[PATCH 3/5] btrace, linux: use data_size and data_offset

[Markus Metzger <markus dot t dot metzger at intel dot com>]

In struct perf_event_mmap_page there are new fields data_size and data_offset
that give the location of the perf_event data buffer relative to the mmap
page.  Use them if they are present.

2015-06-23  Markus Metzger  <markus.t.metzger@intel.com>

	nat/linux-btrace.c (linux_enable_bts): Check for PERF_ATTR_SIZE_VER5.
	Check for data_offset and data_size fields.  Use them.
---
 gdb/nat/linux-btrace.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/gdb/nat/linux-btrace.c b/gdb/nat/linux-btrace.c
index 40915da..2ed167a 100644
--- a/gdb/nat/linux-btrace.c
+++ b/gdb/nat/linux-btrace.c
@@ -644,7 +644,7 @@ linux_enable_bts (ptid_t ptid, const struct btrace_config_bts *conf)
   struct perf_event_mmap_page *header;
   struct btrace_target_info *tinfo;
   struct btrace_tinfo_bts *bts;
-  unsigned long long size, pages;
+  unsigned long long size, pages, data_offset, data_size;
   int pid, pg;
 
   tinfo = xzalloc (sizeof (*tinfo));
@@ -706,16 +706,27 @@ linux_enable_bts (ptid_t ptid, const struct btrace_config_bts *conf)
 	break;
     }
 
-  if (header == MAP_FAILED)
+  if (pages == 0)
     goto err_file;
 
+  data_offset = PAGE_SIZE;
+  data_size = size;
+
+#if defined(PERF_ATTR_SIZE_VER5)
+  if (offsetof(struct perf_event_mmap_page, data_size) <= header->size)
+    {
+      data_offset = header->data_offset;
+      data_size = header->data_size;
+    }
+#endif /* defined(PERF_ATTR_SIZE_VER5) */
+
   bts->header = header;
-  bts->bts.mem = ((const uint8_t *) header) + PAGE_SIZE;
-  bts->bts.size = size;
+  bts->bts.mem = ((const uint8_t *) header) + data_offset;
+  bts->bts.size = data_size;
   bts->bts.data_head = &header->data_head;
   bts->bts.last_head = 0;
 
-  tinfo->conf.bts.size = size;
+  tinfo->conf.bts.size = data_size;
   return tinfo;
 
  err_file:
-- 
1.8.3.1