linux-kernel - [PATCH 1/3] lib/vdso: Delay mask application in do

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Date:   Tue, 25 Jun 2019 17:18:02 +0100
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Cc:     catalin.marinas@....com, will.deacon@....com, arnd@...db.de,
        linux@...linux.org.uk, ralf@...ux-mips.org, paul.burton@...s.com,
        daniel.lezcano@...aro.org, tglx@...utronix.de, salyzyn@...roid.com,
        pcc@...gle.com, shuah@...nel.org, 0x7f454c46@...il.com,
        linux@...musvillemoes.dk, huw@...eweavers.com,
        sthotton@...vell.com, andre.przywara@....com
Subject: [PATCH 1/3] lib/vdso: Delay mask application in do_hres()

do_hres() in the vDSO generic library masks the hw counter value
immediately after reading it.

Postpone the mask application after checking if the syscall fallback is
enabled, in order to be able to detect a possible fallback for the
architectures that have masks smaller than ULLONG_MAX.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@....com>
---
 lib/vdso/gettimeofday.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/lib/vdso/gettimeofday.c b/lib/vdso/gettimeofday.c
index ef28cc5d7bff..ee1221ba1d32 100644
--- a/lib/vdso/gettimeofday.c
+++ b/lib/vdso/gettimeofday.c
@@ -35,12 +35,12 @@ static int do_hres(const struct vdso_data *vd, clockid_t clk,
 
 	do {
 		seq = vdso_read_begin(vd);
-		cycles = __arch_get_hw_counter(vd->clock_mode) &
-			vd->mask;
+		cycles = __arch_get_hw_counter(vd->clock_mode);
 		ns = vdso_ts->nsec;
 		last = vd->cycle_last;
 		if (unlikely((s64)cycles < 0))
 			return clock_gettime_fallback(clk, ts);
+		cycles &= vd->mask;
 		if (cycles > last)
 			ns += (cycles - last) * vd->mult;
 		ns >>= vd->shift;
-- 
2.22.0