[Qemu-devel] Re: [PATCH v2 1/2] Do not register kvmclock savevm section if kvmclock is disabled.

[Marcelo Tosatti]

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?