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 <[e-mail]>
Cc: Will Deacon <[e-mail]>
Cc: Vincenzo Frascino <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Andre Przywara <[e-mail]>
Acked-by: Mark Rutland <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Catalin Marinas <[e-mail]>

arch/arm64/include/asm/vdso/gettimeofday.h

···
 
 #ifndef __ASSEMBLY__
 
+#include <asm/alternative.h>
 #include <asm/barrier.h>
 #include <asm/unistd.h>
+#include <asm/sysreg.h>
 
 #define VDSO_HAS_CLOCK_GETRES		1
 
···
 		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;