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Date:   Wed, 28 Sep 2022 11:40:43 -0700
In-Reply-To: <20220928184043.408364-1-eranian@google.com>
Message-Id: <20220928184043.408364-3-eranian@google.com>
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References: <20220928184043.408364-1-eranian@google.com>
Subject: [PATCH 2/2] perf/x86/amd/lbr: adjust LBR regardless of filtering
From:   Stephane Eranian <eranian@google.com>
To:     linux-kernel@vger.kernel.org
Cc:     peterz@infradead.org, sandipan.das@amd.com, ananth.narayan@amd.com,
        ravi.bangoria@amd.com
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Precedence: bulk

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.

Signed-off-by: Stephane Eranian <eranian@google.com>
---
 arch/x86/events/amd/lbr.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/events/amd/lbr.c b/arch/x86/events/amd/lbr.c
index 2e1c1573efe7..38a75216c12c 100644
--- 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) {
-- 
2.37.3.998.g577e59143f-goog