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Subject: Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future
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To:     "Van De Ven, Arjan" <arjan.van.de.ven@intel.com>,
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From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <aa3fbaf1-0efc-64b1-4f7b-3d00b0493d48@redhat.com>
Date:   Tue, 20 Feb 2018 12:43:37 +0100
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On 20/02/2018 01:00, Van De Ven, Arjan wrote:
>>> the guest is not the problem; guests obviously will already honor
>>> if Enhanced IBRS is enumerated. The problem is mixed migration
>>> pools where the hypervisor
>>> may need to decide to not pass this enumeration through to the
>>> guest.
>> For bonus points:  What should happen to a VM that is live migrated
>> from one hypervisor to another, and the hypervisors have different
>> IBRS support?
>
> Doctor Doctor it hurts when I do this....
> 
> Migration tends to only work between HV's that are relatively
> homogeneous, that's nothing new...

No Arjan, this is just wrong.  Well, I suppose it's right in the present
tense with the IBRS mess on Skylake, but it's _not_ been true until last
year.

Paolo