Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 10/07/09 14:37, Avi Kivity wrote:
> If the guest does a RMW on the version, but the host does not (copying
> it from somewhere else), then the guest RMW can be lost.
>
> Looking at the code, that's what kvm does:
>
>     vcpu->hv_clock.version += 2;
>
>     shared_kaddr = kmap_atomic(vcpu->time_page, KM_USER0);
>
>     memcpy(shared_kaddr + vcpu->time_offset, &vcpu->hv_clock,
>            sizeof(vcpu->hv_clock));
>
> so a guest-side ++version can be lost.

I see, yes.  The Xen code explicitly reads back the guest version and
increments that (I realize now that's what you meant by guest-private
version).  If you were to have a second version number it would have to
be separated as well to avoid being overwritten by the hypervisor.

    J