_sc = sc;
 
+#ifndef HN_USE_TXDESC_BUFRING
 	mtx_init(&txr->hn_txlist_spin, "hn txlist", NULL, MTX_SPIN);
+#endif
 	mtx_init(&txr->hn_tx_lock, "hn tx", NULL, MTX_DEF);
 
 	txr->hn_txdesc_cnt = HN_TX_DESC_CNT;
 	txr->hn_txdesc = malloc(sizeof(struct hn_txdesc) * txr->hn_txdesc_cnt,
 	    M_NETVSC, M_WAITOK | M_ZERO);
+#ifndef HN_USE_TXDESC_BUFRING
 	SLIST_INIT(&txr->hn_txlist);
+#else
+	txr->hn_txdesc_br = buf_ring_alloc(txr->hn_txdesc_cnt, M_NETVSC,
+	    M_WAITOK, &txr->hn_tx_lock);
+#endif
 
 	txr->hn_tx_taskq = sc->hn_tx_taskq;
 	TASK_INIT(&txr->hn_start_task, 0, hn_start_taskfunc, txr);
@@ -2158,7 +2188,11 @@ hn_create_tx_ring(struct hn_softc *sc, int id)
 
 		/* All set, put it to list */
 		txd->flags |= HN_TXD_FLAG_ONLIST;
+#ifndef HN_USE_TXDESC_BUFRING
 		SLIST_INSERT_HEAD(&txr->hn_txlist, txd, link);
+#else
+		buf_ring_enqueue(txr->hn_txdesc_br, txd);
+#endif
 	}
 	txr->hn_txdesc_avail = txr->hn_txdesc_cnt;
 
@@ -2191,6 +2225,20 @@ hn_create_tx_ring(struct hn_softc *sc, int id)
 }
 
 static void
+hn_txdesc_dmamap_destroy(struct hn_txdesc *txd)
+{
+	struct hn_tx_ring *txr = txd->txr;
+
+	KASSERT(txd->m == NULL, ("still has mbuf installed"));
+	KASSERT((txd->flags & HN_TXD_FLAG_DMAMAP) == 0, ("still dma mapped"));
+
+	bus_dmamap_unload(txr->hn_tx_rndis_dtag, txd->rndis_msg_dmap);
+	bus_dmamem_free(txr->hn_tx_rndis_dtag, txd->rndis_msg,
+	    txd->rndis_msg_dmap);
+	bus_dmamap_destroy(txr->hn_tx_data_dtag, txd->data_dmap);
+}
+
+static void
 hn_destroy_tx_ring(struct hn_tx_ring *txr)
 {
 	struct hn_txdesc *txd;
@@ -2198,19 +2246,15 @@ hn_destroy_tx_ring(struct hn_tx_ring *txr)
 	if (txr->hn_txdesc == NULL)
 		return;
 
+#ifndef HN_USE_TXDESC_BUFRING
 	while ((txd = SLIST_FIRST(&txr->hn_txlist)) != NULL) {
-		KASSERT(txd->m == NULL, ("still has mbuf installed"));
-		KASSERT((txd->flags & HN_TXD_FLAG_DMAMAP) == 0,
-		    ("still dma mapped"));
 		SLIST_REMOVE_HEAD(&txr->hn_txlist, link);
-
-		bus_dmamap_unload(txr->hn_tx_rndis_dtag,
-		    txd->rndis_msg_dmap);
-		bus_dmamem_free(txr->hn_tx_rndis_dtag,
-		    txd->rndis_msg, txd->rndis_msg_dmap);
-
-		bus_dmamap_destroy(txr->hn_tx_data_dtag, txd->data_dmap);
+		hn_txdesc_dmamap_destroy(txd);
 	}
+#else
+	while ((txd = buf_ring_dequeue_sc(txr->hn_txdesc_br)) != NULL)
+		hn_txdesc_dmamap_destroy(txd);
+#endif
 
 	if (txr->hn_tx_data_dtag != NULL)
 		bus_dma_tag_destroy(txr->hn_tx_data_dtag);
@@ -2219,7 +2263,9 @@ hn_destroy_tx_ring(struct hn_tx_ring *txr)
 	free(txr->hn_txdesc, M_NETVSC);
 	txr->hn_txdesc = NULL;
 
+#ifndef HN_USE_TXDESC_BUFRING
 	mtx_destroy(&txr->hn_txlist_spin);
+#endif
 	mtx_destroy(&txr->hn_tx_lock);
 }