linux-kernel - Re: [PATCH] KVM: x86: do not reset microcode version on INIT or RESET

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Message-ID: <988c9422-52d2-f544-c703-b3ec6274e2be@redhat.com>
Date:   Wed, 12 Feb 2020 08:44:20 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: do not reset microcode version on INIT or RESET

On 11/02/20 23:38, Sean Christopherson wrote:
> On Tue, Feb 11, 2020 at 07:04:39PM +0100, Paolo Bonzini wrote:
>> The microcode version should be set just once, since it is essentially
>> a CPU feature; so do it on vCPU creation rather than reset.
> I wouldn't call it a CPU feature, CPU features generally can't be
> arbitrarily changed while running.

That was true of CPUID bits too until microcode started adding and
removing them, but I see your point. :)  What I was trying to convey as
"CPU feature" is that KVM will not change it arbitrarily when running;
it can only change as a result of userspace actions, KVM_SET_MSRS in
this case.  But yes, I will improve the text based on your version:

---
Do not initialize the microcode version at RESET or INIT, only on vCPU
creation.   Microcode updates are not lost during INIT, and exact
behavior across a warm RESET is not specified by the architecture.

Since we do not support a microcode update directly from the hypervisor,
but only as a result of userspace setting the microcode version MSR,
it's simpler for userspace if we do nothing in KVM and let userspace
emulate behavior for RESET as it sees fit.
---

Thanks,

Paolo

> I'd prefer to have a changelog that
> at least somewhat ties the change to hardware behavior. 
> 
>   Do not initialize the microcode version at RESET or INIT.   Microcode
>   updates are not lost during INIT, and exact behavior across a warm RESET
>   is microarchitectural, i.e. defer to userspace to emulate behavior for
>   RESET as it sees fit.
> 
> For the code: