Re: [PATCH V3 13/23] perf/x86/intel/lbr: Factor out intel_pmu_store_lbr

[Peter Zijlstra]

On Fri, Jul 03, 2020 at 04:59:49PM -0400, Liang, Kan wrote:
> On 7/3/2020 3:50 PM, Peter Zijlstra wrote:

> > If I'm not mistaken, this correctly deals with LBR_FORMAT_INFO, so can't
> > we also use the intel_pmu_arch_lbr_read() function for that case?
> 
> But the intel_pmu_arch_lbr_read() doesn't have the optimization
> (LBR_NO_INFO) for the LBR_FORMAT_INFO.
> https://lkml.kernel.org/r/tip-b16a5b52eb90d92b597257778e51e1fdc6423e64@xxxxxxxxxxxxxx
> 
> To apply the optimization, we need extra codes as below.

Right, I saw that, but shouldn't we support that for anything with this
format anyway? That is, it's weird and inconsistent to not support
PERF_SAMPLE_BRANCH_NO_{CYCLES,FLAGS} for PEBS/ARCH-LBR output.

Arguably, we should even support NO_CYCLES for FORMAT_TIME. Yes it's
daft, but that's what you get for adding the ABI.