[v3,00/10] target/i386/kvm/pmu: PMU Enhancement, Bugfix and Cleanup

Message

Dongli Zhang March 31, 2025, 1:32 a.m. UTC

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.

Xiaoyao Li (1):
  kvm: Introduce kvm_arch_pre_create_vcpu()

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

 accel/kvm/kvm-all.c        |   5 +
 include/system/kvm.h       |   1 +
 target/arm/kvm.c           |   5 +
 target/i386/cpu.c          |   8 +
 target/i386/cpu.h          |  12 ++
 target/i386/kvm/kvm.c      | 356 ++++++++++++++++++++++++++++++++++------
 target/loongarch/kvm/kvm.c |   5 +
 target/mips/kvm.c          |   5 +
 target/ppc/kvm.c           |   5 +
 target/riscv/kvm/kvm-cpu.c |   5 +
 target/s390x/kvm/kvm.c     |   5 +
 11 files changed, 365 insertions(+), 47 deletions(-)

base-commit: 0f15892acaf3f50ecc20c6dad4b3ebdd701aa93e

Thank you very much!

Dongli Zhang