dv@ suggested coming to the list to request testing for the pvclock(4)
driver. Attached is a patch that corrects several bugs. Most of
these changes will only matter in the non-TSC_STABLE case on a
multiprocessor VM.
Ideally, nothing should break.
- pvclock yields a 64-bit value. The BSD timecounter layer can only
use the lower 32 bits, but internally we need to track the full
64-bit value to allow comparisons with the full value in the
non-TSC_STABLE case. So make pvclock_lastcount a 64-bit quantity.
- In pvclock_get_timecount(), move rdtsc() up into the lockless read
loop to get a more accurate timestamp.
- In pvclock_get_timecount(), use rdtsc_lfence(), not rdtsc().
- In pvclock_get_timecount(), check that our TSC value doesn't predate
ti->ti_tsc_timestamp, otherwise we will produce an enormous value.
- In pvclock_get_timecount(), update pvclock_lastcount in the
non-TSC_STABLE case with more care. On amd64 we can do this with an
atomic_cas_ulong(9) loop because u_long is 64 bits. On i386 we need
to introduce a mutex to protect our comparison and read/write.
Index: pvclock.c
===================================================================
RCS file: /cvs/src/sys/dev/pv/pvclock.c,v
retrieving revision 1.8
diff -u -p -r1.8 pvclock.c
--- pvclock.c 5 Nov 2021 11:38:29 -0000 1.8
+++ pvclock.c 2 Sep 2022 22:54:08 -0000
@@ -27,6 +27,10 @@
#include <sys/timeout.h>
#include <sys/malloc.h>
#include <sys/atomic.h>
+#include <sys/stdint.h>
+#if defined(__i386__)
+#include <sys/mutex.h>
+#endif
#include <machine/cpu.h>
#include <machine/atomic.h>
@@ -35,7 +39,12 @@
#include <dev/pv/pvvar.h>
#include <dev/pv/pvreg.h>
-uint pvclock_lastcount;
+#if defined(__amd64__)
+volatile u_long pvclock_lastcount;
+#elif defined(__i386__)
+struct mutex pvclock_mtx = MUTEX_INITIALIZER(IPL_HIGH);
+uint64_t pvclock_lastcount;
+#endif
struct pvclock_softc {
struct device sc_dev;
@@ -212,7 +221,7 @@ pvclock_get_timecount(struct timecounter
{
struct pvclock_softc *sc = tc->tc_priv;
struct pvclock_time_info *ti;
- uint64_t tsc_timestamp, system_time, delta, ctr;
+ uint64_t system_time, delta, ctr, tsc;
uint32_t version, mul_frac;
int8_t shift;
uint8_t flags;
@@ -220,8 +229,12 @@ pvclock_get_timecount(struct timecounter
ti = sc->sc_time;
do {
version = pvclock_read_begin(ti);
+ tsc = rdtsc_lfence();
+ if (ti->ti_tsc_timestamp < tsc)
+ delta = tsc - ti->ti_tsc_timestamp;
+ else
+ delta = 0;
system_time = ti->ti_system_time;
- tsc_timestamp = ti->ti_tsc_timestamp;
mul_frac = ti->ti_tsc_to_system_mul;
shift = ti->ti_tsc_shift;
flags = ti->ti_flags;
@@ -231,7 +244,6 @@ pvclock_get_timecount(struct timecounter
* The algorithm is described in
* linux/Documentation/virtual/kvm/msr.txt
*/
- delta = rdtsc() - tsc_timestamp;
if (shift < 0)
delta >>= -shift;
else
@@ -241,10 +253,20 @@ pvclock_get_timecount(struct timecounter
if ((flags & PVCLOCK_FLAG_TSC_STABLE) != 0)
return (ctr);
- if (ctr < pvclock_lastcount)
- return (pvclock_lastcount);
-
- atomic_swap_uint(&pvclock_lastcount, ctr);
-
+#if defined(__amd64__)
+ u_long last;
+ do {
+ last = pvclock_lastcount;
+ if (ctr < last)
+ return last;
+ } while (atomic_cas_ulong(&pvclock_lastcount, last, ctr) != last);
+#elif defined(__i386__)
+ mtx_enter(&pvclock_mtx);
+ if (pvclock_lastcount < ctr)
+ pvclock_lastcount = ctr;
+ else
+ ctr = pvclock_lastcount;
+ mtx_leave(&pvclock_mtx);
+#endif
return (ctr);
}
