On 04/02/2015 11:59 AM, Andy Lutomirski wrote:
On Thu, Apr 2, 2015 at 11:44 AM, Radim Krčmář <rkrcmar@xxxxxxxxxx> wrote:
If we were migrated right after __getcpu, but before reading the
migration_count, we wouldn't notice that we read TSC of a different
VCPU, nor that KVM's bug made pvti invalid, as only migration_count
on source VCPU is increased.
Change vdso instead of updating migration_count on destination.
Looks good to me.
Just to check: what tree is this intended to go through? I can take it,
but not until the previous patch makes it into Linus' tree or -tip. Or
I can take both patches.
Marcelo, Paolo?
--Andy
--Andy
Fixes: 0a4e6be9ca17 ("x86: kvm: Revert "remove sched notifier for cross-cpu migrations"")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Radim Krčmář <rkrcmar@xxxxxxxxxx>
---
Because it we'll get a complete rewrite, this series does not
- remove the outdated 'TODO: We can put [...]' comment
- use a proper encapsulation for the inner do-while loop
- optimize the outer do-while loop
(no need to re-read cpu id on version mismatch)
arch/x86/vdso/vclock_gettime.c | 20 ++++++++++++--------
1 file changed, 12 insertions(+), 8 deletions(-)
diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
index 30933760ee5f..40d2473836c9 100644
--- a/arch/x86/vdso/vclock_gettime.c
+++ b/arch/x86/vdso/vclock_gettime.c
@@ -99,21 +99,25 @@ static notrace cycle_t vread_pvclock(int *mode)
* __getcpu() calls (Gleb).
*/
- pvti = get_pvti(cpu);
+ /* Make sure migrate_count will change if we leave the VCPU. */
+ do {
+ pvti = get_pvti(cpu);
+ migrate_count = pvti->migrate_count;
- migrate_count = pvti->migrate_count;
+ cpu1 = cpu;
+ cpu = __getcpu() & VGETCPU_CPU_MASK;
+ } while (unlikely(cpu != cpu1));
version = __pvclock_read_cycles(&pvti->pvti, &ret, &flags);
/*
* Test we're still on the cpu as well as the version.
- * We could have been migrated just after the first
- * vgetcpu but before fetching the version, so we
- * wouldn't notice a version change.
+ * - 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.
*/
- cpu1 = __getcpu() & VGETCPU_CPU_MASK;
- } while (unlikely(cpu != cpu1 ||
- (pvti->pvti.version & 1) ||
+ smp_rmb();
+ } while (unlikely((pvti->pvti.version & 1) ||
pvti->pvti.version != version ||
pvti->migrate_count != migrate_count));
--
2.3.4
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