From: Vincenzo Frascino <vincenzo.frascino@arm.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arch@vger.kernel.org,
LAK <linux-arm-kernel@lists.infradead.org>,
LKML <linux-kernel@vger.kernel.org>,
linux-mips@vger.kernel.org, linux-kselftest@vger.kernel.org,
catalin.marinas@arm.com, Will Deacon <will.deacon@arm.com>,
Arnd Bergmann <arnd@arndb.de>,
linux@armlinux.org.uk, Ralf Baechle <ralf@linux-mips.org>,
paul.burton@mips.com, Daniel Lezcano <daniel.lezcano@linaro.org>,
salyzyn@android.com, pcc@google.com, shuah@kernel.org,
0x7f454c46@gmail.com, linux@rasmusvillemoes.dk,
huw@codeweavers.com, sthotton@marvell.com,
andre.przywara@arm.com, Andy Lutomirski <luto@kernel.org>
Subject: Re: [PATCH 1/3] lib/vdso: Delay mask application in do_hres()
Date: Wed, 26 Jun 2019 10:25:20 +0100 [thread overview]
Message-ID: <4ff43de4-fe51-eed4-a155-31a05edf2f11@arm.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1906260832470.32342@nanos.tec.linutronix.de>
Hi Thomas,
On 26/06/2019 07:38, Thomas Gleixner wrote:
> On Tue, 25 Jun 2019, Thomas Gleixner wrote:
>> On Tue, 25 Jun 2019, Vincenzo Frascino wrote:
>>> 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.
>>
>> Right. This only worked on x86 because the mask is there ULLONG_MAX for all
>> VDSO capable clocksources, i.e. that ever worked just by chance.
>
> But it's actually worse than that:
>
>>> + cycles &= vd->mask;
>>> if (cycles > last)
>>> ns += (cycles - last) * vd->mult;
>>> ns >>= vd->shift;
>
> This is broken for any clocksource which can legitimately wrap around. The
> core timekeeping does the right thing:
>
> (cycles - last) & mask
>
> That makes sure that a wraparound is correctly handled. With the above the
> wrap around would be ignored due to
>
> if (cycles > last)
>
You are right. Thanks for spotting it.
...
--
Regards,
Vincenzo
next prev parent reply other threads:[~2019-06-26 9:25 UTC|newest]
Thread overview: 108+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-21 9:52 [PATCH v7 00/25] Unify vDSOs across more architectures Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 01/25] kernel: Standardize vdso_datapage Vincenzo Frascino
2019-06-24 13:56 ` Catalin Marinas
2019-06-21 9:52 ` [PATCH v7 02/25] kernel: Define gettimeofday vdso common code Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 03/25] kernel: Unify update_vsyscall implementation Vincenzo Frascino
2019-06-21 10:49 ` Huw Davies
2019-06-21 9:52 ` [PATCH v7 04/25] arm64: Substitute gettimeofday with C implementation Vincenzo Frascino
2019-06-24 13:36 ` Will Deacon
2019-06-24 13:59 ` Vincenzo Frascino
2019-06-25 16:18 ` [PATCH 1/3] lib/vdso: Delay mask application in do_hres() Vincenzo Frascino
2019-06-25 16:18 ` [PATCH 2/3] arm64: Fix __arch_get_hw_counter() implementation Vincenzo Frascino
2019-06-25 16:18 ` [PATCH 3/3] arm64: compat: " Vincenzo Frascino
2019-06-25 17:02 ` [PATCH 1/3] lib/vdso: Delay mask application in do_hres() Thomas Gleixner
2019-06-25 18:27 ` Thomas Gleixner
2019-06-25 20:15 ` Andy Lutomirski
2019-06-25 22:24 ` Thomas Gleixner
2019-06-26 6:38 ` Thomas Gleixner
2019-06-26 9:25 ` Vincenzo Frascino [this message]
2019-06-26 10:02 ` lib/vdso: Make delta calculation work correctly Thomas Gleixner
2019-06-26 11:08 ` Vincenzo Frascino
2019-06-24 13:58 ` [PATCH v7 04/25] arm64: Substitute gettimeofday with C implementation Catalin Marinas
2019-06-25 15:33 ` Dave Martin
2019-06-26 13:27 ` Vincenzo Frascino
2019-06-26 16:14 ` Dave Martin
2019-06-26 19:01 ` Vincenzo Frascino
2019-06-27 10:01 ` Dave Martin
2019-06-27 10:57 ` Vincenzo Frascino
2019-06-27 11:27 ` Dave Martin
2019-06-27 11:59 ` Vincenzo Frascino
2019-06-27 14:38 ` Dave Martin
2019-06-27 15:34 ` Vincenzo Frascino
2019-06-25 17:43 ` [PATCH] arm64: vdso: Fix compilation with clang < 8 Vincenzo Frascino
2019-06-26 11:36 ` [PATCH v2] arm64: vdso: Fix compilation with clang older then 8 Vincenzo Frascino
[not found] ` <CGME20190628130921eucas1p239935b0771032c331911eacc1a69dd2e@eucas1p2.samsung.com>
2019-06-28 13:09 ` [PATCH v7 04/25] arm64: Substitute gettimeofday with C implementation Marek Szyprowski
2019-06-28 14:32 ` Vincenzo Frascino
2019-06-28 16:50 ` Sylwester Nawrocki
2019-06-29 6:58 ` Vincenzo Frascino
2019-07-08 12:57 ` Sylwester Nawrocki
2019-07-08 13:09 ` Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 05/25] arm64: Build vDSO with -ffixed-x18 Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 06/25] arm64: compat: Add missing syscall numbers Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 07/25] arm64: compat: Expose signal related structures Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 08/25] arm64: compat: Generate asm offsets for signals Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 09/25] lib: vdso: Add compat support Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 10/25] arm64: compat: Add vDSO Vincenzo Frascino
2019-06-24 14:00 ` Catalin Marinas
2019-07-10 4:02 ` John Stultz
2019-07-10 6:12 ` Thomas Gleixner
2019-07-10 9:48 ` Vincenzo Frascino
2019-07-10 8:27 ` Will Deacon
2019-07-10 8:58 ` Thomas Gleixner
2019-07-10 9:12 ` Will Deacon
2019-07-10 9:47 ` Vincenzo Frascino
2019-07-10 13:41 ` Vincenzo Frascino
2019-07-10 13:04 ` [PATCH] arm64: vdso: Fix ABI regression in compat vdso Vincenzo Frascino
2019-07-10 13:25 ` Will Deacon
2019-07-10 13:42 ` Vincenzo Frascino
2019-07-10 14:01 ` [PATCH v2] " Vincenzo Frascino
2019-07-10 15:44 ` John Stultz
2019-07-10 15:53 ` Vincenzo Frascino
2019-07-11 9:45 ` Will Deacon
2019-07-11 10:34 ` Thomas Gleixner
2019-07-11 11:32 ` Will Deacon
2019-06-21 9:52 ` [PATCH v7 11/25] arm64: Refactor vDSO code Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 12/25] arm64: compat: vDSO setup for compat layer Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 13/25] arm64: elf: vDSO code page discovery Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 14/25] arm64: compat: Get sigreturn trampolines from vDSO Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 15/25] arm64: Add vDSO compat support Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 16/25] arm: Add support for generic vDSO Vincenzo Frascino
2019-12-04 13:51 ` [PATCH v7 16/25] arm: Add support for generic vDSO (causing crash) Guenter Roeck
2019-12-04 13:58 ` Vincenzo Frascino
2019-12-04 16:16 ` Guenter Roeck
2019-12-04 17:15 ` Vincenzo Frascino
2019-12-04 19:39 ` Guenter Roeck
2019-12-05 9:42 ` Philippe Mathieu-Daudé
2019-12-05 10:00 ` Vincenzo Frascino
2019-12-05 11:02 ` Arnd Bergmann
2019-12-05 14:56 ` Philippe Mathieu-Daudé
2019-06-21 9:52 ` [PATCH v7 17/25] arm: Add clock_getres entry point Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 18/25] arm: Add clock_gettime64 " Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 19/25] mips: Add support for generic vDSO Vincenzo Frascino
2019-07-26 5:15 ` Paul Burton
2019-07-26 16:29 ` [PATCH 0/2] mips: vdso: Fix Makefile Vincenzo Frascino
2019-07-26 16:29 ` [PATCH 1/2] mips: vdso: Fix source path Vincenzo Frascino
2019-07-26 16:29 ` [PATCH 2/2] mips: vdso: Fix flip/flop vdso building bug Vincenzo Frascino
2019-07-28 22:20 ` [PATCH 0/2] mips: vdso: Fix Makefile Paul Burton
2019-06-21 9:52 ` [PATCH v7 20/25] mips: Add clock_getres entry point Vincenzo Frascino
2019-07-26 5:15 ` Paul Burton
2019-06-21 9:52 ` [PATCH v7 21/25] mips: Add clock_gettime64 " Vincenzo Frascino
2019-07-26 5:15 ` Paul Burton
2019-06-21 9:52 ` [PATCH v7 22/25] x86: Add support for generic vDSO Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 23/25] x86: Add clock_getres entry point Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 24/25] x86: Add clock_gettime64 " Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 25/25] kselftest: Extend vDSO selftest Vincenzo Frascino
2019-06-24 0:34 ` [PATCH v7 00/25] Unify vDSOs across more architectures Thomas Gleixner
2019-06-24 1:15 ` Andy Lutomirski
2019-06-24 7:42 ` Thomas Gleixner
2019-06-24 13:21 ` Vincenzo Frascino
2019-06-24 14:18 ` Thomas Gleixner
2019-06-24 14:23 ` Russell King - ARM Linux admin
2019-06-24 14:49 ` Catalin Marinas
2019-06-24 16:20 ` Vincenzo Frascino
2019-10-25 11:42 ` Geert Uytterhoeven
2019-06-24 18:41 ` Paul Burton
2019-06-24 23:16 ` Vincenzo Frascino
2019-06-25 17:11 ` Paul Burton
2019-06-25 17:17 ` Vincenzo Frascino
2019-06-24 12:50 ` Andre Przywara
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