Re: [PATCH v2 1/6] KVM: SVM: Use maxphyaddr in emulator RAX check for VMRUN/VMLOAD/VMSAVE

[Sean Christopherson <seanjc@google.com>]

On Wed, Mar 11, 2026, Yosry Ahmed wrote:
> On Wed, Mar 11, 2026 at 11:31 AM Yosry Ahmed <yosry@kernel.org> wrote:
> >
> > On Fri, Mar 6, 2026 at 4:32 PM Sean Christopherson <seanjc@google.com> wrote:
> > >
> > > On Fri, Mar 06, 2026, Yosry Ahmed wrote:
> > > > > > Right, but I am trying to have the #GP check for VMLOAD/VMSAVE behave
> > > > > > consistently with vls=1, whether it's done by the hardware or the
> > > > > > emulator.
> > > > >
> > > > > Consistency should not be an issue, since VLS cannot be enabled when
> > > > > the MAXPHYADDRs differ. VLS doesn't work in that scenario.
> > > >
> > > > Why? It's only broken if VMLOAD/VMSAVE is executed with a GPA that
> > > > exceeds the guest's MAXPHYADDR, but not the host's, right? So only
> > > > broken if the guest is misbehaving.
> > > >
> > > > Taking a step back, I am not disagreeing that VLS should not be used
> > > > with different MAXPHYADDRs, I am just saying it might be.
> > >
> > > KVM straight up doesn't support that (see my other reply).
> >
> > Sean, I intend to send a new version today with 2 main diffs:
> > - Use cpuid_maxphyaddr() here instead of  kvm_host.maxphyaddr.
> > - Use a common helper for checking RAX for SVM instructions for both
> > the emulator and gp_interception() (see response on patch 4).
> >
> > Holler if you want me to wait for further feedback.
> 
> I just realized I cannot just do cpuid_maxphyaddr(ctxt->vcpu) in
> check_svme_pa(), because vcpu is defined as a void pointer in
> x86_emulate_ctxt. Looking at commit c9b8b07cded5 ("KVM: x86:
> Dynamically allocate per-vCPU emulation context"), I cannot tell why.

To prevent dereferencing the vcpu object in emulator code.  It's kinda silly
because common KVM is tightly coupled to the emulator, but we try to contain
what the emulator can do.

> I was going to move emul_to_vcpu() to arch/x86/kvm/kvm_emulate.h, but
> maybe we should just make this a normal struct kvm_vpcu pointer and
> drop emul_to_vcpu() completely?

Heh, talk about scope creep, that'll open a much bigger can of worms and subsequent
discussion.

Honestly, why bother keeping check_svme_pa()?  Can't we just do the checks in
svm_check_intercept()?  E.g. vmx_check_intercept() already "injects" #UD for RDTSCP.

Ha!  And dropping check_svme_pa() would technically be a bug fix, because the #GP
is supposed to have lower priority than the #UD due to EFER.SVME=0.