Re: [PATCH V2 08/23] perf/x86: Expose CPUID enumeration bits for arch LBR

[Peter Zijlstra]

On Tue, Jun 30, 2020 at 11:36:29AM -0400, Liang, Kan wrote:
> > Why keep the full CPUID leaf here? What's wrong with something like:
> > 
> > 	unsigned int	lbr_depth_mask:8;
> > 	unsigned int	lbr_deep_c_reset:1;
> > 	unsigned int	lbr_lip:1;
> > 	unsigned int	lbr_cpl:1;
> > 	unsigned int	lbr_filter:1;
> > 	unsigned int	lbr_call_stack:1;
> > 	unsigned int	lbr_mispred:1;
> > 	unsigned int	lbr_timed_lbr:1;
> > 	unsigned int	lbr_br_type:1;
> > 
> > That's only 2 bytes, instead of 24.
> 
> Yes, it can save the space.
> I will add some codes in patch 14 to initialize these items in
> intel_pmu_arch_lbr_init().

It also makes stuff like:

> +   if (!x86_pmu.lbr_ebx.split.lbr_cpl ||
> +       !x86_pmu.lbr_ebx.split.lbr_filter ||
> +       !x86_pmu.lbr_ebx.split.lbr_call_stack)

eminently more readable.