linux-kernel - Re: [PATCH 0/3] KVM: x86: guest interface for SEV live migration

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Message-ID: <YH9aj8FLQ4z4Po/x@google.com>
Date:   Tue, 20 Apr 2021 22:49:51 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        srutherford@...gle.com, joro@...tes.org, brijesh.singh@....com,
        thomas.lendacky@....com, venu.busireddy@...cle.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...e.de>,
        x86@...nel.org, Ashish Kalra <ashish.kalra@....com>
Subject: Re: [PATCH 0/3] KVM: x86: guest interface for SEV live migration

On Tue, Apr 20, 2021, Paolo Bonzini wrote:
> On 20/04/21 22:16, Sean Christopherson wrote:
> > On Tue, Apr 20, 2021, Sean Christopherson wrote:
> > > On Tue, Apr 20, 2021, Paolo Bonzini wrote:
> > > > In this particular case, if userspace sets the bit in CPUID2 but doesn't
> > > > handle KVM_EXIT_HYPERCALL, the guest will probably trigger some kind of
> > > > assertion failure as soon as it invokes the HC_PAGE_ENC_STATUS hypercall.
> > 
> > Oh!  Almost forgot my hail mary idea.  Instead of a new capability, can we
> > reject the hypercall if userspace has _not_ set KVM_CAP_ENFORCE_PV_FEATURE_CPUID?
> > 
> > 			if (vcpu->arch.pv_cpuid.enforce &&
> > 			    !guest_pv_has(vcpu, KVM_FEATURE_HC_PAGE_ENC_STATUS)
> > 				break;
> 
> Couldn't userspace enable that capability and _still_ copy the supported
> CPUID blindly to the guest CPUID, without supporting the hypercall?

Yes.  I was going to argue that we get to define the behavior, but that's not
true because it would break existing VMMs that blindly copy.  Capability it is...