Messages in this thread | | | Date | Tue, 10 Oct 2023 09:30:45 +0100 | Subject | Re: [PATCH] drivers: perf: arm_pmuv3: Update 'pmc_width' based on actual HW event width | From | James Clark <> |
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On 10/10/2023 09:28, James Clark wrote: > > > On 10/10/2023 04:03, Anshuman Khandual wrote: >> Hi James, >> >> On 10/9/23 15:13, James Clark wrote: >>> >>> >>> On 09/10/2023 05:37, Anshuman Khandual wrote: >>>> This updates 'perf_event_mmap_page->pmc_width' based on actual HW event's >>>> width that are currently missing i.e ARMPMU_EVT_63BIT and ARMPMU_EVT_47BIT. >>>> >>> >>> Might be worth adding why this is needed or what the actual effect is. >> >> To have correct 'pmc_width' visible to the user space ? > > Well yeah, but for example I didn't know what that was. And it's not > clear why it needs updating at this point in time without a link to any > other commit or relevant section from the Arm ARM. So I had a kind of a > "why now" question. > > "To have correct 'pmc_width' visible to the user space" is definitely > more of a what than a why. >
If anything this should at least have a fixes: tag on it. If you're saying that it's now correct.
>> >>> >>>> Cc: Will Deacon <will@kernel.org> >>>> Cc: Mark Rutland <mark.rutland@arm.com> >>>> Cc: linux-arm-kernel@lists.infradead.org >>>> Cc: linux-kernel@vger.kernel.org >>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com> >>>> --- >>>> This applies on v6.6-rc5. >>>> >>>> drivers/perf/arm_pmuv3.c | 4 ++++ >>>> 1 file changed, 4 insertions(+) >>>> >>>> diff --git a/drivers/perf/arm_pmuv3.c b/drivers/perf/arm_pmuv3.c >>>> index fe4db1831662..94723d00548e 100644 >>>> --- a/drivers/perf/arm_pmuv3.c >>>> +++ b/drivers/perf/arm_pmuv3.c >>>> @@ -1375,6 +1375,10 @@ void arch_perf_update_userpage(struct perf_event *event, >>>> if (userpg->cap_user_rdpmc) { >>>> if (event->hw.flags & ARMPMU_EVT_64BIT) >>>> userpg->pmc_width = 64; >>>> + else if (event->hw.flags & ARMPMU_EVT_63BIT) >>>> + userpg->pmc_width = 63; >>>> + else if (event->hw.flags & ARMPMU_EVT_47BIT) >>>> + userpg->pmc_width = 47; >>> >>> Although it doesn't explicitly say it, the bit of the docs about >>> pmc_width in Documentation/arch/arm64/perf.rst loosely implies that this >>> is always either 64 or 32. Now that this isn't the case it could mislead >>> someone in userspace that they don't have to handle the now arbitrary >>> bit widths rather than just whole bytes/ints. >> >> Are you suggesting that the user space would not handle pmc_width correctly >> , once it deviates from a whole bytes/ints format ? In that case user space >> handling might need some fixing. >> > > Not really, I'm just suggesting that anyone writing a new tool and only > reading the docs could make that assumption. Seeing as only 32 and 64 > bit options are mentioned. So it's more to avoid misleading someone in > the future than about fixing any existing code, as updating the docs > wouldn't have that effect. > >>> >>> I think the fix is as simple as adding something like "the width may not >>> match the requested value or necessarily be a multiple of 8". Unless we >>> think this is already widely known and I suppose we could leave it as >>> is. (The existing bit in perf that uses it already handles it correctly). >> >> This is from perf_event_mmap_page definition where it does not assert the >> width to be multiple of bytes or ints. Hence the assumption should not be >> made into the user space tools. >> > > Yeah I know its already ok for Perf which is why I mentioned it. But > there are more tools out there than Perf, and ones that don't even exist > yet, which people would normally read the documentation before writing. > >> /* >> * If cap_user_rdpmc this field provides the bit-width of the value >> * read using the rdpmc() or equivalent instruction. This can be used >> * to sign extend the result like: >> * >> * pmc <<= 64 - width; >> * pmc >>= 64 - width; // signed shift right >> * count += pmc; >> */ >> __u16 pmc_width; >> >> Moreover, on x86 too 'userpg->pmc_width' gets assigned to different values >> although multiple of 8. >> >> userpg->pmc_width = x86_pmu.cntval_bits >> arch/x86/events/amd/core.c: .cntval_bits = 48 >> arch/x86/events/intel/knc.c: .cntval_bits = 40 >> arch/x86/events/intel/p6.c: .cntval_bits = 32 >> >>> >>>> else >>>> userpg->pmc_width = 32; >>>> } > > _______________________________________________ > linux-arm-kernel mailing list > linux-arm-kernel@lists.infradead.org > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
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