Re: [Patch v5 13/18] perf/x86/intel: Refresh LBR stack for non-PEBS LBR events if needed

[Mi, Dapeng]

On 10/30/2026 5:30 PM, Peter Zijlstra wrote:
> On Fri, Oct 30, 2026 at 09:54:02AM +0800, Mi, Dapeng wrote:
>
>>>> @@ -4012,8 +4023,14 @@ static int handle_pmi_common(struct pt_regs *regs, u64 status)
>>>>  
>>>>  		perf_sample_data_init(&data, 0, last_period);
>>>>  
>>>> -		if (has_branch_stack(event))
>>>> +		if (has_branch_stack(event)) {
>>>> +			if (lbr_need_update) {
>>>> +				intel_pmu_lbr_read(&cpuc->lbr_stack,
>>>> +					cpuc->lbr_entries, cpuc->lbr_counters);
>>>> +				lbr_need_update = false;
>>>> +			}
>>>>  			intel_pmu_lbr_save_brstack(&data, cpuc, event);
>>>> +		}
>>>>  
>>>>  		x86_pmu_clear_perf_regs(regs);
>>>>  		x86_pmu_update_perf_regs(event, &data, regs, false);
>>> Or... you move intel_pmu_lbr_read() from handle_irq() into pmi_common()
>>> after we done the PEBS things?
>> Yes, I ever tried to do that, but I found it would break the PEBS LBR
>> sampling on the platforms prior to ICE LAKE. The PEBS doesn't support
>> snapshot LBR stack on these old platforms, so PMI handler has to snapshot
>> the LBR stack before draining the PEBS buffer.
> Argh; we have so many variations :-(
>
> Still, re-reading those MSRs seems wasteful, esp. if most modern chips
> will be affected.
>
> Can't we have a toggle between early and late LBR read or something?

Hmm, yes. I would refactor the code and eliminate the duplicated LBR
reading. Thanks.