[PATCH 4/5] arm64: vdso: Avoid ISB after reading from cntvct_el0

[Will Deacon]

We can avoid the expensive ISB instruction after reading the counter in
the vDSO gettime functions by creating a fake address hazard against a
dummy stack read, just like we do inside the kernel.

Signed-off-by: Will Deacon <will at kernel.org>
---
 arch/arm64/include/asm/arch_timer.h        | 21 ---------------------
 arch/arm64/include/asm/barrier.h           | 19 +++++++++++++++++++
 arch/arm64/include/asm/vdso/gettimeofday.h |  6 +-----
 3 files changed, 20 insertions(+), 26 deletions(-)

diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h
index 9f0ec21d6327..88d20f04c64a 100644
--- a/arch/arm64/include/asm/arch_timer.h
+++ b/arch/arm64/include/asm/arch_timer.h
@@ -165,25 +165,6 @@ static inline void arch_timer_set_cntkctl(u32 cntkctl)
 	isb();
 }
 
-/*
- * Ensure that reads of the counter are treated the same as memory reads
- * for the purposes of ordering by subsequent memory barriers.
- *
- * This insanity brought to you by speculative system register reads,
- * out-of-order memory accesses, sequence locks and Thomas Gleixner.
- *
- * http://lists.infradead.org/pipermail/linux-arm-kernel/2019-February/631195.html
- */
-#define arch_counter_enforce_ordering(val) do {				\
-	u64 tmp, _val = (val);						\
-									\
-	asm volatile(							\
-	"	eor	%0, %1, %1\n"					\
-	"	add	%0, sp, %0\n"					\
-	"	ldr	xzr, [%0]"					\
-	: "=r" (tmp) : "r" (_val));					\
-} while (0)
-
 static __always_inline u64 __arch_counter_get_cntpct_stable(void)
 {
 	u64 cnt;
@@ -224,8 +205,6 @@ static __always_inline u64 __arch_counter_get_cntvct(void)
 	return cnt;
 }
 
-#undef arch_counter_enforce_ordering
-
 static inline int arch_timer_arch_init(void)
 {
 	return 0;
diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
index c3009b0e5239..37d891af8ea5 100644
--- a/arch/arm64/include/asm/barrier.h
+++ b/arch/arm64/include/asm/barrier.h
@@ -70,6 +70,25 @@ static inline unsigned long array_index_mask_nospec(unsigned long idx,
 	return mask;
 }
 
+/*
+ * Ensure that reads of the counter are treated the same as memory reads
+ * for the purposes of ordering by subsequent memory barriers.
+ *
+ * This insanity brought to you by speculative system register reads,
+ * out-of-order memory accesses, sequence locks and Thomas Gleixner.
+ *
+ * http://lists.infradead.org/pipermail/linux-arm-kernel/2019-February/631195.html
+ */
+#define arch_counter_enforce_ordering(val) do {				\
+	u64 tmp, _val = (val);						\
+									\
+	asm volatile(							\
+	"	eor	%0, %1, %1\n"					\
+	"	add	%0, sp, %0\n"					\
+	"	ldr	xzr, [%0]"					\
+	: "=r" (tmp) : "r" (_val));					\
+} while (0)
+
 #define __smp_mb()	dmb(ish)
 #define __smp_rmb()	dmb(ishld)
 #define __smp_wmb()	dmb(ishst)
diff --git a/arch/arm64/include/asm/vdso/gettimeofday.h b/arch/arm64/include/asm/vdso/gettimeofday.h
index 631ab1281633..4b4c0dac0e14 100644
--- a/arch/arm64/include/asm/vdso/gettimeofday.h
+++ b/arch/arm64/include/asm/vdso/gettimeofday.h
@@ -83,11 +83,7 @@ static __always_inline u64 __arch_get_hw_counter(s32 clock_mode,
 	 */
 	isb();
 	asm volatile("mrs %0, cntvct_el0" : "=r" (res) :: "memory");
-	/*
-	 * This isb() is required to prevent that the seq lock is
-	 * speculated.#
-	 */
-	isb();
+	arch_counter_enforce_ordering(res);
 
 	return res;
 }
-- 
2.31.0.rc2.261.g7f71774620-goog