[PATCH v1 05/13] perf/x86/amd: enable branch sampling priv level filtering

[Stephane Eranian]

The AMD Branch Sampling features does not provide hardware filtering
by privilege level. The associated PMU counter does but not the branch
sampling by itself. Given how BRS operates there is a possibility that
BRS captures kernel level branches even though the event is programmed to
count only at the user level. This patch implements a workaround in software
by removing the branches which belong to the wrong privilege level. The privilege
level is evaluated on the target of the branch and not the source so as to be
compatible with other architectures. As a consequence of this patch, the number
of entries in the PERF_RECORD_BRANCH_STACK buffer may be less than the maximum (16).
It could even be zero. Another consequence is that consecutive entries in the
branch stack may not reflect actual code path and may have discontinuities,
in case kernel branches were suppressed. But this is no different than what
happens on other architectures.

Signed-off-by: Stephane Eranian <eranian@xxxxxxxxxx>
---
 arch/x86/events/amd/brs.c | 26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

diff --git a/arch/x86/events/amd/brs.c b/arch/x86/events/amd/brs.c
index 86dbc6d06815..23b969001857 100644
--- a/arch/x86/events/amd/brs.c
+++ b/arch/x86/events/amd/brs.c
@@ -92,10 +92,6 @@ int amd_brs_setup_filter(struct perf_event *event)
 	if ((type & ~PERF_SAMPLE_BRANCH_PLM_ALL) != PERF_SAMPLE_BRANCH_ANY)
 		return -EINVAL;
 
-	/* can only capture at all priv levels due to the way BRS works */
-	if ((type & PERF_SAMPLE_BRANCH_PLM_ALL) != PERF_SAMPLE_BRANCH_PLM_ALL)
-		return -EINVAL;
-
 	return 0;
 }
 
@@ -181,6 +177,21 @@ void amd_brs_disable(void)
 	}
 }
 
+static bool amd_brs_match_plm(struct perf_event *event, u64 to)
+{
+	int type = event->attr.branch_sample_type;
+	int plm_k = PERF_SAMPLE_BRANCH_KERNEL | PERF_SAMPLE_BRANCH_HV;
+	int plm_u = PERF_SAMPLE_BRANCH_USER;
+
+	if (!(type & plm_k) && kernel_ip(to))
+		return 0;
+
+	if (!(type & plm_u) && !kernel_ip(to))
+		return 0;
+
+	return 1;
+}
+
 /*
  * Caller must ensure amd_brs_inuse() is true before calling
  * return:
@@ -237,8 +248,6 @@ void amd_brs_drain(void)
 		if (to == BRS_POISON)
 			break;
 
-		rdmsrl(brs_from(brs_idx), from);
-
 		/*
 		 * Sign-extend SAMP_BR_TO to 64 bits, bits 61-63 are reserved.
 		 * Necessary to generate proper virtual addresses suitable for
@@ -246,6 +255,11 @@ void amd_brs_drain(void)
 		 */
 		to = (u64)(((s64)to << shift) >> shift);
 
+		if (!amd_brs_match_plm(event, to))
+			continue;
+
+		rdmsrl(brs_from(brs_idx), from);
+
 		cpuc->lbr_entries[nr].from = from;
 		cpuc->lbr_entries[nr].to   = to;
 
-- 
2.33.0.153.gba50c8fa24-goog