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

On 10/07/2009 11:51 PM, Jeremy Fitzhardinge wrote:
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,

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.

Yes. We have the space since a cacheline is 64 bytes (minimum) vs 32 bytes of pvclock data.

I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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