[PATCH v2 2/2] perf auxtrace arm: Support compat_auxtrace_mmap__{read_head|write_tail}

[Leo Yan]

When the tool runs with compat mode on Arm platform, the kernel is in
64-bit mode and user space is in 32-bit mode; the user space can use
instructions "ldrd" and "strd" for 64-bit value atomicity.

This patch adds compat_auxtrace_mmap__{read_head|write_tail} for arm
building, it uses "ldrd" and "strd" instructions to ensure accessing
atomicity for aux head and tail.  The file arch/arm/util/auxtrace.c is
built for arm and arm64 building, these two functions are not needed for
arm64, so check the compiler macro "__arm__" to only include them for
arm building.

Signed-off-by: Leo Yan <leo.yan@xxxxxxxxxx>
Reviewed-by: James Clark <james.clark@xxxxxxx>
Tested-by: James Clark <james.clark@xxxxxxx>
---
 tools/perf/arch/arm/util/auxtrace.c | 32 +++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/tools/perf/arch/arm/util/auxtrace.c b/tools/perf/arch/arm/util/auxtrace.c
index b187bddbd01a..c7c7ec0812d5 100644
--- a/tools/perf/arch/arm/util/auxtrace.c
+++ b/tools/perf/arch/arm/util/auxtrace.c
@@ -107,3 +107,35 @@ struct auxtrace_record
 	*err = 0;
 	return NULL;
 }
+
+#if defined(__arm__)
+u64 compat_auxtrace_mmap__read_head(struct auxtrace_mmap *mm)
+{
+	struct perf_event_mmap_page *pc = mm->userpg;
+	u64 result;
+
+	__asm__ __volatile__(
+"	ldrd    %0, %H0, [%1]"
+	: "=&r" (result)
+	: "r" (&pc->aux_head), "Qo" (pc->aux_head)
+	);
+
+	return result;
+}
+
+int compat_auxtrace_mmap__write_tail(struct auxtrace_mmap *mm, u64 tail)
+{
+	struct perf_event_mmap_page *pc = mm->userpg;
+
+	/* Ensure all reads are done before we write the tail out */
+	smp_mb();
+
+	__asm__ __volatile__(
+"	strd    %2, %H2, [%1]"
+	: "=Qo" (pc->aux_tail)
+	: "r" (&pc->aux_tail), "r" (tail)
+	);
+
+	return 0;
+}
+#endif
-- 
2.25.1