linux-kernel - Re: [PATCH 1/2] KVM: SVM: Generate #UD for certain instructions when SVME.EFER is disabled

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Message-ID: <aV2fVaLBrtUsccHJ@google.com>
Date: Tue, 6 Jan 2026 15:48:37 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
Cc: Kevin Cheng <chengkev@...gle.com>, pbonzini@...hat.com, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] KVM: SVM: Generate #UD for certain instructions when
 SVME.EFER is disabled

On Tue, Jan 06, 2026, Yosry Ahmed wrote:
> On Tue, Jan 06, 2026 at 10:21:40AM -0800, Sean Christopherson wrote:
> > So rather than manually handle the intercepts in svm_set_efer() and fight recalcs,
> > trigger KVM_REQ_RECALC_INTERCEPTS and teach svm_recalc_instruction_intercepts()
> > about EFER.SVME handling.
> > 
> > After the dust settles, it might make sense to move the #GP intercept logic into
> > svm_recalc_intercepts() as well, but that's not a priority.
> 
> Unrelated question about the #GP intercept logic, it seems like if
> enable_vmware_backdoor is set, the #GP intercept will be set, even for
> SEV guests, which goes against the in svm_set_efer():
> 
> 	/*
> 	 * Never intercept #GP for SEV guests, KVM can't
> 	 * decrypt guest memory to workaround the erratum.
> 	 */
> 	if (svm_gp_erratum_intercept && !sev_guest(vcpu->kvm))
> 		set_exception_intercept(svm, GP_VECTOR);
> 
> I initially thought if userspace sets enable_vmware_backdoor and runs
> SEV guests it's shooting itself in the foot, but given that
> enable_vmware_backdoor is a module parameter (i.e. global), isn't it
> possible that the host runs some SEV and some non-SEV VMs, where the
> non-SEV VMs require the vmware backdoor?

Commit 29de732cc95c ("KVM: SEV: Move SEV's GP_VECTOR intercept setup to SEV")
moved the override to sev_init_vmcb():

	/*
	 * Don't intercept #GP for SEV guests, e.g. for the VMware backdoor, as
	 * KVM can't decrypt guest memory to decode the faulting instruction.
	 */
	clr_exception_intercept(svm, GP_VECTOR);

I.e. init_vmcb() will set the #GP intercept, then sev_init_vmcb() will immediately
clear it.