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Date: Fri, 11 Nov 2016 12:23:47 -0200
From: Marcelo Tosatti <mtosatti@redhat.com>
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Subject: Re: [Qemu-devel] [QEMU PATCH v2] kvmclock: advance clock by time
 window between vm_stop and pre_save
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>, kvm@vger.kernel.org, qemu-devel <qemu-devel@nongnu.org>, Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>, Juan Quintela <quintela@redhat.com>, Eduardo Habkost <ehabkost@redhat.com>

On Thu, Nov 10, 2016 at 06:57:21PM +0100, Paolo Bonzini wrote:
> 
> 
> On 10/11/2016 12:48, Marcelo Tosatti wrote:
> > Destination has to run the following logic:
> > 
> > If (source has KVM_CAP_ADVANCE_CLOCK)
> >     use KVM_GET_CLOCK value
> > Else
> >    read pvclock from guest
> > 
> > To support migration from older QEMU versions which do not have
> > KVM_CAP_ADVANCE_CLOCK (or new QEMU versions running on old
> > hosts without KVM_CAP_ADVANCE_CLOCK).
> > 
> > I don't see any clean way to give that information, except changing
> > the migration format to pass "host: kvm_cap_advance_clock=true/false"
> > information.
> 
> If you make it only affect new machine types, you could transmit a dummy
> clock value such as -1 if the source does not have KVM_CLOCK_TSC_STABLE.
> 
> Paolo

Prefer a new subsection (which is fine since migration is broken
anyway), because otherwise you have to deal with restoring
s->clock from -1 to what was read at KVM_GET_CLOCK (in case
migration fails).