[PATCH v2 3/3] arm64: vdso: use SYS_CNTVCTSS_EL0 for gettimeofday

[Joey Gouly]

If FEAT_ECV is implemented, the self-synchronized counter CNTVCTSS_EL0 can
be used, removing the need for an ISB.

Signed-off-by: Joey Gouly <joey.gouly at arm.com>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Will Deacon <will at kernel.org>
Cc: Vincenzo Frascino <vincenzo.frascino at arm.com>
Cc: Mark Rutland <mark.rutland at arm.com>
Cc: Andre Przywara <andre.przywara at arm.com>
Acked-by: Mark Rutland <mark.rutland at arm.com>
---
 arch/arm64/include/asm/vdso/gettimeofday.h | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/vdso/gettimeofday.h b/arch/arm64/include/asm/vdso/gettimeofday.h
index 4f7a629df81f..764d13e2916c 100644
--- a/arch/arm64/include/asm/vdso/gettimeofday.h
+++ b/arch/arm64/include/asm/vdso/gettimeofday.h
@@ -7,8 +7,10 @@
 
 #ifndef __ASSEMBLY__
 
+#include <asm/alternative.h>
 #include <asm/barrier.h>
 #include <asm/unistd.h>
+#include <asm/sysreg.h>
 
 #define VDSO_HAS_CLOCK_GETRES		1
 
@@ -78,11 +80,20 @@ static __always_inline u64 __arch_get_hw_counter(s32 clock_mode,
 		return 0;
 
 	/*
-	 * This isb() is required to prevent that the counter value
+	 * If FEAT_ECV is available, use the self-synchronizing counter.
+	 * Otherwise the isb is required to prevent that the counter value
 	 * is speculated.
-	 */
-	isb();
-	asm volatile("mrs %0, cntvct_el0" : "=r" (res) :: "memory");
+	*/
+	asm volatile(
+	ALTERNATIVE("isb\n"
+		    "mrs %0, cntvct_el0",
+		    "nop\n"
+		    __mrs_s("%0", SYS_CNTVCTSS_EL0),
+		    ARM64_HAS_ECV)
+	: "=r" (res)
+	:
+	: "memory");
+
 	arch_counter_enforce_ordering(res);
 
 	return res;
-- 
2.17.1