[PATCH v7 0/9] target/i386/kvm/pmu: PMU Enhancement, Bugfix and Cleanup

Dongli Zhang
2025-11-11T06:20:24Z

This patchset addresses four bugs related to AMD PMU virtualization.

1. The PerfMonV2 is still available if PERCORE if disabled via
"-cpu host,-perfctr-core".

2. The VM 'cpuid' command still returns PERFCORE although "-pmu" is
configured.

3. The third issue is that using "-cpu host,-pmu" does not disable AMD PMU
virtualization. When using "-cpu EPYC" or "-cpu host,-pmu", AMD PMU
virtualization remains enabled. On the VM's Linux side, you might still
see:

[ 0.510611] Performance Events: Fam17h+ core perfctr, AMD PMU driver.

instead of:

[ 0.596381] Performance Events: PMU not available due to virtualization, using software events only.
[ 0.600972] NMI watchdog: Perf NMI watchdog permanently disabled

To address this, KVM_CAP_PMU_CAPABILITY is used to set KVM_PMU_CAP_DISABLE
when "-pmu" is configured.

4. The fourth issue is that unreclaimed performance events (after a QEMU
system_reset) in KVM may cause random, unwanted, or unknown NMIs to be
injected into the VM.

The AMD PMU registers are not reset during QEMU system_reset.

(1) If the VM is reset (e.g., via QEMU system_reset or VM kdump/kexec) while
running "perf top", the PMU registers are not disabled properly.

(2) Despite x86_cpu_reset() resetting many registers to zero, kvm_put_msrs()
does not handle AMD PMU registers, causing some PMU events to remain
enabled in KVM.

(3) The KVM kvm_pmc_speculative_in_use() function consistently returns true,
preventing the reclamation of these events. Consequently, the
kvm_pmc->perf_event remains active.

(4) After a reboot, the VM kernel may report the following error:

[ 0.092011] Performance Events: Fam17h+ core perfctr, Broken BIOS detected, complain to your hardware vendor.
[ 0.092023] [Firmware Bug]: the BIOS has corrupted hw-PMU resources (MSR c0010200 is 530076)

(5) In the worst case, the active kvm_pmc->perf_event may inject unknown
NMIs randomly into the VM kernel:

[...] Uhhuh. NMI received for unknown reason 30 on CPU 0.

To resolve these issues, we propose resetting AMD PMU registers during the
VM reset process

Changed since v1:
- Use feature_dependencies for CPUID_EXT3_PERFCORE and
CPUID_8000_0022_EAX_PERFMON_V2.
- Remove CPUID_EXT3_PERFCORE when !cpu->enable_pmu.
- Pick kvm_arch_pre_create_vcpu() patch from Xiaoyao Li.
- Use "-pmu" but not a global "pmu-cap-disabled" for KVM_PMU_CAP_DISABLE.
- Also use sysfs kvm.enable_pmu=N to determine if PMU is supported.
- Some changes to PMU register limit calculation.
Changed since v2:
- Change has_pmu_cap to pmu_cap.
- Use cpuid_find_entry() instead of cpu_x86_cpuid().
- Rework the code flow of PATCH 07 related to kvm.enable_pmu=N following
Zhao's suggestion.
- Use object_property_get_int() to get CPU family.
- Add support to Zhaoxin.
Changed since v3:
- Re-base on top of Zhao's queued patch.
- Use host_cpu_vendor_fms() from Zhao's patch.
- Pick new version of kvm_arch_pre_create_vcpu() patch from Xiaoyao.
- Re-split the cases into enable_pmu and !enable_pmu, following Zhao's
suggestion.
- Check AMD directly makes the "compat" rule clear.
- Some changes on commit message and comment.
- Bring back global static variable 'kvm_pmu_disabled' read from
/sys/module/kvm/parameters/enable_pmu.
Changed since v4:
- Re-base on top of most recent mainline QEMU.
- Add more Reviewed-by.
- All patches are reviewed.
Changed since v5:
- Re-base on top of most recent mainline QEMU.
- Remove patch "kvm: Introduce kvm_arch_pre_create_vcpu()" as it is
already merged.
- To resolve conflicts in new [PATCH v6 3/9] , move the PMU related code
before the call site of is_tdx_vm().
Changed since v6:
- Re-base on top of most recent mainline QEMU (staging branch).
- Add more Reviewed-by from Dapeng and Sandipan.

Dongli Zhang (9):
target/i386: disable PerfMonV2 when PERFCORE unavailable
target/i386: disable PERFCORE when "-pmu" is configured
target/i386/kvm: set KVM_PMU_CAP_DISABLE if "-pmu" is configured
target/i386/kvm: extract unrelated code out of kvm_x86_build_cpuid()
target/i386/kvm: rename architectural PMU variables
target/i386/kvm: query kvm.enable_pmu parameter
target/i386/kvm: reset AMD PMU registers during VM reset
target/i386/kvm: support perfmon-v2 for reset
target/i386/kvm: don't stop Intel PMU counters

target/i386/cpu.c | 8 +
target/i386/cpu.h | 16 ++
target/i386/kvm/kvm.c | 355 +++++++++++++++++++++++++++++++++++++++------
3 files changed, 332 insertions(+), 47 deletions(-)

branch: remotes/origin/staging
base-commit: 593aee5df98b4a862ff8841a57ea3dbf22131a5f

Thank you very much!

Dongli Zhang