On Mon, Jun 20, 2016 at 08:22:49PM +0300, Roman Kagan wrote:
> On Fri, Jun 17, 2016 at 07:21:21PM -0300, Marcelo Tosatti wrote:
> > On Mon, Jun 13, 2016 at 08:19:30PM +0300, Roman Kagan wrote:
> > > While we're at this:
> > >
> > > According to the comments in the code, the purpose of the masterclock
> > > scheme is to prevent any vCPU from seeing an outdated hv_clock of
> > > another vCPU.
> >
> > It prevents two vcpus from using different hv_clocks:
> >
> > "
> > * To avoid that problem, do not allow visibility of distinct
> > * system_timestamp/tsc_timestamp values simultaneously: use a master
> > * copy of host monotonic time values. Update that master copy
> > * in lockstep.
> > "
> >
> > > However I'm missing how that is achieved. AFAICS the guest entry is
> > > allowed as soon as all vCPUs are kicked from guest with
> > > KVM_REQ_CLOCK_UPDATE set; what stops one vCPU from processing it and
> > > entering the guest before another vCPU even started updating its
> > > hv_clock?
> >
> > static void kvm_gen_update_masterclock(struct kvm *kvm)
> > {
> > #ifdef CONFIG_X86_64
> > int i;
> > struct kvm_vcpu *vcpu;
> > struct kvm_arch *ka = &kvm->arch;
> >
> > spin_lock(&ka->pvclock_gtod_sync_lock);
> > kvm_make_mclock_inprogress_request(kvm);
> > /* no guest entries from this point */
> > pvclock_update_vm_gtod_copy(kvm);
> >
> > kvm_for_each_vcpu(i, vcpu, kvm)
> > kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
> >
> > /* guest entries allowed */
> > kvm_for_each_vcpu(i, vcpu, kvm)
> > clear_bit(KVM_REQ_MCLOCK_INPROGRESS, &vcpu->requests);
> >
> > spin_unlock(&ka->pvclock_gtod_sync_lock);
> > #endif
> > }
>
> Unless I'm missing something obvious again:
>
> 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)
> Is there anything that prevents this?
Guest code confirming both version and cpu do not change across
a kvmclock read. Other than this, no.
>
> Thanks,
> Roman.
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