Re: [PATCH 9/9] perf: arm_pmuv3: Add support for Armv9.4 PMU instruction counter

Mark Rutland
2024-06-10T11:55:32Z

On Fri, Jun 07, 2024 at 02:31:34PM -0600, Rob Herring (Arm) wrote:
> Armv9.4/8.9 PMU adds optional support for a fixed instruction counter
> similar to the fixed cycle counter. Support for the feature is indicated
> in the ID_AA64DFR1_EL1 register PMICNTR field. The counter is not
> accessible in AArch32.
>
> Existing userspace using direct counter access won't know how to handle
> the fixed instruction counter, so we have to avoid using the counter
> when user access is requested.
>
> Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
> ---
> arch/arm/include/asm/arm_pmuv3.h | 21 +++++++++++++++++++++
> arch/arm64/include/asm/arm_pmuv3.h | 29 +++++++++++++++++++++++++++++
> arch/arm64/kvm/pmu.c | 8 ++++++--
> arch/arm64/tools/sysreg | 25 +++++++++++++++++++++++++
> drivers/perf/arm_pmuv3.c | 28 ++++++++++++++++++++++++++--
> include/linux/perf/arm_pmu.h | 8 ++++++--
> include/linux/perf/arm_pmuv3.h | 4 +++-
> 7 files changed, 116 insertions(+), 7 deletions(-)
>
> diff --git a/arch/arm/include/asm/arm_pmuv3.h b/arch/arm/include/asm/arm_pmuv3.h
> index ac2cf37b57e3..b836537ddfbf 100644
> --- a/arch/arm/include/asm/arm_pmuv3.h
> +++ b/arch/arm/include/asm/arm_pmuv3.h
> @@ -10,6 +10,7 @@
> #include <asm/cputype.h>
>
> #define ARMV8_PMU_CYCLE_IDX 31
> +#define ARMV8_PMU_INSTR_IDX 32 /* Not accessible from AArch32 */

As with ARMV8_PMU_CYCLE_IDX, I reckon this should live in
<linux/perf/arm_pmuv3.h> (with the comment above) so that we don't need
separate definitions for arm & arm64.

[...]

> diff --git a/arch/arm64/tools/sysreg b/arch/arm64/tools/sysreg
> index 231817a379b5..8ab6e09871de 100644
> --- a/arch/arm64/tools/sysreg
> +++ b/arch/arm64/tools/sysreg
> @@ -2029,6 +2029,31 @@ Sysreg FAR_EL1 3 0 6 0 0
> Field 63:0 ADDR
> EndSysreg
>
> +Sysreg PMICNTR_EL0 3 3 9 4 0
> +Field 63:0 ICNT
> +EndSysreg

LGTM per ARM DDI 0487K.a, section D23.5.15, pages 8989 to 8992.

> +
> +Sysreg PMICFILTR_EL0 3 3 9 6 0
> +Res0 63:59
> +Field 58 SYNC
> +Field 57:56 VS

The 'VS' field doesn't seem to be in the ARM ARM (ARM DDI 0487K.a); is
that defined in a supplement?

> +Res0 55:32
> +Field 31 P
> +Field 30 U
> +Field 29 NSK
> +Field 28 NSU
> +Field 27 NSH
> +Field 26 M
> +Res0 25
> +Field 24 SH
> +Field 23 T
> +Field 22 RLK
> +Field 21 RLU
> +Field 20 RLH
> +Res0 19:16
> +Field 15:0 evtCount
> +EndSysreg

Aside from 'VS', this LGTM per ARM DDI 0487K.a, section D23.5.14, pages
8981 to 8988.

One important thing to note is that this doesn't have the threshold
controls (TC, TE, TH); so if threshold events make sense for instruction
events, we cannot place those in the dedicated isntruction counter.

[...]

> @@ -931,6 +939,18 @@ static int armv8pmu_get_event_idx(struct pmu_hw_events *cpuc,
> return -EAGAIN;
> }
>
> + /*
> + * Always prefer to place a instruction counter into the instruction counter,
> + * but don't expose the instruction counter to userspace access as userspace
> + * may not know how to handle it.
> + */
> + if (test_bit(ARMV8_PMU_INSTR_IDX, cpu_pmu->cntr_mask) &&
> + (evtype == ARMV8_PMUV3_PERFCTR_INST_RETIRED) &&
> + !armv8pmu_event_want_user_access(event)) {
> + if (!test_and_set_bit(ARMV8_PMU_INSTR_IDX, cpuc->used_mask))
> + return ARMV8_PMU_INSTR_IDX;
> + }

I reckon this'd be a bit clearer if we check the evtype first, as with
cycles, e.g.

/*
* Always prefer to place a instruction counter into the instruction counter,
* but don't expose the instruction counter to userspace access as userspace
* may not know how to handle it.
*/
if (evtype == ARMV8_PMUV3_PERFCTR_INST_RETIRED) {
if (test_bit(ARMV8_PMU_INSTR_IDX, cpu_pmu->cntr_mask) &&
!armv8pmu_event_want_user_access(event) &&
!test_and_set_bit(ARMV8_PMU_INSTR_IDX, cpuc->used_mask))
return ARMV8_PMU_INSTR_IDX;
}

As above, we might need to check for threshold controls, but I'll need
to go page that in.

Mark.