Re: [PATCH v3 0/3] powerpc/perf: Fix vpa-pmu driver to report correct numbers

[Gautam Menghani]

Forgot to add linuxppc-dev

On Fri, Nov 06, 2026 at 09:01:31PM +0530, Gautam Menghani wrote:
> The vpa-pmu driver can sometimes report incorrect context switch latency
> numbers:
> 
> $ perf stat -e vpa_pmu/l1_to_l2_lat/  --tid=2061  -I 1000
>            time             counts unit events
>      1.001022517 18,446,744,073,708,177,408      vpa_pmu/l1_to_l2_lat/
>      2.002120674            345,092      vpa_pmu/l1_to_l2_lat/
>      3.003171582            406,342      vpa_pmu/l1_to_l2_lat/
>      4.004234838            340,836      vpa_pmu/l1_to_l2_lat/
> 
> These invalid numbers are observed because of 2 issues:
> 
> 1. The PACA never points to the correct vCPU running on a host vCPU when
> vpa_pmu_add() is called.
> 
> 2. When get_counter_data() reads the PACA counter values, it does not
> update the event->hw.prev_count.
> 
> 
> Changes in V3:
> 1. Drop the usage of the PACA's vCPU pointer.
> 2. Introduce a new patch to fix an issue seen when a tracepoint and vpa-pmu
> driver concurrently enable/disable access to latency counters in VPA.
> 
> 
> Gautam Menghani (3):
>   Revert "powerpc/perf: Add per-task/process monitoring to vpa_pmu
>     driver"
>   powerpc/perf: Update prev_count of event to get accurate values
>   KVM: PPC: Book3s HV: Ensure reliable usage of latency counters
> 
>  arch/powerpc/include/asm/kvm_book3s_64.h |  3 --
>  arch/powerpc/kvm/book3s_hv.c             | 54 +++---------------------
>  arch/powerpc/perf/vpa-pmu.c              | 16 ++-----
>  3 files changed, 11 insertions(+), 62 deletions(-)
> 
> -- 
> 2.56.0
>