Re: [PATCH V13 - RESEND 06/10] arm64/perf: Enable branch stack events via FEAT_BRBE

[Suzuki K Poulose <suzuki.poulose@arm.com>]

On 25/07/2023 12:42, Anshuman Khandual wrote:
> Hello Yang,
> 
> On 7/25/23 12:42, Yang Shen wrote:
>>> +    if (!(branch_type & PERF_SAMPLE_BRANCH_NO_CYCLES))
>>> +        brbcr |= BRBCR_EL1_CC;
>>
>> Hi Anshuman,
>>
>> Here is problem about enable CYCLES_COUNT. The SPEC defines that the CYCLES_COUNT is only
>>
>> valid when the BRECR_EL1.CC & BRBCR_EL2.CC is true. And here the SPEC also defines that
>>
>> when PSTATE.EL == EL2 and HCR_EL2.E2h == '1', 'MSR BRBCR_EL1, <Xt>' means writing to
>>
>> BRBCR_EL2 actually. So 'armv8pmu_branch_enable' can only set the BRBCR_EL2.CC, while the
>>
>> BRECR_EL1.CC is still 0. The CYCLES_COUNT will be always 0 in records.
> 
> 
> Agreed, this is a valid problem i.e BRBCR_EL1.CC and BRBCR_EL2.CC both needs to be set
> for valid cycle count information regardless if the kernel runs in EL1 or EL2. A simple
> hack in the current code setting BRBCR_EL12.C, which in turn sets BRBCR_EL1.CC when the
> kernel runs in EL2 solves the problem.
> 
>>
>> As a solution, maybe BRBCR_EL12 should be added for driver according to the registers definition.
> 
> Right, will add the definition for BRBCR_EL12 in arch/arm64/tools/sysreg
> 
>>
>> Or, do you have a more standard solution?
> 
> Right, there are some nuances involved here.
> 
> Kernel could boot
> 	
> a. Directly into EL2 and stays in EL2 for good
> b. Directly into EL2 but switches into EL1
> c. Directly into EL1 without ever going into EL2
> 
> In all the above cases BRBCR_EL1.CC and BRBCR_EL2.CC needs to be set when cycle count
> is requested in the perf event interface (event->attr.branch_sample_type) via clearing
> PERF_SAMPLE_BRANCH_NO_CYCLES.
> 
> 
> - For the case as in (c) where kernel boots into EL1 directly and hence cannot ever set
>    EL2 register, BRBCR_EL2.CC would be a booting requirement - updated in booting.rst
> 
> - For the cases as in (a) and (b) kernel boots via EL2, hence there is an opportunity
>    to set both BRBCR_EL1.CC (via accessed BRBCR_EL12.CC) and BRBCR_EL2.CC. Depending on

You don't need to use BRBCR_EL12, if you do it early enough, before
HCR_EL2.E2H == 1 is applied.

Suzuki