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target/i386: add compatibility property for arch_capabilities

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Prior to v10.1, if requested by user, arch-capabilities is always on
despite the fact that CPUID advertises it to be off/unvailable.
This causes a migration issue for VMs that are run on a machine
without arch-capabilities and expect this feature to be present
on the destination host with QEMU 10.1.

Add a compatibility property to restore the legacy behavior for all
machines with version prior to 10.1.

To preserve the functionality (added by 10.1) of turning off
ARCH_CAPABILITIES where Windows does not like it, use directly
the guest CPU vendor: x86_cpu_get_supported_feature_word is not
KVM-specific and therefore should not necessarily use the host
CPUID.

Co-authored-by: Hector Cao <hector.cao@canonical.com>
Signed-off-by: Hector Cao <hector.cao@canonical.com>
Fixes: d3a24134e3 ("target/i386: do not expose ARCH_CAPABILITIES on AMD CPU", 2025-07-17)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2025-09-23 12:22:54 +02:00
parent 2438f18ee0
commit e9efa4a771
4 changed files with 25 additions and 5 deletions
Download patch file Download diff file Expand all files Collapse all files

1
hw/i386/pc.c
View file

@ -87,6 +87,7 @@ const size_t pc_compat_10_1_len = G_N_ELEMENTS(pc_compat_10_1);
GlobalProperty pc_compat_10_0[] = {
{ TYPE_X86_CPU, "x-consistent-cache", "false" },
{ TYPE_X86_CPU, "x-vendor-cpuid-only-v2", "false" },
{ TYPE_X86_CPU, "x-arch-cap-always-on", "true" },
};
const size_t pc_compat_10_0_len = G_N_ELEMENTS(pc_compat_10_0);

17
target/i386/cpu.c
View file

@ -7539,6 +7539,20 @@ uint64_t x86_cpu_get_supported_feature_word(X86CPU *cpu, FeatureWord w)
#endif
break;
case FEAT_7_0_EDX:
/*
* Windows does not like ARCH_CAPABILITIES on AMD machines at all.
* Do not show the fake ARCH_CAPABILITIES MSR that KVM sets up,
* except if needed for migration.
*
* When arch_cap_always_on is removed, this tweak can move to
* kvm_arch_get_supported_cpuid.
*/
if (cpu && IS_AMD_CPU(&cpu->env) && !cpu->arch_cap_always_on) {
unavail = CPUID_7_0_EDX_ARCH_CAPABILITIES;
}
break;
default:
break;
}
@ -10004,6 +10018,9 @@ static const Property x86_cpu_properties[] = {
true),
DEFINE_PROP_BOOL("x-l1-cache-per-thread", X86CPU, l1_cache_per_core, true),
DEFINE_PROP_BOOL("x-force-cpuid-0x1f", X86CPU, force_cpuid_0x1f, false),
DEFINE_PROP_BOOL("x-arch-cap-always-on", X86CPU,
arch_cap_always_on, false),
};
#ifndef CONFIG_USER_ONLY

6
target/i386/cpu.h
View file

@ -2314,6 +2314,12 @@ struct ArchCPU {
/* Forcefully disable KVM PV features not exposed in guest CPUIDs */
bool kvm_pv_enforce_cpuid;
/*
* Expose arch-capabilities unconditionally even on AMD models, for backwards
* compatibility with QEMU <10.1.
*/
bool arch_cap_always_on;
/* Number of physical address bits supported */
uint32_t phys_bits;

6
target/i386/kvm/kvm.c
View file

@ -503,12 +503,8 @@ 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 || !(edx & CPUID_7_0_EDX_ARCH_CAPABILITIES)) {
if (!has_msr_arch_capabs) {
ret &= ~CPUID_7_0_EDX_ARCH_CAPABILITIES;
}
} else if (function == 7 && index == 1 && reg == R_EAX) {