[Patch v5 09/18] perf/x86/intel: Keep LBR frozen throughout PMI handling

[Dapeng Mi]

For the platforms which has 4+ PMU version, streamline LBR freeze is
supported. The PERF_GLOBAL_STATUS.LBR_FRZ bit is set to freeze LBR once
a PMU counter overflow. However, the current PMI handler clears the LBR
freeze bit too early, allowing branch operations that occur during PMI
handling to be recorded into the LBR stack.

This can pollute or overwrite the intended branch snapshot, especially
when bpf_get_branch_snapshot() is invoked from bpf_overflow_handler().
In that case, branches recorded while processing the PMI are mixed into
the original LBR data.

Defer unfreezing until after PMI handling completes so the LBR remains
frozen for the entire PMI path and no branch activity from the handler
is recorded.

Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
Closes: https://lore.kernel.org/all/sashiko-outbox-179823@xxxxxxxxxx/
Fixes: 425507fa5f32 ("perf/x86/intel/lbr: Optimize v4 LBR unfreezing")
Signed-off-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>
---
 arch/x86/events/intel/core.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index 15dcf8b78de9..a67297148bb6 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -4062,7 +4062,13 @@ static int intel_pmu_handle_irq(struct pt_regs *regs)
 	loops = 0;
 again:
 	intel_pmu_lbr_read();
-	intel_pmu_ack_status(status);
+
+	/*
+	 * Do not clear the LBR frozen status here; keep LBR frozen
+	 * throughout PMI handling.
+	 */
+	intel_pmu_ack_status(status & ~GLOBAL_STATUS_LBRS_FROZEN);
+
 	if (++loops > 100) {
 		static bool warned;
 
@@ -4081,10 +4087,13 @@ static int intel_pmu_handle_irq(struct pt_regs *regs)
 	 * Repeat if there is more work to be done:
 	 */
 	status = intel_pmu_get_status();
-	if (status)
+	if (status & ~GLOBAL_STATUS_LBRS_FROZEN)
 		goto again;
 
 done:
+	if (status & GLOBAL_STATUS_LBRS_FROZEN)
+		intel_pmu_ack_status(status & GLOBAL_STATUS_LBRS_FROZEN);
+
 	if (mid_ack)
 		apic_write(APIC_LVTPC, APIC_DM_NMI);
 	/* Only restore PMU state when it's active. See x86_pmu_disable(). */
-- 
2.34.1