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SubjectRe: [PATCH 2/2] KVM: x86: Allow userspace to define what's the microcode version
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On 26/11/2017 17:41, Filippo Sironi wrote:
> ... that the guest should see.
> Guest operating systems may check the microcode version to decide whether
> to disable certain features that are known to be buggy up to certain
> microcode versions. Address the issue by making the microcode version
> that the guest should see settable.

What's the advantage of specifying the microcode version, rather than
relying on userspace to drop the CPUID bit for the buggy feature?

old guest(*) new guest

hide in CPUID good good

use ucode rev BAD good


(*) old guest = doesn't know that the feature is buggy until a given
ucode revision

Thanks,

Paolo

> The rationale for having userspace specifying the microcode version, rather
> than having the kernel picking it, is to ensure consistency for live-migrated
> instances; we don't want them to see a microcode version increase without a
> reset.

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