From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932677AbcAYWgz (ORCPT ); Mon, 25 Jan 2016 17:36:55 -0500 Received: from hauke-m.de ([5.39.93.123]:58290 "EHLO hauke-m.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755329AbcAYWgw (ORCPT ); Mon, 25 Jan 2016 17:36:52 -0500 Subject: Re: [PATCH v3 3/3] MIPS: VDSO: Add implementations of gettimeofday() and clock_gettime() To: Markos Chandras , linux-mips@linux-mips.org References: <1444645496-32046-1-git-send-email-markos.chandras@imgtec.com> <1445417864-31453-1-git-send-email-markos.chandras@imgtec.com> Cc: Alex Smith , linux-kernel@vger.kernel.org From: Hauke Mehrtens X-Enigmail-Draft-Status: N1110 Message-ID: <56A6A380.70708@hauke-m.de> Date: Mon, 25 Jan 2016 23:36:48 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Icedove/38.5.0 MIME-Version: 1.0 In-Reply-To: <1445417864-31453-1-git-send-email-markos.chandras@imgtec.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/21/2015 10:57 AM, Markos Chandras wrote: > From: Alex Smith > > Add user-mode implementations of gettimeofday() and clock_gettime() to > the VDSO. This is currently usable with 2 clocksources: the CP0 count > register, which is accessible to user-mode via RDHWR on R2 and later > cores, or the MIPS Global Interrupt Controller (GIC) timer, which > provides a "user-mode visible" section containing a mirror of its > counter registers. This section must be mapped into user memory, which > is done below the VDSO data page. > > When a supported clocksource is not in use, the VDSO functions will > return -ENOSYS, which causes libc to fall back on the standard syscall > path. > > When support for neither of these clocksources is compiled into the > kernel at all, the VDSO still provides clock_gettime(), as the coarse > realtime/monotonic clocks can still be implemented. However, > gettimeofday() is not provided in this case as nothing can be done > without a suitable clocksource. This causes the symbol lookup to fail > in libc and it will then always use the standard syscall path. > > This patch includes a workaround for a bug in QEMU which results in > RDHWR on the CP0 count register always returning a constant (incorrect) > value. A fix for this has been submitted, and the workaround can be > removed after the fix has been in stable releases for a reasonable > amount of time. > > A simple performance test which calls gettimeofday() 1000 times in a > loop and calculates the average execution time gives the following > results on a Malta + I6400 (running at 20MHz): > > - Syscall: ~31000 ns > - VDSO (GIC): ~15000 ns > - VDSO (CP0): ~9500 ns > > [markos.chandras@imgtec.com: > - Minor code re-arrangements in order for mappings to be made > in the order they appear to the process' address space. > - Move do_{monotonic, realtime} outside of the MIPS_CLOCK_VSYSCALL ifdef > - Use gic_get_usm_range so we can do the GIC mapping in the > arch/mips/kernel/vdso instead of the GIC irqchip driver] > > Cc: linux-kernel@vger.kernel.org > Signed-off-by: Alex Smith > Signed-off-by: Markos Chandras > --- > Changes since v2: > - Do not export VDSO symbols if the toolchain does not have proper support > for the VDSO. > > Changes since v1: > - Use gic_get_usm_range so we can do the GIC mapping in the > arch/mips/kernel/vdso instead of the GIC irqchip driver > --- > arch/mips/Kconfig | 5 + > arch/mips/include/asm/clocksource.h | 29 +++++ > arch/mips/include/asm/vdso.h | 68 +++++++++- > arch/mips/kernel/csrc-r4k.c | 44 +++++++ > arch/mips/kernel/vdso.c | 71 ++++++++++- > arch/mips/vdso/gettimeofday.c | 232 +++++++++++++++++++++++++++++++++++ > arch/mips/vdso/vdso.h | 9 ++ > arch/mips/vdso/vdso.lds.S | 5 + > drivers/clocksource/mips-gic-timer.c | 7 +- > 9 files changed, 460 insertions(+), 10 deletions(-) > create mode 100644 arch/mips/include/asm/clocksource.h > create mode 100644 arch/mips/vdso/gettimeofday.c > .... > + > +int __vdso_clock_gettime(clockid_t clkid, struct timespec *ts) > +{ > + const union mips_vdso_data *data = get_vdso_data(); > + int ret; > + > + switch (clkid) { > + case CLOCK_REALTIME_COARSE: > + ret = do_realtime_coarse(ts, data); > + break; > + case CLOCK_MONOTONIC_COARSE: > + ret = do_monotonic_coarse(ts, data); > + break; > + case CLOCK_REALTIME: > + ret = do_realtime(ts, data); > + break; > + case CLOCK_MONOTONIC: > + ret = do_monotonic(ts, data); > + break; > + default: > + ret = -ENOSYS; > + break; > + } > + > + /* If we return -ENOSYS libc should fall back to a syscall. */ This comment is important. The other architectures (checked arm64, tile, x86) are calling the original syscall instead of returning -ENOSYS here. This will confuse people trying to use this feature like me. When the libc does not call the normal syscall this will cause problems. > + return ret; > +} Hauke