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From:   Heiko Carstens <hca@linux.ibm.com>
To:     Li Wang <liwang@redhat.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Sven Schnelle <svens@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>
Cc:     ltp@lists.linux.it, linux-kernel@vger.kernel.org,
        linux-s390@vger.kernel.org
Subject: [PATCH 1/3] s390/vdso: fix tod clock steering
Date:   Tue, 23 Mar 2021 22:58:17 +0100
Message-Id: <20210323215819.4161164-2-hca@linux.ibm.com>
In-Reply-To: <20210323215819.4161164-1-hca@linux.ibm.com>
References: <YFmUrVOyX4q+8Dy9@osiris>
 <20210323215819.4161164-1-hca@linux.ibm.com>
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The s390 specific vdso function __arch_get_hw_counter() is supposed to
consider tod clock steering.

If a tod clock steering event happens and the tod clock is set to a
new value __arch_get_hw_counter() will not return the real tod clock
value but slowly drift it from the old delta until the returned value
finally matches the real tod clock value again.

When converting the assembler code to C it was forgotten to tell user
space in which direction the clock has to be adjusted.

Worst case is now that instead of drifting the clock slowly it will
jump into the opposite direction by a factor of two.

Fix this by simply providing the missing value to user space.

Fixes: 4bff8cb54502 ("s390: convert to GENERIC_VDSO")
Cc: <stable@vger.kernel.org> # 5.10
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
---
 arch/s390/kernel/time.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/s390/kernel/time.c b/arch/s390/kernel/time.c
index 165da961f901..e37285a5101b 100644
--- a/arch/s390/kernel/time.c
+++ b/arch/s390/kernel/time.c
@@ -382,6 +382,7 @@ static void clock_sync_global(unsigned long delta)
 		      tod_steering_delta);
 	tod_steering_end = now + (abs(tod_steering_delta) << 15);
 	vdso_data->arch_data.tod_steering_end = tod_steering_end;
+	vdso_data->arch_data.tod_steering_delta = tod_steering_delta;
 
 	/* Update LPAR offset. */
 	if (ptff_query(PTFF_QTO) && ptff(&qto, sizeof(qto), PTFF_QTO) == 0)
-- 
2.25.1