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.
So this commit changes the behavior in qemu so that ARCH_CAPABILITIES
is not provided by default on AMD cpus when the hypervisor emulates it,
but it can still be provided by explicitly setting arch-capabilities=on.
Signed-off-by: Alexandre Chartre <alexandre.char...@oracle.com>
---
target/i386/cpu.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)
diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 0d35e95430..7e136c48df 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -8324,6 +8324,20 @@ void x86_cpu_expand_features(X86CPU *cpu, Error **errp)
}
}
+ /*
+ * For years, KVM has inadvertently emulated the ARCH_CAPABILITIES
+ * MSR on AMD although this is an Intel-specific MSR; and KVM will
+ * continue doing so to not change its ABI for existing setups.
+ *
+ * So ensure that the ARCH_CAPABILITIES MSR is disabled on AMD cpus
+ * to prevent providing a cpu with an MSR which is not supposed to
+ * be there, unless it was explicitly requested by the user.
+ */
+ if (IS_AMD_CPU(env) &&
+ !(env->user_features[FEAT_7_0_EDX] & CPUID_7_0_EDX_ARCH_CAPABILITIES))
{
+ env->features[FEAT_7_0_EDX] &= ~CPUID_7_0_EDX_ARCH_CAPABILITIES;
+ }
+
if (x86_threads_per_pkg(&env->topo_info) > 1) {
env->features[FEAT_1_EDX] |= CPUID_HT;
--
2.43.5
