arm64: vdso: use SYS_CNTVCTSS_EL0 for gettimeofday

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@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Andre Przywara <andre.przywara@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20220830104833.34636-4-joey.gouly@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/include/asm/vdso/gettimeofday.h b/arch/arm64/include/asm/vdso/gettimeofday.h
index 4f7a629df81f7416e9797794406029dd3024a355..764d13e2916c559bc8fd7efa032b6f468ac0d4a2 100644 (file)
--- 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;