On Mon, Nov 14, 2016 at 05:43:33PM +0100, Paolo Bonzini wrote:
>
>
> On 14/11/2016 16:40, Marcelo Tosatti wrote:
> > static bool kvmclock_src_use_reliable_get_clock(void *opaque)
> > {
> > KVMClockState *s = opaque;
> >
> > /*
> > * On machine types that support reliable KVM_GET_CLOCK,
> > * if host kernel does provide reliable KVM_GET_CLOCK,
> > * set src_use_reliable_get_clock=true so that destination
> > * avoids reading kvmclock from memory.
> > */
> > if (s->mach_use_reliable_get_clock && kvm_has_adjust_clock_stable())
> > {
> > s->src_use_reliable_get_clock = true;
> > }
> >
> > return s->mach_use_reliable_get_clock;
> > }
> >
> >
> > Ah, OK, done.
>
> s->src_use_reliable_get_clock should not be set with
> KVM_CHECK_EXTENSION, but rather from the flags returned by KVM_GET_CLOCK.
Well, thats not right: What matters is the presence of get_kvmclock_ns
which returns a value that the guest sees.
get_kernel_monotonic_clock() + kvmclock_offset +
(rdtsc() - tsc_timestamp)
IOW what the guest sees. And you changed that in
commit 108b249c453dd7132599ab6dc7e435a7036c193f
Author: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Date: Thu Sep 1 14:21:03 2016 +0200
KVM: x86: introduce get_kvmclock_ns
And the correct behaviour (once KVM_GET_CLOCK is fixed per
previous message to return rdtsc - tsc_timestamp for the
non masterclock case) depends on this commit above,
not on masterclock.
> > So s->src_use_reliable_get_clock is only used to indicate
> > to the destination that: "you can use KVM_GET_CLOCK value,
> > its safe".
>
> Yes, we agree. I was listing all the points, not just those where we
> disagree. Actually I'm not sure where we disagree, except on using
> flags from KVM_CHECK_EXTENSION vs. flags from KVM_GET_CLOCK...
>
> Paolo
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