Re: [kvm-unit-tests patch v3 3/8] x86/pmu: Fix incorrect masking of fixed counters

Sean Christopherson
2025-11-20T22:28:18Z

On Wed, Sep 03, 2025, Dapeng Mi wrote:
> From: dongsheng <dongsheng.x.zhang@intel.com>
>
> The current implementation mistakenly limits the width of fixed counters
> to the width of GP counters. Corrects the logic to ensure fixed counters
> are properly masked according to their own width.
>
> Opportunistically refine the GP counter bitwidth processing code.
>
> Signed-off-by: dongsheng <dongsheng.x.zhang@intel.com>
> Co-developed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
> Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
> Tested-by: Yi Lai <yi1.lai@intel.com>
> ---
> x86/pmu.c | 8 +++-----
> 1 file changed, 3 insertions(+), 5 deletions(-)
>
> diff --git a/x86/pmu.c b/x86/pmu.c
> index 04946d10..44c728a5 100644
> --- a/x86/pmu.c
> +++ b/x86/pmu.c
> @@ -556,18 +556,16 @@ static void check_counter_overflow(void)
> int idx;
>
> cnt.count = overflow_preset;
> - if (pmu_use_full_writes())
> - cnt.count &= (1ull << pmu.gp_counter_width) - 1;
> -
> if (i == pmu.nr_gp_counters) {
> if (!pmu.is_intel)
> break;
>
> cnt.ctr = fixed_events[0].unit_sel;
> - cnt.count = measure_for_overflow(&cnt);

Per commit 7ec3b67a ("x86/pmu: Reset the expected count of the fixed counter 0
when i386"), re-measuring for the fixed counter is necessary when running a 32-bit
guests. I didn't see failures (spotted this by inspection), but I don't see any
point in making this change without good reason.

> - cnt.count &= (1ull << pmu.gp_counter_width) - 1;
> + cnt.count &= (1ull << pmu.fixed_counter_width) - 1;
> } else {
> cnt.ctr = MSR_GP_COUNTERx(i);
> + if (pmu_use_full_writes())
> + cnt.count &= (1ull << pmu.gp_counter_width) - 1;
> }
>
> if (i % 2)
> --
> 2.34.1
>