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Date: Wed, 13 Aug 2014 14:18:51 +0200
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Subject: Re: [Qemu-devel] [PATCH] Qemu: Fix eax for cpuid leaf 0x40000000
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To: Eduardo Habkost <ehabkost@redhat.com>
Cc: Jidong Xiao <jidong.xiao@gmail.com>, qemu-devel@nongnu.org, KVM <kvm@vger.kernel.org>

Il 12/08/2014 21:29, Eduardo Habkost ha scritto:
> On Tue, Aug 12, 2014 at 09:12:00PM +0200, Paolo Bonzini wrote:
>> Il 12/08/2014 20:55, Eduardo Habkost ha scritto:
>>> This makes the CPUID data change under the guest's feet during
>>> live-migration.
>>>
>>> Adding compat code to ensure older machine-types keep the old behavior
>>> is necessary, but in this specific case it is mostly harmless because
>>> 0x0 is documented as being equivalent to 0x40000001.
>>>
>>> (But I don't know how guests are supposed to behave when they see
>>> CPUID[KVM_CPUID_SIGNATURE_NEXT].EAX==0.)
>>
>> The only obvious thing to do would be to treat it as 0x40000101.
> 
> I just want to be sure the guests really do that. If we know guests
> won't do anything different with the CPUID change, I won't mind having
> no compat code for this.
> 

Considering that only two leaves are defined for KVM, and both are
mandatory I don't think current guests have any reason to look at
CPUID[KVM_CPUID_SIGNATURE | kvm_base].EAX at all.

Paolo