Re: [PATCH] KVM: X86: Allow userspace to define the microcode version

Wanpeng Li
2018-02-26T11:02:34Z

2018-02-26 18:49 GMT+08:00 Borislav Petkov <bp@alien8.de>:
> On Mon, Feb 26, 2018 at 06:06:42PM +0800, Wanpeng Li wrote:
>> I think it is the host admin(e.g. cloud provider)'s responsibility to
>> set an expected microcode revision.
>
> + vcpu->arch.microcode_version = 0x1;
>
> That already looks pretty arbitrary and non-sensical to me.

This is the original kvm implementation before the patch.

>
>>In addition, the non-sensical value which is written by the guest will
>>not reflect to guest-visible microcode revision and just be ignored in
>>this implementation.
>
> Huh? How so?
>
> So a guest will have *two* microcode revisions - both of which are most
> likely wrong?!

Just one revision.

>
> This whole thing sounds like the wrong approach to me.
>
>> 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.
>
> It sounds to me like the proper fix is to make the kernel *not* look at
> microcode revisions when running virtualized. The same way we're not
> loading microcode in a guest:
>
> if (native_cpuid_ecx(1) & BIT(31))
>
> Letting userspace control the microcode revision number is revision
> number management SNAFU waiting to happen IMO.

https://lkml.org/lkml/2017/12/9/29 The original discussion explain in
more details.

Regards,
Wanpeng Li