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.
