[PATCH v7 00/20] ARM64 PMU Partitioning

Colton Lewis
2026-05-04T21:18:40Z

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.

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

After a few false starts, meeting with Will Deacon and Mark Rutland to
discuss implementation ideas, and a few more false starts, I finally
have an implementation of dynamic counter reservation that works
without disrupting host perf too much. Now the host only loses access
to the guest counters when a vCPU resides on the CPU.

The key was creating perf_pmu_resched_update, which behaves exactly
like perf_pmu_resched except it takes a callback to call in between
when the perf events are scheduled out and when they are scheduled
back in. That allows us to update the PMU's available counters when we
know they are not currently in use without needing to expose private
perf core functions and triple check they are not being called in a
way that violates existing assumptions.

Because this introduces a possibility of perf reschedule during vCPU
load, I've optimized to only do that operation if there are host
events occupying the intended guest counters at the time of the load.

The kernel command line parameter for the driver still exists, but now
only defines an upper limit of counters the guest might use rather
than taking those counters from the host permanently.

v7:

* Implement dynamic counter reservation as described above. One side
effect is the PMUv3 driver now needs much fewer changes to enforce
the boundary.

* Move register accesses out of fast path for non-FGT hardware. The
performance impact was negligible and this moves bloat out of the
fast path and allows a more reliable design with more code sharing.

* Make PMCCNTR a special case in the context swap again because trying
to access it with PMXEVCNTR is undefined.

* Fix a bug where kvm_pmu_guest_counter_mask was using & instead of |.

* Re-expose the dedicated instruction counter to the host since it was
decided the guest will not own it.

* Change the global armv8pmu_reserved_host_counters to
armv8pmu_is_partitoned because it was only used in boolean checks.

* Fix typo in vcpu attribute commit so the spelling of the flag in the
commit message matches the code.

* Rebase to v7.0-rc7

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 (19):
arm64: cpufeature: Add cpucap for HPMN0
KVM: arm64: Reorganize PMU functions
perf: arm_pmuv3: Generalize counter bitmasks
perf: arm_pmuv3: Check cntr_mask before using pmccntr
perf: arm_pmuv3: Add method to partition the 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: 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
KVM: arm64: selftests: Relax testing for exceptions when partitioned

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

arch/arm/include/asm/arm_pmuv3.h | 18 +
arch/arm64/include/asm/arm_pmuv3.h | 12 +-
arch/arm64/include/asm/kvm_host.h | 17 +-
arch/arm64/include/asm/kvm_types.h | 6 +-
arch/arm64/include/uapi/asm/kvm.h | 2 +
arch/arm64/kernel/cpufeature.c | 8 +
arch/arm64/kvm/Makefile | 2 +-
arch/arm64/kvm/arm.c | 2 +
arch/arm64/kvm/config.c | 41 +-
arch/arm64/kvm/debug.c | 31 +-
arch/arm64/kvm/pmu-direct.c | 494 ++++++++++++
arch/arm64/kvm/pmu-emul.c | 674 +----------------
arch/arm64/kvm/pmu.c | 701 ++++++++++++++++++
arch/arm64/kvm/sys_regs.c | 250 ++++++-
arch/arm64/tools/cpucaps | 1 +
arch/arm64/tools/sysreg | 6 +-
drivers/perf/arm_pmuv3.c | 111 ++-
include/kvm/arm_pmu.h | 110 +++
include/linux/perf/arm_pmu.h | 3 +
include/linux/perf/arm_pmuv3.h | 14 +-
include/linux/perf_event.h | 3 +
kernel/events/core.c | 28 +-
tools/testing/selftests/kvm/Makefile.kvm | 1 +
.../selftests/kvm/arm64/vpmu_counter_access.c | 112 ++-
tools/testing/selftests/kvm/lib/find_bit.c | 1 +
25 files changed, 1861 insertions(+), 787 deletions(-)
create mode 100644 arch/arm64/kvm/pmu-direct.c
create mode 100644 tools/testing/selftests/kvm/lib/find_bit.c

base-commit: 591cd656a1bf5ea94a222af5ef2ee76df029c1d2
--
2.54.0.545.g6539524ca2-goog