6.11-stable review patch. If anyone has any objections, please let me know.
------------------
From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
commit ef493f4b122d6b14a6de111d1acac1eab1d673b0 upstream.
The BPF subsystem may capture LBR data on a counting event. However, the
current implementation assumes that LBR can/should only be used with
sampling events.
For instance, retsnoop tool ([0]) makes an extensive use of this
functionality and sets up perf event as follows:
struct perf_event_attr attr;
memset(&attr, 0, sizeof(attr));
attr.size = sizeof(attr);
attr.type = PERF_TYPE_HARDWARE;
attr.config = PERF_COUNT_HW_CPU_CYCLES;
attr.sample_type = PERF_SAMPLE_BRANCH_STACK;
attr.branch_sample_type = PERF_SAMPLE_BRANCH_KERNEL;
To limit the LBR for a sampling event is to avoid unnecessary branch
stack setup for a counting event in the sample read. Because LBR is only
read in the sampling event's overflow.
Although in most cases LBR is used in sampling, there is no HW limit to
bind LBR to the sampling mode. Allow an LBR setup for a counting event
unless in the sample read mode.
Fixes: 85846b27072d ("perf/x86: Add PERF_X86_EVENT_NEEDS_BRANCH_STACK flag")
Closes: https://lore.kernel.org/lkml/20240905180055.1221620-1-andrii@xxxxxxxxxx/
Reported-by: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
Signed-off-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
Tested-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/20240909155848.326640-1-kan.liang@xxxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/x86/events/intel/core.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -3972,8 +3972,12 @@ static int intel_pmu_hw_config(struct pe
x86_pmu.pebs_aliases(event);
}
- if (needs_branch_stack(event) && is_sampling_event(event))
- event->hw.flags |= PERF_X86_EVENT_NEEDS_BRANCH_STACK;
+ if (needs_branch_stack(event)) {
+ /* Avoid branch stack setup for counting events in SAMPLE READ */
+ if (is_sampling_event(event) ||
+ !(event->attr.sample_type & PERF_SAMPLE_READ))
+ event->hw.flags |= PERF_X86_EVENT_NEEDS_BRANCH_STACK;
+ }
if (branch_sample_counters(event)) {
struct perf_event *leader, *sibling;
