perf auxtrace: Add compat_auxtrace_mmap__{read_head|write_tail}

When perf runs in compat mode (kernel in 64-bit mode and the perf is in
32-bit mode), the 64-bit value atomicity in the user space cannot be
assured, E.g. on some architectures, the 64-bit value accessing is split
into two instructions, one is for the low 32-bit word accessing and
another is for the high 32-bit word.

This patch introduces weak functions compat_auxtrace_mmap__read_head()
and compat_auxtrace_mmap__write_tail(), as their naming indicates, when
perf tool works in compat mode, it uses these two functions to access
the AUX head and tail.  These two functions can allow the perf tool to
work properly in certain conditions, e.g. when perf tool works in
snapshot mode with only using AUX head pointer, or perf tool uses the
AUX buffer and the incremented tail is not bigger than 4GB.

When perf tool cannot handle the case when the AUX tail is bigger than
4GB, the function compat_auxtrace_mmap__write_tail() returns -1 and
tells the caller to bail out for the error.

These two functions are declared as weak attribute, this allows to
implement arch specific functions if any arch can support the 64-bit
value atomicity in compat mode.

Suggested-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: John Garry <john.garry@huawei.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Mike Leach <mike.leach@linaro.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Russell King (oracle)" <linux@armlinux.org.uk>
Cc: Suzuki Poulouse <suzuki.poulose@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: coresight@lists.linaro.org
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lore.kernel.org/lkml/20210829102238.19693-2-leo.yan@linaro.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

1 files changed, 19 insertions, 3 deletions

diff --git a/tools/perf/util/auxtrace.h b/tools/perf/util/auxtrace.h
index d68a5e80b217..5f383908ca6e 100644
--- a/tools/perf/util/auxtrace.h
+++ b/tools/perf/util/auxtrace.h
@@ -440,23 +440,39 @@ struct auxtrace_cache;
 
 #ifdef HAVE_AUXTRACE_SUPPORT
 
-static inline u64 auxtrace_mmap__read_head(struct auxtrace_mmap *mm)
+u64 compat_auxtrace_mmap__read_head(struct auxtrace_mmap *mm);
+int compat_auxtrace_mmap__write_tail(struct auxtrace_mmap *mm, u64 tail);
+
+static inline u64 auxtrace_mmap__read_head(struct auxtrace_mmap *mm,
+					   int kernel_is_64_bit __maybe_unused)
 {
 	struct perf_event_mmap_page *pc = mm->userpg;
-	u64 head = READ_ONCE(pc->aux_head);
+	u64 head;
+
+#if BITS_PER_LONG == 32
+	if (kernel_is_64_bit)
+		return compat_auxtrace_mmap__read_head(mm);
+#endif
+	head = READ_ONCE(pc->aux_head);
 
 	/* Ensure all reads are done after we read the head */
 	smp_rmb();
 	return head;
 }
 
-static inline void auxtrace_mmap__write_tail(struct auxtrace_mmap *mm, u64 tail)
+static inline int auxtrace_mmap__write_tail(struct auxtrace_mmap *mm, u64 tail,
+					    int kernel_is_64_bit __maybe_unused)
 {
 	struct perf_event_mmap_page *pc = mm->userpg;
 
+#if BITS_PER_LONG == 32
+	if (kernel_is_64_bit)
+		return compat_auxtrace_mmap__write_tail(mm, tail);
+#endif
 	/* Ensure all reads are done before we write the tail out */
 	smp_mb();
 	WRITE_ONCE(pc->aux_tail, tail);
+	return 0;
 }
 
 int auxtrace_mmap__mmap(struct auxtrace_mmap *mm,