Re: [PATCH 2/2] KVM: x86: Allow userspace to define what's the microcode version

[Sironi, Filippo]

> On 27. Nov 2017, at 02:40, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> 
> 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

On C5 and M5 instances, we're basically exposing the host CPUID with few
exceptions.  Linux (among the others) has checks to make sure that certain
features aren't enabled on a certain family/model/stepping if the microcode
version isn't greater than or equal to a known good version.
apic_check_deadline_errata() in arch/x86/kernel/apic/apic.c is the most
recent example in Linux that I know of (by now you've updated it ;) but
when we got the original bug report that triggered this patch, this wasn't
the case).

By exposing the real microcode version, we're preventing buggy guests that
don't check that they are running virtualized (i.e., they should trust the
hypervisor) from disabling features that are effectively not buggy.

Filippo

>> 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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