perf/x86/amd/lbr: Adjust LBR regardless of filtering

Stephane Eranian · Peter Zijlstra · commit 3f9a1b359100 · 2022-09-29T12:20:57.000+02:00
In case of fused compare and taken branch instructions, the AMD LBR points to the compare instruction instead of the branch. Users of LBR usually expects the from address to point to a branch instruction. The kernel has code to adjust the from address via get_branch_type_fused(). However this correction is only applied when a branch filter is applied. That means that if no filter is present, the quality of the data is lower. Fix the problem by applying the adjustment regardless of the filter setting, bringing the AMD LBR to the same level as other LBR implementations. Fixes: 245268c ("perf/x86/amd/lbr: Use fusion-aware branch classifier") Signed-off-by: Stephane Eranian <eranian@google.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Sandipan Das <sandipan.das@amd.com> Link: https://lore.kernel.org/r/20220928184043.408364-3-eranian@google.com
diff --git a/arch/x86/events/amd/lbr.c b/arch/x86/events/amd/lbr.c
@@ -99,12 +99,13 @@ static void amd_pmu_lbr_filter(void)
 	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
 	int br_sel = cpuc->br_sel, offset, type, i, j;
 	bool compress = false;
+	bool fused_only = false;
 	u64 from, to;
 
 	/* If sampling all branches, there is nothing to filter */
 	if (((br_sel & X86_BR_ALL) == X86_BR_ALL) &&
 	    ((br_sel & X86_BR_TYPE_SAVE) != X86_BR_TYPE_SAVE))
-		return;
+		fused_only = true;
 
 	for (i = 0; i < cpuc->lbr_stack.nr; i++) {
 		from = cpuc->lbr_entries[i].from;
@@ -116,8 +117,11 @@ static void amd_pmu_lbr_filter(void)
 		 * fusion where it points to an instruction preceding the
 		 * actual branch
 		 */
-		if (offset)
+		if (offset) {
 			cpuc->lbr_entries[i].from += offset;
+			if (fused_only)
+				continue;
+		}
 
 		/* If type does not correspond, then discard */
 		if (type == X86_BR_NONE || (br_sel & type) != type) {
