Re: [PATCH v7 2/9] target/i386: disable PERFCORE when "-pmu" is configured

Zhao Liu
2025-11-19T10:44:36Z

Hi Dongli,

> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index 3653f8953e..4fcade89bc 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -8360,6 +8360,10 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
> !(env->hflags & HF_LMA_MASK)) {
> *edx &= ~CPUID_EXT2_SYSCALL;
> }
> +
> + if (!cpu->enable_pmu) {
> + *ecx &= ~CPUID_EXT3_PERFCORE;
> + }

Learned the lessons from PDCM [*]:

[*] https://lore.kernel.org/qemu-devel/20250923104136.133875-3-pbonzini@redhat.com/

Directly masking CPUID bit off is not a good idea... modifying the
CPUID, even when fixing or adding dependencies, can easily break
migration.

So a safe way is to add a compat option. And I think it would be better
if patch 1 also has a compat option. A single compat option could cover
patch 1 & 2.

I have these thoughts:

* For "-pmu" dependency, it can be checked as [*] did.
* For normal feature bit dependency (patch 1), it seems possible to add
compat_prop condition in feature_dependencies[].

I attached a draft for discussion (which is on the top of the whole
series).

Note, we are currently in the RC phase for v10.2, and the
pc_compat_10_2[] array is not yet added, which will be added before the
release of v10.2. Therefore, we have enough time for discussion I think.

If you think it's fine, I'll post compat_prop support separately as an
RFC. The specific compat option can be left to this series.

Although wait for a while, in a way, it is lucky :) - there's an
opportunity to avoid making mistakes for us.

Regards,
Zhao

------------------------------------------------
diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index f8b919cb6c47..d2b8e83e556d 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -83,6 +83,7 @@

GlobalProperty pc_compat_10_1[] = {
{ "mch", "extended-tseg-mbytes", "16" },
+ { TYPE_X86_CPU, "x-amd-perfmon-always-on", "true" }, // This should be added to pc_compat_10_2!
};
const size_t pc_compat_10_1_len = G_N_ELEMENTS(pc_compat_10_1);

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 4fcade89bcea..8c56789eb296 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -1997,6 +1997,8 @@ static FeatureDep feature_dependencies[] = {
{
.from = { FEAT_8000_0001_ECX, CPUID_EXT3_PERFCORE },
.to = { FEAT_8000_0022_EAX, CPUID_8000_0022_EAX_PERFMON_V2 },
+ .compat_prop =
+ { "x-amd-perfmon-always-on", "false" },
},
};

@@ -8998,15 +9000,36 @@ void x86_cpu_expand_features(X86CPU *cpu, Error **errp)
}
}

- if (!cpu->pdcm_on_even_without_pmu) {
+ if (!cpu->enable_pmu) {
/* PDCM is fixed1 bit for TDX */
- if (!cpu->enable_pmu && !is_tdx_vm()) {
+ if (!cpu->pdcm_on_even_without_pmu && !is_tdx_vm()) {
env->features[FEAT_1_ECX] &= ~CPUID_EXT_PDCM;
}
+
+ if (!cpu->amd_perfmon_always_on) {
+ env->features[FEAT_8000_0001_ECX] &= ~CPUID_EXT3_PERFCORE;
+ }
}

for (i = 0; i < ARRAY_SIZE(feature_dependencies); i++) {
FeatureDep *d = &feature_dependencies[i];
+ bool compat_omit = false;
+ PropValue *prop = &d->compat_prop;
+
+ if (prop->value) {
+ char *v = object_property_print(OBJECT(cpu), prop->prop, false, NULL);
+ if (v) {
+ compat_omit = !strcmp(prop->value, v);
+ printf("feature_dependencies: v: %s, value: %s, omit: %d\n",
+ v, prop->value, compat_omit);
+ g_free(v);
+ }
+
+ if (compat_omit) {
+ continue;
+ }
+ }
+
if (!(env->features[d->from.index] & d->from.mask)) {
uint64_t unavailable_features = env->features[d->to.index] & d->to.mask;

@@ -10065,6 +10088,8 @@ static const Property x86_cpu_properties[] = {
arch_cap_always_on, false),
DEFINE_PROP_BOOL("x-pdcm-on-even-without-pmu", X86CPU,
pdcm_on_even_without_pmu, false),
+ DEFINE_PROP_BOOL("x-amd-perfmon-always-on", X86CPU,
+ amd_perfmon_always_on, false),
};

#ifndef CONFIG_USER_ONLY
diff --git a/target/i386/cpu.h b/target/i386/cpu.h
index 6d78f3995b6b..ac8dd92efc59 100644
--- a/target/i386/cpu.h
+++ b/target/i386/cpu.h
@@ -696,8 +696,14 @@ typedef struct FeatureMask {
uint64_t mask;
} FeatureMask;

+typedef struct PropValue {
+ const char *prop, *value;
+} PropValue;
+
typedef struct FeatureDep {
FeatureMask from, to;
+ /* Once set, only when prop is satisfied, dependency can be checked. */
+ PropValue compat_prop;
} FeatureDep;

typedef uint64_t FeatureWordArray[FEATURE_WORDS];
@@ -2353,6 +2359,16 @@ struct ArchCPU {
*/
bool pdcm_on_even_without_pmu;

+ /*
+ * Backwards compatibility with QEMU <10.2. (TODO: change 10.2 to 11.0!)
+ * This flag covers 2 parts:
+ * - The PERFCORE feature is now disabled when PMU is not available, but prior to
+ * 10.2 it was enabled even if PMU is off.
+ * - The PerfMonV2 feature is now disabled when PERFCORE is disabled, but prior to
+ * 10.2 it was enabled even if PERFCORE is off.
+ */
+ bool amd_perfmon_always_on;
+
/* Number of physical address bits supported */
uint32_t phys_bits;

@@ -2579,9 +2595,7 @@ bool cpu_x86_xrstor(CPUX86State *s, void *host, size_t len, uint64_t rbfm);
/* cpu.c */
void x86_cpu_vendor_words2str(char *dst, uint32_t vendor1,
uint32_t vendor2, uint32_t vendor3);
-typedef struct PropValue {
- const char *prop, *value;
-} PropValue;
+
void x86_cpu_apply_props(X86CPU *cpu, PropValue *props);

void x86_cpu_after_reset(X86CPU *cpu);