Messages in this thread | | | Date | Thu, 17 Nov 2022 11:15:10 +0530 | Subject | Re: [PATCH V5 6/7] arm64/perf: Add BRBE driver | From | Anshuman Khandual <> |
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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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