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

[Paolo Bonzini]

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.
>