perf: Fix capabilities bitfield compatibility in 'struct perf_event_mmap_page'

Solve the problems around the broken definition of perf_event_mmap_page::
cap_usr_time and cap_usr_rdpmc fields which used to overlap, partially
fixed by:

  860f085b74e9 ("perf: Fix broken union in 'struct perf_event_mmap_page'")

The problem with the fix (merged in v3.12-rc1 and not yet released
officially), noticed by Vince Weaver is that the new behavior is
not detectable by new user-space, and that due to the reuse of the
field names it's easy to mis-compile a binary if old headers are used
on a new kernel or new headers are used on an old kernel.

To solve all that make this change explicit, detectable and self-contained,
by iterating the ABI the following way:

 - Always clear bit 0, and rename it to usrpage->cap_bit0, to at least not
   confuse old user-space binaries. RDPMC will be marked as unavailable
   to old binaries but that's within the ABI, this is a capability bit.

 - Rename bit 1 to ->cap_bit0_is_deprecated and always set it to 1, so new
   libraries can reliably detect that bit 0 is deprecated and perma-zero
   without having to check the kernel version.

 - Use bits 2, 3, 4 for the newly defined, correct functionality:

	cap_user_rdpmc		: 1, /* The RDPMC instruction can be used to read counts */
	cap_user_time		: 1, /* The time_* fields are used */
	cap_user_time_zero	: 1, /* The time_zero field is used */

 - Rename all the bitfield names in perf_event.h to be different from the
   old names, to make sure it's not possible to mis-compile it
   accidentally with old assumptions.

The 'size' field can then be used in the future to add new fields and it
will act as a natural ABI version indicator as well.

Also adjust tools/perf/ userspace for the new definitions, noticed by
Adrian Hunter.

Reported-by: Vince Weaver <vincent.weaver@maine.edu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Also-Fixed-by: Adrian Hunter <adrian.hunter@intel.com>
Link: http://lkml.kernel.org/n/tip-zr03yxjrpXesOzzupszqglbv@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 3 insertions, 3 deletions

diff --git a/tools/perf/arch/x86/util/tsc.c b/tools/perf/arch/x86/util/tsc.c
index 9570c2b..b2519e4 100644
--- a/tools/perf/arch/x86/util/tsc.c
+++ b/tools/perf/arch/x86/util/tsc.c
@@ -32,7 +32,7 @@ u64 tsc_to_perf_time(u64 cyc, struct perf_tsc_conversion *tc)
 int perf_read_tsc_conversion(const struct perf_event_mmap_page *pc,
 			     struct perf_tsc_conversion *tc)
 {
-	bool cap_usr_time_zero;
+	bool cap_user_time_zero;
 	u32 seq;
 	int i = 0;
 
@@ -42,7 +42,7 @@ int perf_read_tsc_conversion(const struct perf_event_mmap_page *pc,
 		tc->time_mult = pc->time_mult;
 		tc->time_shift = pc->time_shift;
 		tc->time_zero = pc->time_zero;
-		cap_usr_time_zero = pc->cap_usr_time_zero;
+		cap_user_time_zero = pc->cap_user_time_zero;
 		rmb();
 		if (pc->lock == seq && !(seq & 1))
 			break;
@@ -52,7 +52,7 @@ int perf_read_tsc_conversion(const struct perf_event_mmap_page *pc,
 		}
 	}
 
-	if (!cap_usr_time_zero)
+	if (!cap_user_time_zero)
 		return -EOPNOTSUPP;
 
 	return 0;