Re: [PATCH V4 0/4] Align SVM with APM defined behaviors

[Sean Christopherson <seanjc@google.com>]

On Mon, Mar 02, 2026, Yosry Ahmed wrote:
> On Fri, Feb 27, 2026 at 7:33 PM Kevin Cheng <chengkev@google.com> wrote:
> >
> > The APM lists the following behaviors
> >   - The VMRUN, VMLOAD, VMSAVE, CLGI, VMMCALL, and INVLPGA instructions
> >     can be used when the EFER.SVME is set to 1; otherwise, these
> >     instructions generate a #UD exception.
> >   - If VMMCALL instruction is not intercepted, the instruction raises a
> >     #UD exception.
> >
> > The patches in this series fix current SVM bugs that do not adhere to
> > the APM listed behaviors.
> >
> > v3 -> v4:
> >   - Dropped "KVM: SVM: Inject #UD for STGI if EFER.SVME=0 and SVM Lock
> >     and DEV are not available" as per Sean
> >   - Added back STGI and CLGI intercept clearing in init_vmcb to maintain
> >     previous behavior on intel guests. Previously intel guests always
> >     had STGI and CLGI intercepts cleared if vgif was enabled. In V3,
> >     because the clearing of the intercepts was moved from init_vmcb() to
> >     the !guest_cpuid_is_intel_compatible() case in
> >     svm_recalc_instruction_intercepts(), the CLGI intercept would be
> >     indefinitely set on intel guests. I added back the clearing to
> >     init_vmcb() to retain intel guest behavior before this patch.
> 
> I am a bit confused by this. v4 kept initializing the intercepts as
> cleared for all guests, but we still set the CLGI/STGI intercepts for
> Intel-compatible guests in svm_recalc_instruction_intercepts() patch
> 3. So what difference did this make?
> 
> Also taking a step back, I am not really sure what's the right thing
> to do for Intel-compatible guests here. It also seems like even if we
> set the intercept, svm_set_gif() will clear the STGI intercept, even
> on Intel-compatible guests.
> 
> Maybe we should leave that can of worms alone, go back to removing
> initializing the CLGI/STGI intercepts in init_vmcb(), and in
> svm_recalc_instruction_intercepts() set/clear these intercepts based
> on EFER.SVME alone, irrespective of Intel-compatibility?

Ya, guest_cpuid_is_intel_compatible() should only be applied to VMLOAD/VMSAVE.
KVM intercepts VMLOAD/VMSAVE to fixup SYSENTER MSRs, not to inject #UD.  I.e. KVM
is handling (the absoutely absurd) case that FMS reports an Intel CPU, but the
guest enables and uses SVM.

	/*
	 * Intercept VMLOAD if the vCPU model is Intel in order to emulate that
	 * VMLOAD drops bits 63:32 of SYSENTER (ignoring the fact that exposing
	 * SVM on Intel is bonkers and extremely unlikely to work).
	 */
	if (guest_cpuid_is_intel_compatible(vcpu))
		guest_cpu_cap_clear(vcpu, X86_FEATURE_V_VMSAVE_VMLOAD);

Sorry for not catching this in previous versions.