linux-kernel - Re: [RFC 1/9] RISC-V: Define a helper function to probe number of hardware counters

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Date:   Tue, 1 Nov 2022 13:30:08 +0100
From:   Andrew Jones <ajones@...tanamicro.com>
To:     Atish Patra <atishp@...osinc.com>
Cc:     linux-kernel@...r.kernel.org, Albert Ou <aou@...s.berkeley.edu>,
        Anup Patel <anup@...infault.org>,
        Atish Patra <atishp@...shpatra.org>,
        Guo Ren <guoren@...nel.org>, kvm-riscv@...ts.infradead.org,
        kvm@...r.kernel.org, linux-riscv@...ts.infradead.org,
        Mark Rutland <mark.rutland@....com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [RFC  1/9] RISC-V: Define a helper function to probe number of
 hardware counters

On Mon, Jul 18, 2022 at 10:01:57AM -0700, Atish Patra wrote:
> KVM module needs to know how many hardware counters the platform supports.
> Otherwise, it will not be able to show optimal value of virtual
                                        ^ the
> counters to the guest.
> 
> Signed-off-by: Atish Patra <atishp@...osinc.com>
> ---
>  drivers/perf/riscv_pmu_sbi.c   | 23 +++++++++++++++++------
>  include/linux/perf/riscv_pmu.h |  4 ++++
>  2 files changed, 21 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/perf/riscv_pmu_sbi.c b/drivers/perf/riscv_pmu_sbi.c
> index 24124546844c..1723af68ffa1 100644
> --- a/drivers/perf/riscv_pmu_sbi.c
> +++ b/drivers/perf/riscv_pmu_sbi.c
> @@ -27,6 +27,7 @@
>   */
>  static union sbi_pmu_ctr_info *pmu_ctr_list;
>  static unsigned int riscv_pmu_irq;
> +static struct riscv_pmu *rvpmu;

Do we really need rvpmu? From a quick scan of the series it's only used
for num_hw_counters, which has to be added to struct riscv_pmu, and
num_counters. How about instead creating a static global for num_counters
and then getting num_hw_counters by iterating pmu_ctr_list. If we want
riscv_pmu_sbi_get_num_hw_ctrs() to be faster, then we can cache the value
in a static variable in the function.

Thanks,
drew