linux-kernel - Re: [PATCH] KVM: x86: Allow XSAVES on CPUs where host doesn't use it due to an errata

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Date:   Thu, 30 Nov 2023 19:24:45 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Allow XSAVES on CPUs where host doesn't use
 it due to an errata

On Mon, 2023-11-27 at 09:24 -0800, Sean Christopherson wrote:
> On Thu, Nov 23, 2023, Maciej S. Szmigiero wrote:
> > From: "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>
> > 
> > Since commit b0563468eeac ("x86/CPU/AMD: Disable XSAVES on AMD family 0x17")
> > kernel unconditionally clears the XSAVES CPU feature bit on Zen1/2 CPUs.
> > 
> > Since KVM CPU caps are initialized from the kernel boot CPU features this
> > makes the XSAVES feature also unavailable for KVM guests in this case, even
> > though they might want to decide on their own whether they are affected by
> > this errata.
> > 
> > Allow KVM guests to make such decision by setting the XSAVES KVM CPU
> > capability bit based on the actual CPU capability
> 
> This is not generally safe, as the guest can make such a decision if and only if
> the Family/Model/Stepping information is reasonably accurate.

Another thing that really worries me is that the XSAVES errata is really nasty one - 
AFAIK it silently corrupts some registers.

Is it better to let a broken CPU boot a broken OS (OS which demands XSAVES blindly),
and let a silent data corruption happen than refuse to boot it completely?

I mean I understand that it is technically OS fault in this case (assuming that we
do provide it the correct CPU family info), but still this seems like the wrong thing to do.

I guess this is one of those few cases when it makes sense for the userspace to
override KVM's CPUID caps and force a feature - in this case at least that
won't be KVM's fault.


Best regards,
	Maxim Levitsky

> 
> > This fixes booting Hyper-V enabled Windows Server 2016 VMs with more than
> > one vCPU on Zen1/2 CPUs.
> 
> How/why does lack of XSAVES break a multi-vCPU setup?  Is Windows blindly doing
> XSAVES based on FMS?
>