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From:   Wanpeng Li <kernellwp@gmail.com>
Date:   Mon, 26 Feb 2018 19:02:29 +0800
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Subject: Re: [PATCH] KVM: X86: Allow userspace to define the microcode version
To:     Borislav Petkov <bp@alien8.de>
Cc:     LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>
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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