linux-kernel - Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK

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Date:   Mon, 17 Sep 2018 15:00:39 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Arnd Bergmann <arnd@...db.de>
cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, matt@...trans.com.au,
        Stephen Boyd <sboyd@...nel.org>,
        John Stultz <john.stultz@...aro.org>,
        Florian Weimer <fweimer@...hat.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>, vkuznets@...hat.com,
        devel@...uxdriverproject.org,
        virtualization@...ts.linux-foundation.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Deepa Dinamani <deepa.kernel@...il.com>
Subject: Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI
 support

On Fri, 14 Sep 2018, Arnd Bergmann wrote:
> On Fri, Sep 14, 2018 at 2:52 PM Thomas Gleixner <tglx@...utronix.de> wrote:
> A couple of architectures (s390, ia64, riscv, powerpc, arm64)
> implement the vdso as assembler code at the moment, so they
> won't be as easy to consolidate (other than outright replacing all
> the code).
> 
> The other five:
> arch/x86/entry/vdso/vclock_gettime.c
> arch/sparc/vdso/vclock_gettime.c
> arch/nds32/kernel/vdso/gettimeofday.c
> arch/mips/vdso/gettimeofday.c
> arch/arm/vdso/vgettimeofday.c
>
> are basically all minor variations of the same code base and could be
> consolidated to some degree.
> Any suggestions here? Should we plan to do that consolitdation based on
> your new version, or just add clock_gettime64 in arm32 and x86-32, and then
> be done with it? The other ones will obviously still be fast for 32-bit time_t
> and will have a working non-vdso sys_clock_getttime64().

In principle consolidating all those implementations should be possible to
some extent and probably worthwhile. What's arch specific are the actual
accessors to the hardware clocks.

> I also wonder about clock_getres(): half the architectures seem to implement
> it in vdso, but notably arm32 and x86 don't, and I had not expected it to be
> performance critical given that the result is easily cached in user space.

getres() is not really performance critical, but adding it does not create
a huge problem either.

Thanks,

	tglx