Re: [qemu patch 2/2] kvmclock: reduce kvmclock difference on migration

[Marcelo Tosatti <mtosatti@redhat.com>]

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@redhat.com>
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