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From:   Andy Lutomirski <luto@kernel.org>
Date:   Thu, 4 Oct 2018 13:32:51 -0700
Message-ID: <CALCETrUii1XGHhe7BN+LG0_jjyjOcHHnBgcOvh-wZXGxdMotPg@mail.gmail.com>
Subject: Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Andrew Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Matt Rickard <matt@softrans.com.au>,
        Stephen Boyd <sboyd@kernel.org>,
        John Stultz <john.stultz@linaro.org>,
        Florian Weimer <fweimer@redhat.com>,
        KY Srinivasan <kys@microsoft.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        devel@linuxdriverproject.org,
        Linux Virtualization <virtualization@lists.linux-foundation.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>, Juergen Gross <jgross@suse.com>
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On Fri, Sep 14, 2018 at 5:52 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> Matt attempted to add CLOCK_TAI support to the VDSO clock_gettime()
> implementation, which extended the clockid switch case and added yet
> another slightly different copy of the same code.
>
> Especially the extended switch case is problematic as the compiler tends to
> generate a jump table which then requires to use retpolines. If jump tables
> are disabled it adds yet another conditional to the existing maze.
>
> This series takes a different approach by consolidating the almost
> identical functions into one implementation for high resolution clocks and
> one for the coarse grained clock ids by storing the base data for each
> clock id in an array which is indexed by the clock id.
>
> This completely eliminates the switch case and allows further
> simplifications of the code base, which at the end all together gain a few
> cycles performance or at least stay on par with todays code. The resulting
> performance depends heavily on the micro architecture and the compiler.

tglx, please consider this whole series Acked-by: Andy Lutomirski
<luto@kernel.org>

Please feel free to push it top tip:x86/vdso, as long as you pull in
tip/x86/urgent first.  Once it lands, I'll email out a couple of
follow-up patches.

--Andy