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McKenney" References: <20181001175845.168430-1-salyzyn@android.com> <20181001175845.168430-12-salyzyn@android.com> From: Mark Salyzyn Message-ID: Date: Mon, 1 Oct 2018 13:52:32 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-GB Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/01/2018 11:15 AM, Thomas Gleixner wrote: > On Mon, 1 Oct 2018, Mark Salyzyn wrote: >> >> +static notrace int do_boottime(const struct vdso_data *vd, struct timespec *ts) >> +{ >> + u32 seq, mult, shift; >> + u64 nsec, cycle_last; >> + vdso_wtm_clock_nsec_t wtm_nsec; >> + >> + /* open coding timespec_add_ns to save a ts->tv_nsec = 0 */ >> + ts->tv_sec = sec + __iter_div_u64_rem(nsec, NSEC_PER_SEC, &nsec); >> + ts->tv_nsec = nsec; >> + >> + return 0; >> +} >> + > Instead of adding yet another copy of the same code you might want to look > at the rework I did for the x86 vdso in order to support CLOCK_TAI w/o > running into the issue of the clock switch case being compiled into a jump > table and then the compile asking for retpoline. > > http://lkml.kernel.org/r/20180917124533.329334911@linutronix.de > > Thanks, > > tglx Great idea. Thanks! The point of the first 12 patches is to _align_ the arm code to match the assembler for arm64 exactly 1:1. Then switch arm64 assembler to use the _same_ code in 'C' as a library. No performance degradation. Next extend the vdso framework on arm64 to _also_ use that library for vDSO32 (arm32 compat on 64-bit). At this point we achieve a 0.4% power reduction on Android. At that point, we would be ready for a rework that fixes all three (ARM vDSO, ARM64 vDSO and ARM64 vDSO32) to get the compiler to handle the switch statement better. One step at a time. -- Mark