target/i386: do not expose ARCH_CAPABILITIES on AMD CPU

KVM emulates the ARCH_CAPABILITIES on x86 for both Intel and AMD
cpus, although the IA32_ARCH_CAPABILITIES MSR is an Intel-specific
MSR and it makes no sense to emulate it on AMD.

As a consequence, VMs created on AMD with qemu -cpu host and using
KVM will advertise the ARCH_CAPABILITIES feature and provide the
IA32_ARCH_CAPABILITIES MSR. This can cause issues (like Windows BSOD)
as the guest OS might not expect this MSR to exist on such cpus (the
AMD documentation specifies that ARCH_CAPABILITIES feature and MSR
are not defined on the AMD architecture).

A fix was proposed in KVM code, however KVM maintainers don't want to
change this behavior that exists for 6+ years and suggest changes to be
done in QEMU instead.  Therefore, hide the bit from "-cpu host":
migration of -cpu host guests is only possible between identical host
kernel and QEMU versions, therefore this is not a problematic breakage.

If a future AMD machine does include the MSR, that would re-expose the
Windows guest bug; but it would not be KVM/QEMU's problem at that
point, as we'd be following a genuine physical CPU impl.

Reported-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index e8c8be09bae955b67418340da9d3a1a2c11a96bb..369626f8c8d78bc205cc9d8ff022f35ce3e239f8 100644 (file)
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -503,8 +503,12 @@ uint32_t kvm_arch_get_supported_cpuid(KVMState *s, uint32_t function,
          * Linux v4.17-v4.20 incorrectly return ARCH_CAPABILITIES on SVM hosts.
          * We can detect the bug by checking if MSR_IA32_ARCH_CAPABILITIES is
          * returned by KVM_GET_MSR_INDEX_LIST.
+         *
+         * But also, because Windows does not like ARCH_CAPABILITIES on AMD
+         * mcahines at all, do not show the fake ARCH_CAPABILITIES MSR that
+         * KVM sets up.
          */
-        if (!has_msr_arch_capabs) {
+        if (!has_msr_arch_capabs || !(edx & CPUID_7_0_EDX_ARCH_CAPABILITIES)) {
             ret &= ~CPUID_7_0_EDX_ARCH_CAPABILITIES;
         }
     } else if (function == 7 && index == 1 && reg == R_EAX) {