2015-03-23 20:21-0300, Marcelo Tosatti:
>
> The following point:
>
> 2. per-CPU pvclock time info is updated if the
> underlying CPU changes.
>
> Is not true anymore since "KVM: x86: update pvclock area conditionally,
> on cpu migration".
>
> Add task migration notification back.
>
> Problem noticed by Andy Lutomirski.
>
> Signed-off-by: Marcelo Tosatti <mtosa...@redhat.com>
> CC: sta...@kernel.org # 3.11+
Revert contains a bug that got pointed out in the discussion:
> diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
> do {
> cpu = __getcpu() & VGETCPU_CPU_MASK;
>
> pvti = get_pvti(cpu);
We can migrate to 'other cpu' here.
> + migrate_count = pvti->migrate_count;
> +
> version = __pvclock_read_cycles(&pvti->pvti, &ret, &flags);
And migrate back to 'cpu' here.
rdtsc was executed on different cpu, so pvti and tsc might not be in
sync, but migrate_count hasn't changed.
> cpu1 = __getcpu() & VGETCPU_CPU_MASK;
(Reading cpuid here is useless.)
> } while (unlikely(cpu != cpu1 ||
> (pvti->pvti.version & 1) ||
> - pvti->pvti.version != version));
> + pvti->pvti.version != version ||
> + pvti->migrate_count != migrate_count));
We can workaround the bug with,
cpu = __getcpu() & VGETCPU_CPU_MASK;
pvti = get_pvti(cpu);
migrate_count = pvti->migrate_count;
if (cpu != (__getcpu() & VGETCPU_CPU_MASK))
continue;
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