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

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Date:   Mon, 26 Feb 2018 13:16:56 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Wanpeng Li <kernellwp@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH] KVM: X86: Allow userspace to define the microcode version

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.

Thanks,

Paolo

>> It has already been fixed for a few months, and fixing it is indeed the
>> right thing to do independent of this patch.
> 
> Yap.
>