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

[Glauber Costa]

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.