[PATCH v9 00/22] ARM64 PMU Partitioning

[Colton Lewis <coltonlewis@google.com>]

This series creates a new PMU scheme on ARM, a partitioned PMU that
allows reserving a subset of counters for more direct guest access,
significantly reducing overhead. More details, including performance
benchmarks, can be read in the v1 cover letter linked below.

There is no longer a kernel command line parameter
(`arm_pmuv3.reserved_host_counters`); PMU partitioning is now completely
controlled via the KVM API using `KVM_ARM_VCPU_PMU_V3_ENABLE_PARTITION`
and `KVM_ARM_VCPU_PMU_V3_SET_NR_COUNTERS` vCPU device attributes. When
partitioning is enabled for a VM, userspace must explicitly configure a
guest event counter count strictly less than the maximum general-purpose
counters implemented by the PMU (leaving at least one general-purpose
counter for the host) prior to calling `KVM_ARM_VCPU_PMU_V3_INIT`. A QEMU
patch demonstrating how to use the uAPI is sent separately.

An overview of what this series accomplishes was presented at KVM
Forum 2025. Slides [1] and video [2] are linked below.

v9:

* Rebase on top of v7.3-rc4.

* Drop the `arm_pmuv3.reserved_host_counters` module parameter so
  partitioning is completely controlled via the KVM vCPU device
  attribute uAPI (`KVM_ARM_VCPU_PMU_V3_ENABLE_PARTITION` and
  `KVM_ARM_VCPU_PMU_V3_SET_NR_COUNTERS`), and document the new
  attribute and counter allocation rules in
  `Documentation/virt/kvm/devices/vcpu.rst` (James Clark).

* Move the dynamic counter allocation mask (`cntr_mask`) from global
  `struct arm_pmu` to per-CPU `struct pmu_hw_events` (`cpuc->cntr_mask`)
  in a dedicated `drivers/perf` patch, fixing multi-pCPU counter
  reservation clobbering on vCPU migration and eliminating the
  `cpu_pm_pmu_setup()` cpuidle `WARN_ON_ONCE` (Zide Chen, James Clark).

* Synchronously propagate trapped guest writes to `PMEVTYPER<n>_EL0`
  and `PMCCFILTR_EL0` to hardware via `kvm_pmu_apply_single_event_filter()`
  when guest-owned, fixing in-guest `perf stat` event count skew across
  runs (James Clark, Sashiko AI Review).

* Refactor `armv8pmu_can_use_pmccntr()` and `armv8pmu_get_event_idx()`
  to check `cntr_mask` inside `armv8pmu_can_use_pmccntr()` and un-nest
  the 64-bit user-access check so cycle events fall back cleanly to
  general-purpose counters when `PMCCNTR_EL0` is reserved by a guest
  (Robin Murphy).

* Restrict the lazy transition to `VCPU_PMU_ACCESS_GUEST_OWNED` to when
  the guest actively enables counting (`PMCR_EL0.E = 1` or setting guest
  counter bits in `PMCNTENSET_EL0` / `PMINTENSET_EL1`), preventing guest
  boot-time PMU probing from prematurely claiming hardware counters and
  triggering spurious host counter preemption warnings (James Clark).

* Fix patch dependency ordering and series bisectability across all
  commits, removing intermediate `max_guest_counters` / `hw_cntr_impl`
  churn and squashing the selftest exception relaxation into the
  Partitioned PMU selftest patch (James Clark).

* Fix compiler warning for `struct arm_pmu` declaration in
  `include/kvm/arm_pmu.h` (kernel test robot) and guard
  `kvm_pmu_host_counter_mask()` when KVM is compiled in but not active
  (wuyifan).

* Track physical CPU PMU residency in `vcpu->arch.pmu.loaded_on_cpu`
  separately from `VCPU_PMU_ACCESS_GUEST_OWNED`, and toggle
  `MDCR_EL2.HPME` via `kvm_pmu_host_start()` / `kvm_pmu_host_stop()`
  instead of `PMCR_EL0.E` when starting/stopping host perf events while
  a partitioned guest is loaded.

* Allow `kvm_vcpu_pmu_resync_el0()` to resynchronize VHE EL0 event
  filters (`PMEVTYPER<n>_EL0.U`) in process context and order
  `kvm_pmu_put()` before `kvm_vcpu_pmu_restore_host()` in
  `kvm_arch_vcpu_put()`.

* Address additional Sashiko AI Review findings:
  - Check `idx - 1` against `cpuc->cntr_mask` in `armv8pmu_get_chain_idx()`
    to prevent 64-bit chained host events from crossing an odd `HPMN`
    partition boundary, and use `cpuc->cntr_mask` in
    `armv8pmu_enable_user_access()`.
  - Latch live hardware `PMOVSSET_EL0` overflow bits for guest counters
    with IRQs disabled (`local_irq_save()`) in `kvm_pmu_part_overflow_status()`
    and during trapped guest accesses to `PMOVS{SET,CLR}_EL0`.
  - Add mandatory `isb()` barriers after control-plane system register
    writes (`mdcr_el2`, `pmcntenclr_el0`, `pmintenclr_el1`), preserve
    guest `PMSELR_EL0` / `PMUSERENR_EL0` when `MDCR_EL2.TPM == 0`, and
    restore host `PMCR_EL0` control flags on `kvm_pmu_put()`.

v8:
https://lore.kernel.org/kvmarm/20260612192909.1153907-1-coltonlewis@google.com/

v7:
https://lore.kernel.org/kvmarm/20260504211813.1804997-1-coltonlewis@google.com/

v6:
https://lore.kernel.org/kvmarm/20260209221414.2169465-1-coltonlewis@google.com/

v5:
https://lore.kernel.org/kvmarm/20251209205121.1871534-1-coltonlewis@google.com/

v4:
https://lore.kernel.org/kvmarm/20250714225917.1396543-1-coltonlewis@google.com/

v3:
https://lore.kernel.org/kvm/20250626200459.1153955-1-coltonlewis@google.com/

v2:
https://lore.kernel.org/kvm/20250620221326.1261128-1-coltonlewis@google.com/

v1:
https://lore.kernel.org/kvm/20250602192702.2125115-1-coltonlewis@google.com/

[1] https://gitlab.com/qemu-project/kvm-forum/-/raw/main/_attachments/2025/Optimizing__itvHkhc.pdf
[2] https://www.youtube.com/watch?v=YRzZ8jMIA6M&list=PLW3ep1uCIRfxwmllXTOA2txfDWN6vUOHp&index=9

Colton Lewis (21):
  arm64: cpufeature: Add cpucap for HPMN0
  KVM: arm64: Reorganize PMU functions
  perf: arm_pmuv3: Generalize counter bitmasks
  perf: arm_pmuv3: Move counter allocation mask to per-CPU struct
    pmu_hw_events
  perf: arm_pmuv3: Check cntr_mask before using pmccntr
  perf: arm_pmuv3: Allocate counter indices from high to low
  KVM: arm64: Add initial scaffolding for Partitioned PMU
  KVM: arm64: Set up FGT for Partitioned PMU
  KVM: arm64: Add Partitioned PMU register trap handlers
  KVM: arm64: Set up MDCR_EL2 to handle a Partitioned PMU
  KVM: arm64: Context swap Partitioned PMU guest registers
  KVM: arm64: Enforce PMU event filter at vcpu_load()
  perf: Add perf_pmu_resched_update()
  KVM: arm64: Allow kvm_vcpu_pmu_resync_el0() to resync filters in
    process context
  KVM: arm64: Apply dynamic guest counter reservations
  KVM: arm64: Implement lazy PMU context swaps
  perf: arm_pmuv3: Handle IRQs for Partitioned PMU guest counters
  KVM: arm64: Detect overflows for the Partitioned PMU
  KVM: arm64: Add vCPU device attr to partition the PMU
  KVM: selftests: Add find_bit to KVM library
  KVM: arm64: selftests: Add test case for Partitioned PMU

Marc Zyngier (1):
  KVM: arm64: Reorganize PMU includes

 Documentation/virt/kvm/devices/vcpu.rst       |  42 +-
 arch/arm/include/asm/arm_pmuv3.h              |  16 +
 arch/arm64/include/asm/arm_pmuv3.h            |   7 +-
 arch/arm64/include/asm/kvm_host.h             |  18 +-
 arch/arm64/include/asm/kvm_types.h            |   6 +-
 arch/arm64/include/uapi/asm/kvm.h             |   2 +
 arch/arm64/kernel/cpufeature.c                |  10 +-
 arch/arm64/kvm/Makefile                       |   2 +-
 arch/arm64/kvm/arm.c                          |   4 +-
 arch/arm64/kvm/config.c                       |  49 +-
 arch/arm64/kvm/debug.c                        |  41 +-
 arch/arm64/kvm/pmu-direct.c                   | 636 ++++++++++++++
 arch/arm64/kvm/pmu-emul.c                     | 718 +---------------
 arch/arm64/kvm/pmu.c                          | 787 +++++++++++++++++-
 arch/arm64/kvm/sys_regs.c                     | 334 ++++++--
 arch/arm64/tools/cpucaps                      |   1 +
 arch/arm64/tools/sysreg                       |   6 +-
 drivers/perf/arm_pmu.c                        |   7 +-
 drivers/perf/arm_pmuv3.c                      |  97 ++-
 include/kvm/arm_pmu.h                         |  93 ++-
 include/linux/perf/arm_pmu.h                  |   2 +
 include/linux/perf/arm_pmuv3.h                |  14 +-
 include/linux/perf_event.h                    |   3 +
 kernel/events/core.c                          |  31 +-
 tools/include/perf/arm_pmuv3.h                |  12 +-
 tools/testing/selftests/kvm/Makefile.kvm      |   1 +
 .../selftests/kvm/arm64/vpmu_counter_access.c | 129 ++-
 tools/testing/selftests/kvm/lib/find_bit.c    |   2 +
 28 files changed, 2200 insertions(+), 870 deletions(-)
 create mode 100644 arch/arm64/kvm/pmu-direct.c
 create mode 100644 tools/testing/selftests/kvm/lib/find_bit.c


base-commit: 93f51579e7df248780214094418f205253383cc5
-- 
2.56.0.rc1.310.g51773c2048-goog