Re: x86: kvm: Revert "remove sched notifier for cross-cpu migrations"

["Radim Krčmář" <rkrcmar@redhat.com>]

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 <mtosatti@redhat.com>
> CC: stable@kernel.org # 3.11+

Please improve the commit message.
"KVM: x86: update pvclock area conditionally [...]" was merged half a
year before the patch we are reverting and is completely unrelated to
the bug we are fixing now, (reverted patch just was just wrong)

Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>

> diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
> @@ -82,18 +82,15 @@ static notrace cycle_t vread_pvclock(int *mode)
>  	/*
> -	 * 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.
> -	 *
> +	 * 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.

(All points from the original comment need to hold -- it would be nicer
 to keep both.)