linux-kernel - Re: [PATCH v2]: arch: arm64: vdso: export the symbols for time()

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Date:   Tue, 23 Jun 2020 15:44:59 +0100
From:   Will Deacon <will@...nel.org>
To:     Mark Salyzyn <salyzyn@...roid.com>
Cc:     linux-kernel@...r.kernel.org, kernel-team@...roid.com,
        Chiawei Wang <chiaweiwang@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Enrico Weigelt <info@...ux.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alexios Zavras <alexios.zavras@...el.com>,
        linux-arm-kernel@...ts.infradead.org,
        Allison Randal <allison@...utok.net>
Subject: Re: [PATCH v2]: arch: arm64: vdso: export the symbols for time()

On Mon, Jun 15, 2020 at 07:38:24AM -0700, Mark Salyzyn wrote:
> From: Chiawei Wang <chiaweiwang@...gle.com>
> 
> __cvdso_time() can be found in vDSO implementation,
> but the symbols for time() are not exported.
> 
> Export the symbols and run bionic-benchmarks.
> 
> BEFORE:
> bionic-benchmarks32 --bionic_extra BM_time_time
> -----------------------------------------------------
> Benchmark           Time             CPU   Iterations
> -----------------------------------------------------
> BM_time_time     83.6 ns         83.5 ns      8385964
> 
> bionic-benchmarks64 --bionic_extra BM_time_time
> -----------------------------------------------------
> Benchmark           Time             CPU   Iterations
> -----------------------------------------------------
> BM_time_time     63.5 ns         63.4 ns     11037509
> 
> AFTER:
> bionic-benchmarks32 --bionic_extra BM_time_time
> -----------------------------------------------------
> Benchmark           Time             CPU   Iterations
> -----------------------------------------------------
> BM_time_time     8.57 ns         8.56 ns     81887312
> 
> bionic-benchmarks64 --bionic_extra BM_time_time
> -----------------------------------------------------
> Benchmark           Time             CPU   Iterations
> -----------------------------------------------------
> BM_time_time     7.52 ns         7.51 ns     93253809
> 
> Signed-off-by: Chiawei Wang <chiaweiwang@...gle.com>
> Signed-off-by: Mark Salyzyn <salyzyn@...roid.com>
> Cc: linux-kernel@...r.kernel.org
> Cc: kernel-team@...roid.com
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Vincenzo Frascino <vincenzo.frascino@....com>
> Cc: Enrico Weigelt <info@...ux.net>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Alexios Zavras <alexios.zavras@...el.com>
> Cc: linux-arm-kernel@...ts.infradead.org

I don't understand this: neither arm nor arm64 offer the time() syscall
afaict, so the C library should be implementing it in terms of
clock_gettime() anyway, which _is_ implemented in the vDSO.

What's am I missing?

Will