linux-kernel - Re: [PATCH 17/28] KVM: SVM: Manually recalc all MSR intercepts on userspace MSR filter change

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Message-ID: <aEHINux8hnYC6HC7@google.com>
Date: Thu, 5 Jun 2025 09:39:18 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Chao Gao <chao.gao@...el.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Borislav Petkov <bp@...en8.de>, Xin Li <xin@...or.com>, Dapeng Mi <dapeng1.mi@...ux.intel.com>
Subject: Re: [PATCH 17/28] KVM: SVM: Manually recalc all MSR intercepts on
 userspace MSR filter change

On Thu, Jun 05, 2025, Chao Gao wrote:
> >+static void svm_recalc_msr_intercepts(struct kvm_vcpu *vcpu)
> >+{
> >+	struct vcpu_svm *svm = to_svm(vcpu);
> >+
> >+	svm_vcpu_init_msrpm(vcpu);
> >+
> >+	if (lbrv)
> >+		svm_recalc_lbr_msr_intercepts(vcpu);
> >+
> >+	if (boot_cpu_has(X86_FEATURE_IBPB))
> >+		svm_set_intercept_for_msr(vcpu, MSR_IA32_PRED_CMD, MSR_TYPE_W,
> >+					  !guest_has_pred_cmd_msr(vcpu));
> >+
> >+	if (boot_cpu_has(X86_FEATURE_FLUSH_L1D))
> >+		svm_set_intercept_for_msr(vcpu, MSR_IA32_FLUSH_CMD, MSR_TYPE_W,
> >+					  !guest_cpu_cap_has(vcpu, X86_FEATURE_FLUSH_L1D));
> >+
> >+	/*
> >+	 * Unconditionally disable interception of SPEC_CTRL if V_SPEC_CTRL is
> >+	 * supported, i.e. if VMRUN/#VMEXIT context switch MSR_IA32_SPEC_CTRL.
> >+	 */

Tangentially related, the comment above isn't quite correct.  MSR_IA32_SPEC_CTRL
isn't purely context switched, the CPU merges together host and guest values.
Same end result though: KVM doesn't need to manually context switch the MSR.

> >+	if (boot_cpu_has(X86_FEATURE_V_SPEC_CTRL))
> >+		svm_disable_intercept_for_msr(vcpu, MSR_IA32_SPEC_CTRL, MSR_TYPE_RW);
> 
> I think there is a bug in the original code. KVM should inject #GP when guests
> try to access unsupported MSRs. Specifically, a guest w/o spec_ctrl support
> should get #GP when it tries to access the MSR regardless of V_SPEC_CTRL
> support on the host.
> 
> So, here should be 
> 
> 	if (boot_cpu_has(X86_FEATURE_V_SPEC_CTRL))
> 		svm_set_intercept_for_msr(vcpu, MSR_IA32_SPEC_CTRL, MSR_TYPE_RW,
> 					  !guest_has_spec_ctrl_msr(vcpu));

Right you are!  I'll slot in a patch earlier in the series, and then it'll end up
looking like this.

I'll also post a KUT testcase.