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Diffstat (limited to 'arch/x86/vdso/vclock_gettime.c')
-rw-r--r--arch/x86/vdso/vclock_gettime.c34
1 files changed, 15 insertions, 19 deletions
diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
index 40d2473836c9..9793322751e0 100644
--- a/arch/x86/vdso/vclock_gettime.c
+++ b/arch/x86/vdso/vclock_gettime.c
@@ -82,15 +82,18 @@ static notrace cycle_t vread_pvclock(int *mode)
cycle_t ret;
u64 last;
u32 version;
- u32 migrate_count;
u8 flags;
unsigned cpu, cpu1;
/*
- * When looping to get a consistent (time-info, tsc) pair, we
- * also need to deal with the possibility we can switch vcpus,
- * so make sure we always re-fetch time-info for the current vcpu.
+ * Note: hypervisor must guarantee that:
+ * 1. cpu ID number maps 1:1 to per-CPU pvclock time info.
+ * 2. that per-CPU pvclock time info is updated if the
+ * underlying CPU changes.
+ * 3. that version is increased whenever underlying CPU
+ * changes.
+ *
*/
do {
cpu = __getcpu() & VGETCPU_CPU_MASK;
@@ -99,27 +102,20 @@ static notrace cycle_t vread_pvclock(int *mode)
* __getcpu() calls (Gleb).
*/
- /* Make sure migrate_count will change if we leave the VCPU. */
- do {
- pvti = get_pvti(cpu);
- migrate_count = pvti->migrate_count;
-
- cpu1 = cpu;
- cpu = __getcpu() & VGETCPU_CPU_MASK;
- } while (unlikely(cpu != cpu1));
+ pvti = get_pvti(cpu);
version = __pvclock_read_cycles(&pvti->pvti, &ret, &flags);
/*
* Test we're still on the cpu as well as the version.
- * - We must read TSC of pvti's VCPU.
- * - KVM doesn't follow the versioning protocol, so data could
- * change before version if we left the VCPU.
+ * We could have been migrated just after the first
+ * vgetcpu but before fetching the version, so we
+ * wouldn't notice a version change.
*/
- smp_rmb();
- } while (unlikely((pvti->pvti.version & 1) ||
- pvti->pvti.version != version ||
- pvti->migrate_count != migrate_count));
+ cpu1 = __getcpu() & VGETCPU_CPU_MASK;
+ } while (unlikely(cpu != cpu1 ||
+ (pvti->pvti.version & 1) ||
+ pvti->pvti.version != version));
if (unlikely(!(flags & PVCLOCK_TSC_STABLE_BIT)))
*mode = VCLOCK_NONE;