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Subject: Re: [PATCH] KVM: X86: Allow userspace to define the microcode version
From:   Paolo Bonzini <pbonzini@redhat.com>
To:     Borislav Petkov <bp@alien8.de>
Cc:     Wanpeng Li <kernellwp@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>
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Message-ID: <3866d359-0ef8-6a99-6254-84890be62b93@redhat.com>
Date:   Mon, 26 Feb 2018 13:18:07 +0100
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On 26/02/2018 13:16, Paolo Bonzini wrote:
> On 26/02/2018 13:15, Borislav Petkov wrote:
>> On Mon, Feb 26, 2018 at 12:54:52PM +0100, Paolo Bonzini wrote:
>>> I don't understand how one thing follows from the other.  How are writes
>>> to 0x8B related to having a virtualized microcode loaded (which is a
>>> concept that actually makes no sense at all)?
>>
>> I'm questioning the whole idea. 0x8b is the MSR which gives you the
>> microcode revision. Most CPUs don't even allow writing to it, AFAICT.
>> (SDM says "may prevent writing" on VM transitions.)
>>
>> So how is that host-initiated write to 0x8b is even going to work, in
>> reality? kvm module writes the microcode version in there? How does the
>> admin work around that?
> 
> In this context, "host-initiated" write means written by KVM userspace
> with ioctl(KVM_SET_MSR).  It generally happens only on VM startup, reset
> or live migration.

To be clear, the target of the write is still the vCPU's emulated MSR.

Paolo