From: Anshuman Khandual <anshuman.khandual@arm.com>
To: James Clark <james.clark@arm.com>
Cc: Mark Brown <broonie@kernel.org>, Rob Herring <robh@kernel.org>,
Marc Zyngier <maz@kernel.org>,
Suzuki Poulose <suzuki.poulose@arm.com>,
Ingo Molnar <mingo@redhat.com>,
linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org,
linux-arm-kernel@lists.infradead.org, peterz@infradead.org,
acme@kernel.org, mark.rutland@arm.com, will@kernel.org,
catalin.marinas@arm.com
Subject: Re: [PATCH V5 6/7] arm64/perf: Add BRBE driver
Date: Thu, 17 Nov 2022 11:15:10 +0530 [thread overview]
Message-ID: <a065d948-3b9c-86bf-4a10-45d9c47e7ea1@arm.com> (raw)
In-Reply-To: <1d06f05c-0e7f-7648-6b1b-b78e999bc65f@arm.com>
On 11/16/22 22:12, James Clark wrote:
>
>
> On 07/11/2022 06:25, Anshuman Khandual wrote:
> [...]
>
>> +static void perf_branch_to_brbcr(struct pmu_hw_events *cpuc, int branch_type)
>> +{
>> + cpuc->brbcr = (BRBCR_EL1_CC | BRBCR_EL1_MPRED);
>> +
>> + if (branch_type & PERF_SAMPLE_BRANCH_USER)
>> + cpuc->brbcr |= BRBCR_EL1_E0BRE;
>> +
>> + if (branch_type & PERF_SAMPLE_BRANCH_NO_CYCLES)
>> + cpuc->brbcr &= ~BRBCR_EL1_CC;
>> +
>> + if (branch_type & PERF_SAMPLE_BRANCH_NO_FLAGS)
>> + cpuc->brbcr &= ~BRBCR_EL1_MPRED;
>> +
>> + if (branch_type & PERF_SAMPLE_BRANCH_KERNEL)
>> + cpuc->brbcr |= BRBCR_EL1_E1BRE;
>> + else
>> + return;
>> +
>> + /*
>> + * The exception and exception return branches could be
>> + * captured only when the event has necessary privilege
>> + * indicated via branch type PERF_SAMPLE_BRANCH_KERNEL,
>> + * which has been ascertained in generic perf. Please
>> + * refer perf_copy_attr() for more details.
>> + */
>> + if (branch_type & PERF_SAMPLE_BRANCH_ANY) {
>> + cpuc->brbcr |= BRBCR_EL1_EXCEPTION;
>> + cpuc->brbcr |= BRBCR_EL1_ERTN;
>
> Because this comes after the PERF_SAMPLE_BRANCH_KERNEL check, it's
> impossible to get syscall records from userspace. When you enable kernel
> branch records, the buffer always fills up before it gets to userspace.
Just to summerize.
System call [user_addr -> kernel_addr] and return [kernel_addr -> user_addr]
records are impossible to be captured, because
- Without PERF_SAMPLE_BRANCH_KERNEL, BRBCR_EL1_EXCEPTION/ERTN are not set
- With PERF_SAMPLE_BRANCH_KERNEL, buffer fills up with in kernel branches
Did you try with latest fix, that clears the paused BRBE after reading branch
records during PMU interrupt ? That fix creates much more samples than before.
>
> Can you move this to the top so that it can be set if either
> PERF_SAMPLE_BRANCH_USER or PERF_SAMPLE_BRANCH_KERNEL is set. The
Why should they depend on privilege filters i.e PERF_SAMPLE_BRANCH_USER/KERNEL
rather than just branch filters PERF_SAMPLE_BRANCH_ANY/ANY_CALL/ANY_RETURN ?
> hardware already handles the security by giving partial records with the
> kernel part zeroed out so I don't think the driver needs to add any
> additional rules other than setting BRBCR_EL1_E1BRE or BRBCR_EL1_E0BRE.
Basically BRBCR_EL1_EXCEPTION/BRBCR_EL1_ERTN should be treated like any other
branch filter rather than privilege filters as is the case now ?
>
> For example I moved it to the top, removed the return below and then I
> get syscall partial records:
>
> .... 5: 0000000000745d0c -> 0000000000000000 0 cycles P 9fbfbfbf SYSCALL
>
> I also get ERETS but with only the userspace part set:
>
> ..... 4: 0000000000000000 -> 0000000000745d10 0 cycles P 9fbfbfbf ERET
But with both user and kernel privilege filters being set, these should have
been complete branch records containing both user and kernel addresses ?
>
>> + return;
>> + }
>> +
>> + if (branch_type & PERF_SAMPLE_BRANCH_ANY_CALL)
>> + cpuc->brbcr |= BRBCR_EL1_EXCEPTION;
>> +
>> + if (branch_type & PERF_SAMPLE_BRANCH_ANY_RETURN)
>> + cpuc->brbcr |= BRBCR_EL1_ERTN;
>> +}
>> +
[....]
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next prev parent reply other threads:[~2022-11-17 5:46 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-07 6:25 [PATCH V5 0/7] arm64/perf: Enable branch stack sampling Anshuman Khandual
2022-11-07 6:25 ` [PATCH V5 1/7] arm64/perf: Add BRBE registers and fields Anshuman Khandual
2022-11-07 15:15 ` Mark Brown
2022-11-07 6:25 ` [PATCH V5 2/7] arm64/perf: Update struct arm_pmu for BRBE Anshuman Khandual
2022-11-09 11:30 ` Suzuki K Poulose
2022-11-18 6:39 ` Anshuman Khandual
2022-11-18 17:47 ` Mark Rutland
2022-11-29 6:06 ` Anshuman Khandual
2022-11-07 6:25 ` [PATCH V5 3/7] arm64/perf: Update struct pmu_hw_events " Anshuman Khandual
2022-11-07 6:25 ` [PATCH V5 4/7] driver/perf/arm_pmu_platform: Add support for BRBE attributes detection Anshuman Khandual
2022-11-18 18:01 ` Mark Rutland
2022-11-21 6:36 ` Anshuman Khandual
2022-11-21 11:39 ` Mark Rutland
2022-11-28 8:24 ` Anshuman Khandual
2022-11-07 6:25 ` [PATCH V5 5/7] arm64/perf: Drive BRBE from perf event states Anshuman Khandual
2022-11-18 18:15 ` Mark Rutland
2022-11-29 6:26 ` Anshuman Khandual
2022-11-07 6:25 ` [PATCH V5 6/7] arm64/perf: Add BRBE driver Anshuman Khandual
2022-11-09 3:08 ` Anshuman Khandual
2022-11-16 16:42 ` James Clark
2022-11-17 5:45 ` Anshuman Khandual [this message]
2022-11-17 10:09 ` James Clark
2022-11-18 6:14 ` Anshuman Khandual
2022-11-29 15:53 ` James Clark
2022-11-30 4:49 ` Anshuman Khandual
2022-11-30 16:56 ` James Clark
2022-12-06 17:05 ` James Clark
2022-11-07 6:25 ` [PATCH V5 7/7] arm64/perf: Enable branch stack sampling Anshuman Khandual
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