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

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Date:   Mon, 24 Sep 2018 23:08:42 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Thomas Gleixner <tglx@...utronix.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 Mon, Sep 17, 2018 at 3:00 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> 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.

Ok.

> > 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.

Right, I'd just not add a getres_time64() to the vdso then.

        Arnd