[PATCH 2/4] perf, tool: Carry perf_event_attr bitfield throught different endians

[Jiri Olsa]

When the perf data file is read cross architectures, the perf_event__attr_swap
function takes care about endianness of all the struct fields except the
bitfield flags.

The bitfield flags need to be transformed as well, since the bitfield
binary storage differs for both endians.

ABI says:
  Bit-fields are allocated from right to left (least to most significant)
  on little-endian implementations and from left to right (most to least
  significant) on big-endian implementations.

The above seems to be byte specific, so we need to reverse each
byte of the bitfield. 'Internet' also says this might be implementation
specific and we probably need proper fix and carry perf_event_attr
bitfield flags in separate data file FEAT_ section. Thought this seems
to work for now.

Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
---
 tools/perf/util/session.c |   34 ++++++++++++++++++++++++++++++++++
 1 files changed, 34 insertions(+), 0 deletions(-)

diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
index 1efd3be..07fda7c 100644
--- a/tools/perf/util/session.c
+++ b/tools/perf/util/session.c
@@ -481,6 +481,38 @@ static void perf_event__read_swap(union perf_event *event)
 	event->read.id		 = bswap_64(event->read.id);
 }
 
+static u8 revbyte(u8 b)
+{
+	int rev = (b >> 4) | ((b & 0xf) << 4);
+	rev = ((rev & 0xcc) >> 2) | ((rev & 0x33) << 2);
+	rev = ((rev & 0xaa) >> 1) | ((rev & 0x55) << 1);
+	return (u8) rev;
+}
+
+/*
+ * XXX this is hack in attempt to carry flags bitfield
+ * throught endian village. ABI says:
+ *
+ * Bit-fields are allocated from right to left (least to most significant)
+ * on little-endian implementations and from left to right (most to least
+ * significant) on big-endian implementations.
+ *
+ * The above seems to be byte specific, so we need to reverse each
+ * byte of the bitfield. 'Internet' also says this might be implementation
+ * specific and we probably need proper fix and carry perf_event_attr
+ * bitfield flags in separate data file FEAT_ section. Thought this seems
+ * to work for now.
+ */
+static void swap_bitfield(u8 *p, unsigned len)
+{
+	unsigned i;
+
+	for (i = 0; i < len; i++) {
+		*p = revbyte(*p);
+		p++;
+	}
+}
+
 /* exported for swapping attributes in file header */
 void perf_event__attr_swap(struct perf_event_attr *attr)
 {
@@ -494,6 +526,8 @@ void perf_event__attr_swap(struct perf_event_attr *attr)
 	attr->bp_type		= bswap_32(attr->bp_type);
 	attr->bp_addr		= bswap_64(attr->bp_addr);
 	attr->bp_len		= bswap_64(attr->bp_len);
+
+	swap_bitfield((u8 *) (&attr->read_format + 1), sizeof(u64));
 }
 
 static void perf_event__hdr_attr_swap(union perf_event *event)
-- 
1.7.7.6

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