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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Fri, 6 Jul 2018 11:31:38 +0200
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Subject: Re: [Qemu-devel] [PATCH] i386: Add support to get/set/migrate MSR
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To: "=?UTF-8?Q?Daniel_P._Berrang=c3=a9?=" <berrange@redhat.com>, Eduardo Habkost <ehabkost@redhat.com>
Cc: Jingqi Liu <jingqi.liu@intel.com>, wei.w.wang@intel.com, mtosatti@redhat.com, qemu-devel@nongnu.org, rth@twiddle.net

On 06/07/2018 10:43, Daniel P. Berrang=C3=A9 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.

Paolo