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SubjectRe: [PATCH v2 4/4] perf tools: arm64: Add support for VG register
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On 17/05/2022 14:19, Leo Yan wrote:
> On Tue, May 17, 2022 at 11:20:05AM +0100, James Clark wrote:
>> Add the name of the VG register so it can be used in --user-regs
>>
>> The event will fail to open if the register is requested but not
>> available so only add it to the mask if the kernel supports sve and also
>> if it supports that specific register.
>>
>> Signed-off-by: James Clark <james.clark@arm.com>
>> ---
>> tools/perf/arch/arm64/util/perf_regs.c | 34 ++++++++++++++++++++++++++
>> tools/perf/util/perf_regs.c | 2 ++
>> 2 files changed, 36 insertions(+)
>>
>> diff --git a/tools/perf/arch/arm64/util/perf_regs.c b/tools/perf/arch/arm64/util/perf_regs.c
>> index 476b037eea1c..c0a921512a90 100644
>> --- a/tools/perf/arch/arm64/util/perf_regs.c
>> +++ b/tools/perf/arch/arm64/util/perf_regs.c
>> @@ -2,9 +2,11 @@
>> #include <errno.h>
>> #include <regex.h>
>> #include <string.h>
>> +#include <sys/auxv.h>
>> #include <linux/kernel.h>
>> #include <linux/zalloc.h>
>>
>> +#include "../../../perf-sys.h"
>> #include "../../../util/debug.h"
>> #include "../../../util/event.h"
>> #include "../../../util/perf_regs.h"
>> @@ -43,6 +45,7 @@ const struct sample_reg sample_reg_masks[] = {
>> SMPL_REG(lr, PERF_REG_ARM64_LR),
>> SMPL_REG(sp, PERF_REG_ARM64_SP),
>> SMPL_REG(pc, PERF_REG_ARM64_PC),
>> + SMPL_REG(vg, PERF_REG_ARM64_VG),
>> SMPL_REG_END
>> };
>>
>> @@ -131,3 +134,34 @@ int arch_sdt_arg_parse_op(char *old_op, char **new_op)
>>
>> return SDT_ARG_VALID;
>> }
>> +
>> +uint64_t arch__user_reg_mask(void)
>> +{
>> + struct perf_event_attr attr = {
>> + .type = PERF_TYPE_HARDWARE,
>> + .config = PERF_COUNT_HW_CPU_CYCLES,
>> + .sample_type = PERF_SAMPLE_REGS_USER,
>> + .disabled = 1,
>> + .exclude_kernel = 1,
>> + .sample_period = 1,
>> + .sample_regs_user = PERF_REGS_MASK
>> + };
>> + int fd;
>> +
>> + if (getauxval(AT_HWCAP) & HWCAP_SVE)
>> + attr.sample_regs_user |= SMPL_REG_MASK(PERF_REG_ARM64_VG);
>> +
>> + /*
>> + * Check if the pmu supports perf extended regs, before
>> + * returning the register mask to sample.
>> + */
>> + if (attr.sample_regs_user != PERF_REGS_MASK) {
>> + event_attr_init(&attr);
>> + fd = sys_perf_event_open(&attr, 0, -1, -1, 0);
>> + if (fd != -1) {
>> + close(fd);
>> + return attr.sample_regs_user;
>> + }
>> + }
>
> Just curious, since we can know SVE is supported from reading
> auxiliary value, can we directly return the register mask as below?
>
> PERF_REGS_MASK | SMPL_REG_MASK(PERF_REG_ARM64_VG);

I was trying to cover the case where the system supports SVE, but
the kernel doesn't have my changes to add the VG register yet.

Technically I could just attempt to open the event without checking
for SVE first and see if it works or not. But I preferred to be
explicit so it's obvious why we're doing that.

James

>
> Except this question, this patch looks good to me.
>
> Thanks,
> Leo
>
>> + return PERF_REGS_MASK;
>> +}
>> diff --git a/tools/perf/util/perf_regs.c b/tools/perf/util/perf_regs.c
>> index a982e40ee5a9..872dd3d38782 100644
>> --- a/tools/perf/util/perf_regs.c
>> +++ b/tools/perf/util/perf_regs.c
>> @@ -103,6 +103,8 @@ static const char *__perf_reg_name_arm64(int id)
>> return "lr";
>> case PERF_REG_ARM64_PC:
>> return "pc";
>> + case PERF_REG_ARM64_VG:
>> + return "vg";
>> default:
>> return NULL;
>> }
>> --
>> 2.28.0
>>

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