[PATCH v4] kasan: speed up mte_set_mem_tag_range

[Evgenii Stepanov]

Use DC GVA / DC GZVA to speed up KASan memory tagging in HW tags mode.

The first cacheline is always tagged using STG/STZG even if the address is
cacheline-aligned, as benchmarks show it is faster than a conditional
branch.

Signed-off-by: Evgenii Stepanov <eugenis@xxxxxxxxxx>
Co-developed-by: Peter Collingbourne <pcc@xxxxxxxxxx>
Signed-off-by: Peter Collingbourne <pcc@xxxxxxxxxx>
---
Changelog since v1:
- Added Co-developed-by.

Changelog since v2:
- Added Signed-off-by.

Changelog since v3:
- Move the implementation back to C with a bit of inline asm.

 arch/arm64/include/asm/mte-kasan.h | 98 +++++++++++++++++++++---------
 1 file changed, 70 insertions(+), 28 deletions(-)

diff --git a/arch/arm64/include/asm/mte-kasan.h b/arch/arm64/include/asm/mte-kasan.h
index ddd4d17cf9a0..34e23886f346 100644
--- a/arch/arm64/include/asm/mte-kasan.h
+++ b/arch/arm64/include/asm/mte-kasan.h
@@ -48,43 +48,85 @@ static inline u8 mte_get_random_tag(void)
 	return mte_get_ptr_tag(addr);
 }
 
+static inline u64 __stg_post(u64 p)
+{
+	asm volatile(__MTE_PREAMBLE "stg %0, [%0], #16"
+		     : "+r"(p)
+		     :
+		     : "memory");
+	return p;
+}
+
+static inline u64 __stzg_post(u64 p)
+{
+	asm volatile(__MTE_PREAMBLE "stzg %0, [%0], #16"
+		     : "+r"(p)
+		     :
+		     : "memory");
+	return p;
+}
+
+static inline void __dc_gva(u64 p)
+{
+	asm volatile(__MTE_PREAMBLE "dc gva, %0" : : "r"(p) : "memory");
+}
+
+static inline void __dc_gzva(u64 p)
+{
+	asm volatile(__MTE_PREAMBLE "dc gzva, %0" : : "r"(p) : "memory");
+}
+
 /*
  * Assign allocation tags for a region of memory based on the pointer tag.
  * Note: The address must be non-NULL and MTE_GRANULE_SIZE aligned and
- * size must be non-zero and MTE_GRANULE_SIZE aligned.
+ * size must be MTE_GRANULE_SIZE aligned.
  */
-static inline void mte_set_mem_tag_range(void *addr, size_t size,
-						u8 tag, bool init)
+static inline void mte_set_mem_tag_range(void *addr, size_t size, u8 tag,
+					 bool init)
 {
-	u64 curr, end;
+	u64 curr, DCZID, mask, line_size, end1, end2, end3;
 
-	if (!size)
-		return;
+	/* Read DC G(Z)VA store size from the register. */
+	__asm__ __volatile__(__MTE_PREAMBLE "mrs %0, dczid_el0"
+			     : "=r"(DCZID)::);
+	line_size = 4ul << (DCZID & 0xf);
 
 	curr = (u64)__tag_set(addr, tag);
-	end = curr + size;
-
-	/*
-	 * 'asm volatile' is required to prevent the compiler to move
-	 * the statement outside of the loop.
+	mask = line_size - 1;
+	/* STG/STZG up to the end of the first cache line. */
+	end1 = curr | mask;
+	end3 = curr + size;
+	/* DC GVA / GZVA in [end1, end2) */
+	end2 = end3 & ~mask;
+
+	/* The following code uses STG on the first cache line even if the start
+	 * address is cache line aligned - it appears to be faster than an
+	 * alignment check + conditional branch. Also, if the size is at least 2
+	 * cache lines, the first two loops can use post-condition to save one
+	 * branch each.
 	 */
-	if (init) {
-		do {
-			asm volatile(__MTE_PREAMBLE "stzg %0, [%0]"
-				     :
-				     : "r" (curr)
-				     : "memory");
-			curr += MTE_GRANULE_SIZE;
-		} while (curr != end);
-	} else {
-		do {
-			asm volatile(__MTE_PREAMBLE "stg %0, [%0]"
-				     :
-				     : "r" (curr)
-				     : "memory");
-			curr += MTE_GRANULE_SIZE;
-		} while (curr != end);
-	}
+#define SET_MEMTAG_RANGE(stg_post, dc_gva)		\
+	do {						\
+		if (size >= 2 * line_size) {		\
+			do {				\
+				curr = stg_post(curr);	\
+			} while (curr < end1);		\
+							\
+			do {				\
+				dc_gva(curr);		\
+				curr += line_size;	\
+			} while (curr < end2);		\
+		}					\
+							\
+		while (curr < end3)			\
+			curr = stg_post(curr);		\
+	} while (0)
+
+	if (init)
+		SET_MEMTAG_RANGE(__stzg_post, __dc_gzva);
+	else
+		SET_MEMTAG_RANGE(__stg_post, __dc_gva);
+#undef SET_MEMTAG_RANGE
 }
 
 void mte_enable_kernel_sync(void);
-- 
2.31.1.751.gd2f1c929bd-goog