arm64: vdso: Avoid ISB after reading from cntvct_el0

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@kernel.org>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://lore.kernel.org/r/20210318170738.7756-5-will@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 1 insertions, 5 deletions

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;
 }