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A guest started under an older > kernel has spec-ctrl in its CPUID, and QEMU filters it on the new > host, so migration, restore and snapshot revert fail (VSTOR-151770). > > Add x-spec-ctrl-compat and turn it on in pc_vz_9_2_compat: an AMD > guest keeps a requested spec-ctrl when the host still provides > amd-ibpb and one of amd-ibrs, amd-stibp and amd-ssbd. KVM then keeps > MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD available to the guest > through those bits (kernel commits 39485ed95d6b, 841c2be09fe4). > > Signed-off-by: Denis V. Lunev > > diff --git a/hw/i386/pc.c b/hw/i386/pc.c > index 25b17866103..95ff2e7860e 100644 > --- a/hw/i386/pc.c > +++ b/hw/i386/pc.c > @@ -550,6 +550,7 @@ const size_t pc_vz_compat_len = G_N_ELEMENTS(pc_vz_compat); > > GlobalProperty pc_vz_9_2_compat[] = { > { TYPE_X86_CPU, "x-tsx-compat", "on" }, > + { TYPE_X86_CPU, "x-spec-ctrl-compat", "on" }, > }; > const size_t pc_vz_9_2_compat_len = G_N_ELEMENTS(pc_vz_9_2_compat); > > diff --git a/target/i386/cpu.c b/target/i386/cpu.c > index 5ff5644dee9..872c00fc983 100644 > --- a/target/i386/cpu.c > +++ b/target/i386/cpu.c > @@ -9116,6 +9116,19 @@ static uint64_t x86_cpu_compat_kept_features(X86CPU *cpu, FeatureWord w) > kept = CPUID_7_0_EBX_HLE | CPUID_7_0_EBX_RTM; > } > break; > + case FEAT_7_0_EDX: > + if (cpu->spec_ctrl_compat && IS_AMD_CPU(env)) { > + uint32_t ebx = kvm_arch_get_supported_cpuid(kvm_state, 0x80000008, > + 0, R_EBX); > + > + if ((ebx & CPUID_8000_0008_EBX_IBPB) && > + (ebx & (CPUID_8000_0008_EBX_IBRS | > + CPUID_8000_0008_EBX_STIBP | > + CPUID_8000_0008_EBX_AMD_SSBD))) { > + kept = CPUID_7_0_EDX_SPEC_CTRL; > + } > + } > + break; Failure case. -cpu EPYC-IBPB,spec-ctrl=on on an old node: KVM offered SPEC_CTRL there, so the guest booted with it and used the MSR freely. Move it to a Zen 1 to 3 node on 6.12. The host still has amd-ibpb plus amd-ssbd, so the guard keeps spec-ctrl and libvirt is happy. But EPYC-IBPB carries only ibpb in leaf 0x80000008 EBX, no stibp, ibrs or ssbd. Every guest wrmsr IA32_SPEC_CTRL now returns 1 from svm_set_msr and the guest takes #GP where it used to succeed. Linux logs "unchecked MSR access error" once per CPU at AP bring-up and on any later STIBP or SSBD toggle. A host-model or host-passthrough guest is not affected, since it always carries the host's AMD bits. Proposed fix: require the same bits in the guest's requested features, not only in the host's: uint64_t guest = env->features[FEAT_8000_0008_EBX]; uint32_t host = kvm_arch_get_supported_cpuid(kvm_state, 0x80000008, 0, R_EBX); uint64_t msrs = guest & host; if ((msrs & CPUID_8000_0008_EBX_IBPB) && (msrs & (CPUID_8000_0008_EBX_IBRS | CPUID_8000_0008_EBX_STIBP | CPUID_8000_0008_EBX_AMD_SSBD))) { kept = CPUID_7_0_EDX_SPEC_CTRL; } -- Andrey Drobyshev