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

Dongli Zhang
2025-11-19T02:08:51Z

Hi Paolo,

Apologies if this causes any inconvenience.

Would you consider this patchset? It resolves several QEMU AMD vPMU issues, and
each patch has received a Reviewed-by from at least two reviewers.

Thank you very much!

Dongli Zhang

On 11/10/25 10:14 PM, Dongli Zhang wrote:
> 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
>
>