On Tue, Jun 16, 2026 at 8:57 AM Puranjay Mohan <[email protected]> wrote:
>
> perf_clear_branch_entry_bitfields() resets the bitfields of struct
> perf_branch_entry one at a time and deliberately leaves from/to alone,
> since callers overwrite those immediately. The list of assignments has to
> be kept in sync with the struct by hand, and it has already drifted:
> new_type and priv were added to perf_branch_entry but never cleared here,
> so stale values can leak into the records handed to userspace.
>
> Clear the entry with a single struct assignment instead:
>
> *br = (struct perf_branch_entry){ };
>
> Every caller writes from/to right after the clear, so zeroing them as well
> is harmless and the dead stores are elided on the x86 LBR read paths.
> There is no longer anything to keep in sync when a field is added.
>
> The helper no longer touches only the bitfields, so rename it to
> perf_clear_branch_entry() and update the callers, fixing up the
> br+nr/br+out spacing on the touched lines while at it.
>
> Fixes: b190bc4ac9e6 ("perf: Extend branch type classification")
> Fixes: 5402d25aa571 ("perf: Capture branch privilege information")
> Suggested-by: James Clark <[email protected]>
> Signed-off-by: Puranjay Mohan <[email protected]>
> ---
> arch/x86/events/amd/brs.c | 2 +-
> arch/x86/events/amd/lbr.c | 2 +-
> arch/x86/events/intel/lbr.c | 6 +++---
> drivers/perf/arm_brbe.c | 2 +-
> include/linux/perf_event.h | 16 ++--------------
> 5 files changed, 8 insertions(+), 20 deletions(-)
>
Another one for tip/perf probably?... Puranjay, maybe send the first
two patches separately?
> diff --git a/arch/x86/events/amd/brs.c b/arch/x86/events/amd/brs.c
> index 06f35a6b58a5..68c5f42965e9 100644
> --- a/arch/x86/events/amd/brs.c
> +++ b/arch/x86/events/amd/brs.c
> @@ -343,7 +343,7 @@ void amd_brs_drain(void)
>
> rdmsrq(brs_from(brs_idx), from);
>
> - perf_clear_branch_entry_bitfields(br+nr);
> + perf_clear_branch_entry(br + nr);
>
> br[nr].from = from;
> br[nr].to = to;
> diff --git a/arch/x86/events/amd/lbr.c b/arch/x86/events/amd/lbr.c
> index d24da377df77..08401fd60585 100644
> --- a/arch/x86/events/amd/lbr.c
> +++ b/arch/x86/events/amd/lbr.c
> @@ -181,7 +181,7 @@ void amd_pmu_lbr_read(void)
> entry.to.split.reserved)
> continue;
>
> - perf_clear_branch_entry_bitfields(br + out);
> + perf_clear_branch_entry(br + out);
>
> br[out].from = sign_ext_branch_ip(entry.from.split.ip);
> br[out].to = sign_ext_branch_ip(entry.to.split.ip);
> diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
> index 72f2adcda7c6..295da179fa74 100644
> --- a/arch/x86/events/intel/lbr.c
> +++ b/arch/x86/events/intel/lbr.c
> @@ -755,7 +755,7 @@ void intel_pmu_lbr_read_32(struct cpu_hw_events *cpuc)
>
> rdmsrq(x86_pmu.lbr_from + lbr_idx, msr_lastbranch.lbr);
>
> - perf_clear_branch_entry_bitfields(br);
> + perf_clear_branch_entry(br);
>
> br->from = msr_lastbranch.from;
> br->to = msr_lastbranch.to;
> @@ -846,7 +846,7 @@ void intel_pmu_lbr_read_64(struct cpu_hw_events *cpuc)
> if (abort && x86_pmu.lbr_double_abort && out > 0)
> out--;
>
> - perf_clear_branch_entry_bitfields(br+out);
> + perf_clear_branch_entry(br + out);
> br[out].from = from;
> br[out].to = to;
> br[out].mispred = mis;
> @@ -920,7 +920,7 @@ static void intel_pmu_store_lbr(struct cpu_hw_events
> *cpuc,
> to = rdlbr_to(i, lbr);
> info = rdlbr_info(i, lbr);
>
> - perf_clear_branch_entry_bitfields(e);
> + perf_clear_branch_entry(e);
>
> e->from = from;
> e->to = to;
> diff --git a/drivers/perf/arm_brbe.c b/drivers/perf/arm_brbe.c
> index ba554e0c846c..effbdeacfcbb 100644
> --- a/drivers/perf/arm_brbe.c
> +++ b/drivers/perf/arm_brbe.c
> @@ -604,7 +604,7 @@ static bool perf_entry_from_brbe_regset(int index, struct
> perf_branch_entry *ent
> return false;
>
> brbinf = bregs.brbinf;
> - perf_clear_branch_entry_bitfields(entry);
> + perf_clear_branch_entry(entry);
> if (brbe_record_is_complete(brbinf)) {
> entry->from = bregs.brbsrc;
> entry->to = bregs.brbtgt;
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 48d851fbd8ea..e034be4a473a 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -1467,21 +1467,9 @@ static inline u32 perf_sample_data_size(struct
> perf_sample_data *data,
> return size;
> }
>
> -/*
> - * Clear all bitfields in the perf_branch_entry.
> - * The to and from fields are not cleared because they are
> - * systematically modified by caller.
> - */
> -static inline void perf_clear_branch_entry_bitfields(struct
> perf_branch_entry *br)
> +static inline void perf_clear_branch_entry(struct perf_branch_entry *br)
> {
> - br->mispred = 0;
> - br->predicted = 0;
> - br->in_tx = 0;
> - br->abort = 0;
> - br->cycles = 0;
> - br->type = 0;
> - br->spec = PERF_BR_SPEC_NA;
> - br->reserved = 0;
> + *br = (struct perf_branch_entry){ };
> }
>
> extern void perf_output_sample(struct perf_output_handle *handle,
> --
> 2.53.0-Meta
>
