On 1.07.2025 10:23, Xiaoyao Li wrote:
On 6/30/2025 9:30 PM, Alexandre Chartre wrote:
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;
+ }
This changes the result for the existing usage of "-cpu host" on AMD. So it
will need a compat_prop to keep the old behavior for old machine.
Do we really add machine compat props for CPUID flags?
I thought CPUID flags aren't live migrated but the target QEMU uses whatever it
was launched with (and the target machine exposes)?
But I would like discuss if we really want to do it in QEMU. ARCH_CAPABILITIES is not the
only one KVM emulates unconditionally. We have TSC_DEADLINE_TIMER as well. So why to
treat them differently? just because some Windows cannot boot? To me, it looks just the
bug of Windows. So please fix Windows. And to run with the buggy Windows, we have the
workaround: "-cpu host,-arch-capabilities"
As Alexandre wrote moments ago, I think "-cpu host" is supposed to be a
passthrough
of the host CPU features and an AMD host CPU obviously does not have
ARCH_CAPABILITIES.
In this case this broke a real guest, in a way that's hard to debug
(it's not Linux where one can look into kernel sources) and
provides poor user experience to people expecting that if their guest works on
their bare metal then just passing through the host CPU will make it work too.
Thanks,
Maciej
