arm64: Fix __arch_get_hw_counter() implementation

Provide the following fixes for the __arch_get_hw_counter()
implementation on arm64:
 - Fallback on syscall when an unstable counter is detected.
 - Introduce isb()s before and after the counter read to avoid
   speculation of the counter value and of the seq lock
   respectively.
   The second isb() is a temporary solution that will be revisited
   in 5.3-rc1.

These fixes restore the semantics that __arch_counter_get_cntvct()
had on arm64.
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arch@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mips@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org
Cc: catalin.marinas@arm.com
Cc: will.deacon@arm.com
Cc: arnd@arndb.de
Cc: linux@armlinux.org.uk
Cc: ralf@linux-mips.org
Cc: paul.burton@mips.com
Cc: daniel.lezcano@linaro.org
Cc: salyzyn@android.com
Cc: pcc@google.com
Cc: shuah@kernel.org
Cc: 0x7f454c46@gmail.com
Cc: linux@rasmusvillemoes.dk
Cc: huw@codeweavers.com
Cc: sthotton@marvell.com
Cc: andre.przywara@arm.com
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Link: https://lkml.kernel.org/r/20190625161804.38713-2-vincenzo.frascino@arm.com

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

@@ -10,6 +10,8 @@
 #include <asm/unistd.h>
 #include <uapi/linux/time.h>
 
+#define __VDSO_USE_SYSCALL		ULLONG_MAX
+
 #define VDSO_HAS_CLOCK_GETRES		1
 
 static __always_inline
@@ -68,7 +70,24 @@ static __always_inline u64 __arch_get_hw_counter(s32 clock_mode)
 {
 	u64 res;
 
+	/*
+	 * clock_mode == 0 implies that vDSO are enabled otherwise
+	 * fallback on syscall.
+	 */
+	if (clock_mode)
+		return __VDSO_USE_SYSCALL;
+
+	/*
+	 * This isb() is required to prevent that the counter value
+	 * is speculated.
+	 */
+	isb();
 	asm volatile("mrs %0, cntvct_el0" : "=r" (res) :: "memory");
+	/*
+	 * This isb() is required to prevent that the seq lock is
+	 * speculated.#
+	 */
+	isb();
 
 	return res;
 }