Re: [PATCH 0/7] KVM: arm64: PMU: Allow userspace to limit the number of PMCs on vCPU

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Thu, 29 Dec 2022 19:59:21 -0800
Reiji Watanabe <reijiw@google.com> wrote:

> The goal of this series is to allow userspace to limit the number
> of PMU event counters on the vCPU.

Hi Rieji,

Why do you want to do this?

I can conjecture a bunch of possible reasons, but they may not
match up with your use case. It would be useful to have that information
in the cover letter.

Jonathan

> 
> The number of PMU event counters is indicated in PMCR_EL0.N.
> For a vCPU with PMUv3 configured, its value will be the same as
> the host value by default. Userspace can set PMCR_EL0.N for the
> vCPU to a lower value than the host value, using KVM_SET_ONE_REG.
> However, it is practically unsupported, as KVM resets PMCR_EL0.N
> to the host value on vCPU reset and some KVM code uses the host
> value to identify (un)implemented event counters on the vCPU.
> 
> This series will ensure that the PMCR_EL0.N value is preserved
> on vCPU reset and that KVM doesn't use the host value
> to identify (un)implemented event counters on the vCPU.
> This allows userspace to limit the number of the PMU event
> counters on the vCPU.
> 
> Patch 1 fixes reset_pmu_reg() to ensure that (RAZ) bits of
> {PMCNTEN,PMOVS}{SET,CLR}_EL1 corresponding to unimplemented event
> counters on the vCPU are reset to zero even when PMCR_EL0.N for
> the vCPU is different from the host.
> 
> Patch 2 is a minor refactoring to use the default PMU register reset
> function (reset_pmu_reg()) for PMUSERENR_EL0 and PMCCFILTR_EL0.
> (With the Patch 1 change, reset_pmu_reg() can now be used for
> those registers)
> 
> Patch 3 fixes reset_pmcr() to preserve PMCR_EL0.N for the vCPU on
> vCPU reset.
> 
> Patch 4-7 adds a selftest to verify reading and writing PMU registers
> for implemented or unimplemented PMU event counters on the vCPU.
> 
> The series is based on kvmarm/fixes at the following commit:
>   commit aff234839f8b ("KVM: arm64: PMU: Fix PMCR_EL0 reset value")
> 
> Reiji Watanabe (7):
>   KVM: arm64: PMU: Have reset_pmu_reg() to clear a register
>   KVM: arm64: PMU: Use reset_pmu_reg() for PMUSERENR_EL0 and
>     PMCCFILTR_EL0
>   KVM: arm64: PMU: Preserve vCPU's PMCR_EL0.N value on vCPU reset
>   tools: arm64: Import perf_event.h
>   KVM: selftests: aarch64: Introduce vpmu_counter_access test
>   KVM: selftests: aarch64: vPMU register test for implemented counters
>   KVM: selftests: aarch64: vPMU register test for unimplemented counters
> 
>  arch/arm64/kvm/pmu-emul.c                     |   6 +
>  arch/arm64/kvm/sys_regs.c                     |  18 +-
>  tools/arch/arm64/include/asm/perf_event.h     | 258 ++++++++
>  tools/testing/selftests/kvm/.gitignore        |   1 +
>  tools/testing/selftests/kvm/Makefile          |   1 +
>  .../kvm/aarch64/vpmu_counter_access.c         | 613 ++++++++++++++++++
>  .../selftests/kvm/include/aarch64/processor.h |   1 +
>  7 files changed, 886 insertions(+), 12 deletions(-)
>  create mode 100644 tools/arch/arm64/include/asm/perf_event.h
>  create mode 100644 tools/testing/selftests/kvm/aarch64/vpmu_counter_access.c
> 
> 
> base-commit: aff234839f8b80ac101e6c2f14d0e44b236efa48


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