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

Zhao Liu
2025-12-16T08:01:09Z

On Mon, Nov 10, 2025 at 10:14:49PM -0800, Dongli Zhang wrote:
> Date: Mon, 10 Nov 2025 22:14:49 -0800
> From: Dongli Zhang <dongli.zhang@oracle.com>
> Subject: [PATCH v7 0/9] target/i386/kvm/pmu: PMU Enhancement, Bugfix and
> Cleanup
> X-Mailer: git-send-email 2.43.5
>
> 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

Hi Dongli,

Except for Patch 1 & 2 which need compatibility options (if you think
it's okay, I could help take these 2 and fix them when v11.0's compat
array is ready).

The other patches still LGTM. Maybe it's better to have a v8 excluding
patch 1 & 2?

Regards,
Zhao