On 3/4/2025 2:45 AM, dongli.zhang@xxxxxxxxxx wrote:
Hi Xiaoyao,
On 3/2/25 5:59 PM, Xiaoyao Li wrote:
On 3/3/2025 6:00 AM, Dongli Zhang wrote:
Currently, AMD PMU support isn't determined based on CPUID, that is, the
"-pmu" option does not fully disable KVM AMD PMU virtualization.
To minimize AMD PMU features, remove PERFCORE when "-pmu" is configured.
To completely disable AMD PMU virtualization will be implemented via
KVM_CAP_PMU_CAPABILITY in upcoming patches.
As a reminder, neither CPUID_EXT3_PERFCORE nor
CPUID_8000_0022_EAX_PERFMON_V2 is removed from env->features[] when "-pmu"
is configured. Developers should query whether they are supported via
cpu_x86_cpuid() rather than relying on env->features[] in future patches.
I don't think it is the correct direction to go.
env->features[] should be finalized before cpu_x86_cpuid() and env-
features[] needs to be able to be exposed to guest directly. This ensures
guest and QEMU have the same view of CPUIDs and it simplifies things.
We can adjust env->features[] by filtering all PMU related CPUIDs based on
cpu->enable_pmu in x86_cpu_realizefn().
Thank you very much for suggestion.
I see code like below in x86_cpu_realizefn() to edit env->features[].
7982 /* On AMD CPUs, some CPUID[8000_0001].EDX bits must match the bits on
7983 * CPUID[1].EDX.
7984 */
7985 if (IS_AMD_CPU(env)) {
7986 env->features[FEAT_8000_0001_EDX] &= ~CPUID_EXT2_AMD_ALIASES;
7987 env->features[FEAT_8000_0001_EDX] |= (env->features[FEAT_1_EDX]
7988 & CPUID_EXT2_AMD_ALIASES);
7989 }
I may do something similar to them for CPUID_EXT3_PERFCORE and
CPUID_8000_0022_EAX_PERFMON_V2.
I just sent a series for CPUID_EXT_PDCM[1]. I think you can put
CPUID_EXT3_PERFCORE and CPUID_8000_0022_EAX_PERFMON_V2 at the same place.
[1]
https://lore.kernel.org/qemu-devel/20250304052450.465445-1-xiaoyao.li@xxxxxxxxx/T/#m31c6777131b6361d7c3af22b09532bdc785dbc06
Dongli Zhang
Suggested-by: Zhao Liu <zhao1.liu@xxxxxxxxx>
Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
---
target/i386/cpu.c | 4 ++++
1 file changed, 4 insertions(+)
diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index b6d6167910..61a671028a 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -7115,6 +7115,10 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t
index, uint32_t count,
!(env->hflags & HF_LMA_MASK)) {
*edx &= ~CPUID_EXT2_SYSCALL;
}
+
+ if (kvm_enabled() && IS_AMD_CPU(env) && !cpu->enable_pmu) {
+ *ecx &= ~CPUID_EXT3_PERFCORE;
+ }
break;
case 0x80000002:
case 0x80000003:
