Re: [PATCH v6 08/24] drivers/perf: riscv: Implement SBI PMU snapshot function

[Andrew Jones]

On Wed, Apr 10, 2024 at 05:07:36PM -0700, Atish Patra wrote:
> SBI v2.0 SBI introduced PMU snapshot feature which adds the following
> features.
> 
> 1. Read counter values directly from the shared memory instead of
> csr read.
> 2. Start multiple counters with initial values with one SBI call.
> 
> These functionalities optimizes the number of traps to the higher
> privilege mode. If the kernel is in VS mode while the hypervisor
> deploy trap & emulate method, this would minimize all the hpmcounter
> CSR read traps. If the kernel is running in S-mode, the benefits
> reduced to CSR latency vs DRAM/cache latency as there is no trap
> involved while accessing the hpmcounter CSRs.
> 
> In both modes, it does saves the number of ecalls while starting
> multiple counter together with an initial values. This is a likely
> scenario if multiple counters overflow at the same time.
> 
> Acked-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
> Reviewed-by: Anup Patel <anup@xxxxxxxxxxxxxx>
> Reviewed-by: Conor Dooley <conor.dooley@xxxxxxxxxxxxx>
> Signed-off-by: Atish Patra <atishp@xxxxxxxxxxxx>
> ---
>  drivers/perf/riscv_pmu.c       |   1 +
>  drivers/perf/riscv_pmu_sbi.c   | 224 +++++++++++++++++++++++++++++++--
>  include/linux/perf/riscv_pmu.h |   6 +
>  3 files changed, 219 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/perf/riscv_pmu.c b/drivers/perf/riscv_pmu.c
> index b4efdddb2ad9..36d348753d05 100644
> --- a/drivers/perf/riscv_pmu.c
> +++ b/drivers/perf/riscv_pmu.c
> @@ -408,6 +408,7 @@ struct riscv_pmu *riscv_pmu_alloc(void)
>  		cpuc->n_events = 0;
>  		for (i = 0; i < RISCV_MAX_COUNTERS; i++)
>  			cpuc->events[i] = NULL;
> +		cpuc->snapshot_addr = NULL;
>  	}
>  	pmu->pmu = (struct pmu) {
>  		.event_init	= riscv_pmu_event_init,
> diff --git a/drivers/perf/riscv_pmu_sbi.c b/drivers/perf/riscv_pmu_sbi.c
> index f23501898657..e2881415ca0a 100644
> --- a/drivers/perf/riscv_pmu_sbi.c
> +++ b/drivers/perf/riscv_pmu_sbi.c
> @@ -58,6 +58,9 @@ PMU_FORMAT_ATTR(event, "config:0-47");
>  PMU_FORMAT_ATTR(firmware, "config:63");
>  
>  static bool sbi_v2_available;
> +static DEFINE_STATIC_KEY_FALSE(sbi_pmu_snapshot_available);
> +#define sbi_pmu_snapshot_available() \
> +	static_branch_unlikely(&sbi_pmu_snapshot_available)
>  
>  static struct attribute *riscv_arch_formats_attr[] = {
>  	&format_attr_event.attr,
> @@ -508,14 +511,109 @@ static int pmu_sbi_event_map(struct perf_event *event, u64 *econfig)
>  	return ret;
>  }
>  
> +static void pmu_sbi_snapshot_free(struct riscv_pmu *pmu)
> +{
> +	int cpu;
> +
> +	for_each_possible_cpu(cpu) {
> +		struct cpu_hw_events *cpu_hw_evt = per_cpu_ptr(pmu->hw_events, cpu);
> +
> +		if (!cpu_hw_evt->snapshot_addr)
> +			continue;
> +
> +		free_page((unsigned long)cpu_hw_evt->snapshot_addr);
> +		cpu_hw_evt->snapshot_addr = NULL;
> +		cpu_hw_evt->snapshot_addr_phys = 0;
> +	}
> +}
> +
> +static int pmu_sbi_snapshot_alloc(struct riscv_pmu *pmu)
> +{
> +	int cpu;
> +	struct page *snapshot_page;
> +
> +	for_each_possible_cpu(cpu) {
> +		struct cpu_hw_events *cpu_hw_evt = per_cpu_ptr(pmu->hw_events, cpu);
> +
> +		if (cpu_hw_evt->snapshot_addr)
> +			continue;
> +
> +		snapshot_page = alloc_page(GFP_ATOMIC | __GFP_ZERO);
> +		if (!snapshot_page) {
> +			pmu_sbi_snapshot_free(pmu);
> +			return -ENOMEM;
> +		}
> +		cpu_hw_evt->snapshot_addr = page_to_virt(snapshot_page);
> +		cpu_hw_evt->snapshot_addr_phys = page_to_phys(snapshot_page);
> +	}
> +
> +	return 0;
> +}
> +
> +static int pmu_sbi_snapshot_disable(void)
> +{
> +	struct sbiret ret;
> +
> +	ret = sbi_ecall(SBI_EXT_PMU, SBI_EXT_PMU_SNAPSHOT_SET_SHMEM, -1,
> +			-1, 0, 0, 0, 0);

The SBI_SHMEM_DISABLE patch got moved in front of this patch, but looks
like it was forgotten to apply it.

Otherwise,

Reviewed-by: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>

Thanks,
drew