Re: [Patch v2 4/9] perf/x86/intel: Fallback to sw branch type decoding if no hw decoding

[Peter Zijlstra]

On Tue, Jun 09, 2026 at 01:02:17PM +0800, Dapeng Mi wrote:
> intel_pmu_lbr_filter() currently assumes Arch LBR provides hardware
> branch-type decoding and skips software decoding on that path.
> 
> However, Arch LBR may not always expose branch-type information. In
> that case, treating entries as hardware-decoded can misclassify sampled
> branches (for example, defaulting to JCC), which breaks branch-type
> filtering results.
> 
> Fix this by using software branch-type decoding when hardware
> branch-type decoding is unavailable (that is, when x86_lbr_type is not
> enabled). This keeps branch classification and filtering behavior
> correct across Arch LBR configurations.
> 
> Signed-off-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/events/intel/lbr.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
> index 72f2adcda7c6..d4c0ed85e1fb 100644
> --- a/arch/x86/events/intel/lbr.c
> +++ b/arch/x86/events/intel/lbr.c
> @@ -1232,6 +1232,7 @@ intel_pmu_lbr_filter(struct cpu_hw_events *cpuc)
>  		 * OTHER_BRANCH branch type still rely on software decoding.
>  		 */
>  		if (static_cpu_has(X86_FEATURE_ARCH_LBR) &&
> +		    static_branch_likely(&x86_lbr_type) &&
>  		    type <= ARCH_LBR_BR_TYPE_KNOWN_MAX) {
>  			to_plm = kernel_ip(to) ? X86_BR_KERNEL : X86_BR_USER;
>  			type = arch_lbr_br_type_map[type] | to_plm;

Now you have two static branches in concert. Best to make sure
x86_lbr_type covers both conditions, no?