Re: [PATCH] x86/kvm: fix condition to update kvm master clocks

[Roman Kagan <rkagan@virtuozzo.com>]

On Mon, Jun 20, 2016 at 06:29:10PM -0300, Marcelo Tosatti wrote:
> On Mon, Jun 20, 2016 at 08:22:49PM +0300, Roman Kagan wrote:
> > The per-vcpu hv_clock is updated when the vcpu processes
> > KVM_REQ_CLOCK_UPDATE request.
> 
> Yes.
> 
> > Once kvm_gen_update_masterclock() sets KVM_REQ_CLOCK_UPDATE and
> > clears KVM_REQ_MCLOCK_INPROGRESS for all vcpus, one vcpu can process the
> > requests, enter the guest, and read another vcpu's hv_clock, before that
> > other vcpu had a chance to process its KVM_REQ_CLOCK_UPDATE request.
> 
> Yes. But guest code should be reading its local kvmclock area:
> 
>                 /*
>                  * 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.
>                  */
>                 cpu1 = __getcpu() & VGETCPU_CPU_MASK;
> 
> (vclock_gettime.c)

This code is from an older version.  The latest always reads the clock
of the CPU #0:

        /*
         * Note: The kernel and hypervisor must guarantee that cpu ID
         * number maps 1:1 to per-CPU pvclock time info.
         *
         * Because the hypervisor is entirely unaware of guest userspace
         * preemption, it cannot guarantee that per-CPU pvclock time
         * info is updated if the underlying CPU changes or that that
         * version is increased whenever underlying CPU changes.
         *
         * On KVM, we are guaranteed that pvti updates for any vCPU are
         * atomic as seen by *all* vCPUs.  This is an even stronger
         * guarantee than we get with a normal seqlock.
         *
         * On Xen, we don't appear to have that guarantee, but Xen still
         * supplies a valid seqlock using the version field.
         *
         * We only do pvclock vdso timing at all if
         * PVCLOCK_TSC_STABLE_BIT is set, and we interpret that bit to
         * mean that all vCPUs have matching pvti and that the TSC is
         * synced, so we can just look at vCPU 0's pvti.
         */

> > Is there anything that prevents this?
> 
> Guest code confirming both version and cpu do not change across 
> a kvmclock read. Other than this, no.

So is the code reading another vcpu's hv_clock wrong?

Roman.