[PATCH RFC v3 3/6] perf/x86: Annotate a BTS error with extended error reporting

[Alexander Shishkin <alexander.shishkin@linux.intel.com>]

Old-style BTS driver does not support kernel tracing, which normally
would be visible to userspace as -EOPNOTSUPP, or from the tool's user's
perspective:

> # perf record -e branches -c1 ls
> Error:
> No hardware sampling interrupt available.
> No APIC? If so then you can boot the kernel with the "lapic" boot parameter to force-enable it.

but with the new way of reporting errors, they will see the following:

> # perf record -e branches -c1 ls
> Error:
> Syscall returned -95, becasue BTS sampling not allowed for kernel space.
> Offending attribute field: "exclude_kernel"

which is somewhat more to the point of what has actually transpired.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
---
 arch/x86/kernel/cpu/perf_event.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index f56cf074d0..e3d953ef49 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -12,6 +12,8 @@
  *  For licencing details see kernel-base/COPYING
  */
 
+#define EXTERR_MODNAME "perf/x86"
+
 #include <linux/perf_event.h>
 #include <linux/capability.h>
 #include <linux/notifier.h>
@@ -426,7 +428,8 @@ int x86_setup_perfctr(struct perf_event *event)
 
 		/* BTS is currently only allowed for user-mode. */
 		if (!attr->exclude_kernel)
-			return -EOPNOTSUPP;
+			return perf_err(-EOPNOTSUPP, exclude_kernel,
+					"BTS sampling not allowed for kernel space");
 
 		/* disallow bts if conflicting events are present */
 		if (x86_add_exclusive(x86_lbr_exclusive_lbr))
-- 
2.5.1