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Date: Wed, 29 Aug 2012 16:40:21 +0300
From: Gleb Natapov <gleb@redhat.com>
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Subject: Re: [Qemu-devel] [PATCHv4 3/4] cpuid: disable pv eoi for 1.1 and
 older compat types
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: kvm@vger.kernel.org, "Michael S. Tsirkin" <mst@redhat.com>, mtosatti@redhat.com, qemu-devel@nongnu.org, Blue Swirl <blauwirbel@gmail.com>, Jan Kiszka <jan.kiszka@web.de>, avi@redhat.com

On Wed, Aug 29, 2012 at 08:36:30AM -0500, Anthony Liguori wrote:
> "Michael S. Tsirkin" <mst@redhat.com> writes:
> 
> > In preparation for adding PV EOI support, disable PV EOI by default for
> > 1.1 and older machine types, to avoid CPUID changing during migration.
> >
> > PV EOI can still be enabled/disabled by specifying it explicitly.
> >     Enable for 1.1
> >     -M pc-1.1 -cpu kvm64,+kvm_pv_eoi
> >     Disable for 1.2
> >     -M pc-1.2 -cpu kvm64,-kvm_pv_eoi
> >
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> 
> As best I can tell, we're papering over an ABI breakage in KVM.
> 
> If an old QEMU attempts to do a live migration on a new kernel,
> migrating to an QEMU on a different box with an older kernel, it will
> fail because there is state that isn't being migrated.
> 
> This ought to be fixed in the kernel by making these features
> whitelisted by userspace.
> 
What do you mean. Userspace and only userspace decides what cpuid bits
will be seen by a guest. Currently userspace enables all PV cpuid bits
it finds.

--
			Gleb.