Implement a workaround for a long-standing problem in

libi386's time(), caused by a qemu bug.  The bug might
be present in other BIOSes, too.

qemu either does not simulate the AT RTC correctly or
has a broken BIOS 1A/02 implementation, and will return
an incorrect value if the RTC is read while it is being
updated.

The effect is worsened by the fact that qemu's INT 15/86
function ("wait" a.k.a. usleep) is non-implmeneted or
broken and returns immediately, causing beastie.4th to
spin in a tight loop calling the "read RTC" function
millions of times, triggering the problem quickly.

Therefore, we keep reading the BIOS value until we get
the same result twice.  This change fixes beastie.4th's
countdown under qemu.

Approved by:	des (mentor)

1 files changed, 32 insertions, 8 deletions

diff --git a/sys/boot/i386/libi386/time.c b/sys/boot/i386/libi386/time.c
index 1db9a4a..183ac3c 100644
--- a/sys/boot/i386/libi386/time.c
+++ b/sys/boot/i386/libi386/time.c
@@ -32,18 +32,16 @@ __FBSDID("$FreeBSD$");
 #include "bootstrap.h"
 #include "libi386.h"
 
+static int	bios_seconds(void);
+
 /*
- * Return the time in seconds since the beginning of the day.
- *
- * If we pass midnight, don't wrap back to 0.
+ * Return the BIOS time-of-day value.
  *
  * XXX uses undocumented BCD support from libstand.
  */
-
-time_t
-time(time_t *t)
+static int
+bios_seconds(void)
 {
-    static time_t	lasttime, now;
     int			hr, minute, sec;
     
     v86.ctl = 0;
@@ -55,7 +53,33 @@ time(time_t *t)
     minute = bcd2bin(v86.ecx & 0xff);		/* minute in %cl */
     sec = bcd2bin((v86.edx & 0xff00) >> 8);	/* second in %dh */
     
-    now = hr * 3600 + minute * 60 + sec;
+    return (hr * 3600 + minute * 60 + sec);
+}
+
+/*
+ * Return the time in seconds since the beginning of the day.
+ *
+ * Some BIOSes (notably qemu) don't correctly read the RTC
+ * registers in an atomic way, sometimes returning bogus values.
+ * Therefore we "debounce" the reading by accepting it only when
+ * we got two identical values in succession.
+ *
+ * If we pass midnight, don't wrap back to 0.
+ */
+time_t
+time(time_t *t)
+{
+    static time_t lasttime;
+    time_t now, check;
+    int try;
+
+    try = 0;
+    check = bios_seconds();
+    do {
+	now = check;
+	check = bios_seconds();
+    } while (now != check && ++try < 1000);
+
     if (now < lasttime)
 	now += 24 * 3600;
     lasttime = now;