Re: [Qemu-devel] [PATCH] i386: Add support to get/set/migrate MSR (33H)

[Eduardo Habkost <ehabkost@redhat.com>]

On Fri, Jul 06, 2018 at 11:31:38AM +0200, Paolo Bonzini wrote:
> On 06/07/2018 10:43, Daniel P. Berrangé wrote:
> >> Based on the Linux patch at [1], guests may try to detect the
> >> feature by writing to the MSR unconditionally.
> >>
> >> If this happens, KVM needs to provide a mechanism to
> >> enable/disable the MSR emulation.  Otherwise users will end up
> >> with VMs that can't be migrated to older hosts even if they are
> >> using older machine-types.
> > Is there really no CPUID flag that can be used to detect the feature ?
> > Unconditionally probing for existance of arbitrary MSRs seems to be
> > just re-inventing CPUID feature detection, but worse because as you
> > say we need to now invent a way to control existance of individual
> > MSRs too :-(
> 
> Now that I know that no silicon exists for this feature, the solution is
> simple.  The feature will be rejected until a CPUID bit exists.

This sounds even better than requiring a new mechanism to
enable/disable MSRs in KVM.

-- 
Eduardo