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SubjectRE: [PATCH 4/4] x86: properly handle kvm emulation of hyperv
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What I'm suggesting is exactly that except that the native hypervisor is later in CPUID space.

KY Srinivasan <kys@microsoft.com> wrote:
>
>
>> -----Original Message-----
>> From: H. Peter Anvin [mailto:hpa@zytor.com]
>> Sent: Wednesday, July 24, 2013 11:14 AM
>> To: KY Srinivasan; Paolo Bonzini; Jason Wang
>> Cc: tglx@linutronix.de; mingo@redhat.com; x86@kernel.org;
>gleb@redhat.com;
>> kvm@vger.kernel.org; linux-kernel@vger.kernel.org
>> Subject: RE: [PATCH 4/4] x86: properly handle kvm emulation of hyperv
>>
>> I don't see how this solves the A emulates B, B emulates A problem?
>
>As Paolo suggested if there were some priority encoded, the guest could
>make an
>informed decision. If the guest under question can run on both
>hypervisors A and B,
>we would rather the guest discover hypervisor A when running on A and
>hypervisor B
>when running on B. The priority encoding could be as simple as
>surfacing the native hypervisor
>signature earlier in the CPUID space.
>
>K. Y
>>
>> KY Srinivasan <kys@microsoft.com> wrote:
>> >
>> >
>> >> -----Original Message-----
>> >> From: Paolo Bonzini [mailto:paolo.bonzini@gmail.com] On Behalf Of
>> >Paolo
>> >> Bonzini
>> >> Sent: Wednesday, July 24, 2013 3:07 AM
>> >> To: Jason Wang
>> >> Cc: H. Peter Anvin; KY Srinivasan; tglx@linutronix.de;
>> >mingo@redhat.com;
>> >> x86@kernel.org; gleb@redhat.com; kvm@vger.kernel.org; linux-
>> >> kernel@vger.kernel.org
>> >> Subject: Re: [PATCH 4/4] x86: properly handle kvm emulation of
>hyperv
>> >>
>> >> Il 24/07/2013 08:54, Jason Wang ha scritto:
>> >> > On 07/24/2013 12:48 PM, H. Peter Anvin wrote:
>> >> >> On 07/23/2013 09:37 PM, Jason Wang wrote:
>> >> >>> On 07/23/2013 10:48 PM, H. Peter Anvin wrote:
>> >> >>>> On 07/23/2013 06:55 AM, KY Srinivasan wrote:
>> >> >>>>> This strategy of hypervisor detection based on some
>detection
>> >order
>> >> IMHO is not
>> >> >>>>> a robust detection strategy. The current scheme works since
>the
>> >only
>> >> hypervisor emulated
>> >> >>>>> (by other hypervisors happens to be Hyper-V). What if this
>were
>> >to
>> >> change.
>> >> >>>>>
>> >> >>>> One strategy would be to pick the *last* one in the CPUID
>list,
>> >since
>> >> >>>> the ones before it are logically the one(s) being emulated...
>> >> >>>>
>> >> >>>> -hpa
>> >> >>>>
>> >> >>> How about simply does a reverse loop from 0x40010000 to
>> >0x40010000?
>> >> >>>
>> >> >> Not all systems like being poked too far into hyperspace. Just
>> >remember
>> >> >> the last match and walk the list.
>> >> >>
>> >> >> -hpa
>> >> >>
>> >> >
>> >> > Ok, but it raises a question - how to know it was the 'last'
>match
>> >> > without knowing all signatures of other hyper-visor?
>> >>
>> >> You can return a "priority" value from the .detect function. The
>> >> priority value can simply be the CPUID leaf where the signature
>was
>> >> found (or a low value such as 1 if detection was done with DMI).
>> >>
>> >> Then you can pick the hypervisor with the highest priority instead
>of
>> >> hard-coding the order.
>> >
>> >I like this idea; this allows some guest level control that is what
>we
>> >want
>> >when we have hypervisors emulating each other.
>> >
>> >
>> >Regards,
>> >
>> >K. Y
>> >>
>> >> Paolo
>> >>
>> >>
>>
>> --
>> Sent from my mobile phone. Please excuse brevity and lack of
>formatting.
>>
>>

--
Sent from my mobile phone. Please excuse brevity and lack of formatting.


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