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[Qemu-devel] Re: [PATCH v2 1/2] Do not register kvmclock savevm section
From: |
Glauber Costa |
Subject: |
[Qemu-devel] Re: [PATCH v2 1/2] Do not register kvmclock savevm section if kvmclock is disabled. |
Date: |
Mon, 13 Dec 2010 11:40:37 -0200 |
On Wed, 2010-12-08 at 17:31 -0200, Marcelo Tosatti wrote:
> On Tue, Dec 07, 2010 at 03:12:36PM -0200, Glauber Costa wrote:
> > On Mon, 2010-12-06 at 19:04 -0200, Marcelo Tosatti wrote:
> > > On Mon, Dec 06, 2010 at 09:03:46AM -0500, Glauber Costa wrote:
> > > > Usually nobody usually thinks about that scenario (me included and
> > > > specially),
> > > > but kvmclock can be actually disabled in the host.
> > > >
> > > > It happens in two scenarios:
> > > > 1. host too old.
> > > > 2. we passed -kvmclock to our -cpu parameter.
> > > >
> > > > In both cases, we should not register kvmclock savevm section. This
> > > > patch
> > > > achives that by registering this section only if kvmclock is actually
> > > > currently enabled in cpuid.
> > > >
> > > > The only caveat is that we have to register the savevm section a little
> > > > bit
> > > > later, since we won't know the final kvmclock state before cpuid gets
> > > > parsed.
> > >
> > > What is the problem of registering the section? Restoring the value if
> > > the host does not support it returns an error?
> > >
> > > Can't you ignore the error if kvmclock is not reported in cpuid, in the
> > > restore handler?
> >
> > We can change the restore handler, but not the restore handler of
> > binaries that are already out there. The motivation here is precisely to
> > address migration to hosts without kvmclock, so it's better to have
> > a way to disable, than to count on the fact that the other side will be
> > able to ignore it.
>
> OK. Can't you register conditionally on kvmclock cpuid bit at the end of
> kvm_arch_init_vcpu, in target-i386/kvm.c?
Haven't looked at it, but will today. Actually, tsc has (obviously) the
same problem and I plan to respin the patch today including a fix for it
as well.
Thanks!
[Qemu-devel] [PATCH v2 2/2] make kvmclock value idempotent for stopped machine, Glauber Costa, 2010/12/06
[Qemu-devel] Re: [PATCH v2 0/2] savevm odness related to kvmclock, Paolo Bonzini, 2010/12/06