UPSTREAM: arm64: vdso: Avoid ISB after reading from cntvct_el0

commit 77ec462536a13d4b428a1eead725c4818a49f0b1 upstream.
(The upstream patch was not marked as fixed but this can fix
Fixes: 28b1a824 ("arm64: vdso: Substitute gettimeofday() with C implementation")
sysbench memory comparison:
- Before: 3072.00 MB transferred (2601.11 MB/sec)
- After:  3072.00 MB transferred (3217.86 MB/sec)
)

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.

Bug: 195968646
Fixes: 28b1a824 ("arm64: vdso: Substitute gettimeofday() with C implementation")
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>
CC: stable@vger.kernel.org
(cherry picked from commit 77ec462536a13d4b428a1eead725c4818a49f0b1)
Signed-off-by: Chanho Park <chanho61.park@samsung.com>
Change-Id: I891873626c27060e7ead724754096a7c5f59e4e6