Re: [PATCH] kvm: x86: Fix kvm clock versioning.

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, Nov 03, 2014 at 04:57:18PM -0800, Owen Hofmann wrote:
> kvm updates the version number for the guest paravirt clock structure by
> incrementing the version of its private copy. It does not read the guest
> version, so will write version = 2 in the first update for every new VM,
> including after restoring a saved state. If guest state is saved during
> reading the clock, it could read and accept struct fields and guest TSC
> from two different updates. This changes the code to increment the guest
> version and write it back.
> 
> Signed-off-by: Owen Hofmann <osh@google.com>
> ---
>  arch/x86/kvm/x86.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 0033df3..c25c9d8 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1637,16 +1637,16 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
>  	vcpu->hv_clock.system_time = kernel_ns + v->kvm->arch.kvmclock_offset;
>  	vcpu->last_guest_tsc = tsc_timestamp;
>  
> +	if (unlikely(kvm_read_guest_cached(v->kvm, &vcpu->pv_time,
> +		&guest_hv_clock, sizeof(guest_hv_clock))))
> +		return 0;
> +
>  	/*
>  	 * The interface expects us to write an even number signaling that the
>  	 * update is finished. Since the guest won't see the intermediate
>  	 * state, we just increase by 2 at the end.
>  	 */
> -	vcpu->hv_clock.version += 2;
> -
> -	if (unlikely(kvm_read_guest_cached(v->kvm, &vcpu->pv_time,
> -		&guest_hv_clock, sizeof(guest_hv_clock))))
> -		return 0;
> +	vcpu->hv_clock.version = guest_hv_clock.version + 2;
>  
>  	/* retain PVCLOCK_GUEST_STOPPED if set in guest copy */
>  	pvclock_flags = (guest_hv_clock.flags & PVCLOCK_GUEST_STOPPED);
> -- 
> 2.1.0.rc2.206.gedb03e5

Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>