Re: [PATCH v3 6/9] KVM: SVM: Add support for IBS Virtualization

[Sean Christopherson <seanjc@google.com>]

On Thu, Oct 08, 2026, Jim Mattson wrote:
> On Thu, Oct 8, 2026 at 12:19 PM Jim Mattson <jmattson@google.com> wrote:
> >
> > On Tue, Mar 10, 2026 at 06:00:18AM +0000, Manali Shukla wrote:
> > > +     /*
> > > +      * When IBS virtualization is enabled, guest reads from
> > > +      * MSR_AMD64_IBSFETCHPHYSAD and MSR_AMD64_IBSDCPHYSAD must return 0.

This is incorrect.  Hardware returns 0 because it (presumably) doesn't want to
translate the address on the fly.  KVM isn't bound by such a restriction.

> > > +      * This is done for security reasons, as guests should not be allowed to
> > > +      * access or infer any information about the system's physical
> > > +      * addresses.
> > > +      */
> > > +     case MSR_AMD64_IBSDCPHYSAD:
> > > +     case MSR_AMD64_IBSFETCHPHYSAD:
> > > +             msr_info->data = 0;
> > > +             break;
> > > +
> >
> > These new cases should be conditional. Before this patch, KVM synthesized
> > a #GP for a guest without X86_FEATURE_IBS. Now it returns 0.
> >
> > But, is this even necessary? APM vol. 2, section 15.38, says that when IBS
> > virtualization is enabled in the VMCB, the processor returns 0 on guest
> > reads from IbsFetchPhysAd and IbsDcPhysAd and ignores guest writes to
> > them. Can't we just disable interception of these MSRs when VIBS is enabled
> > for the vCPU, and let the hardware handle them?
> 
> Doh! As I realized later in the series, KVM_FEP requires that KVM must
> be able to emulate reads and writes of every virtualized MSR...even if
> virtualization just means "ignore."

Not just FEP.  As proposed, the MSR is intercepted and so what hardware does is
irrelevant.

I'm confused by (at least) one thing: why can software write these MSRs?  IIUC,
they are outputs from the CPU.

If these were read-only, couldn't KVM use gva_to_gpa() with MSR_AMD64_IBSDCLINAD
and MSR_AMD64_IBSFETCHLINAD to get the GPA for the guest?  The MSRs being writable
is bizarre.