linux-kernel - Re: [patch 11/11] x66/vdso: Add CLOCK

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Message-ID: <alpine.DEB.2.21.1809141625040.10480@nanos.tec.linutronix.de>
Date:   Fri, 14 Sep 2018 16:27:47 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...capital.net>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Matt Rickard <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>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        devel@...uxdriverproject.org,
        virtualization@...ts.linux-foundation.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Arnd Bergmann <arnd@...db.de>, Juergen Gross <jgross@...e.com>
Subject: Re: [patch 11/11] x66/vdso: Add CLOCK_TAI support

On Fri, 14 Sep 2018, Andy Lutomirski wrote:
> > On Sep 14, 2018, at 5:50 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > 
> > With the storage array in place it's now trivial to support CLOCK_TAI in
> > the vdso. Instead of extending the array to accomodate CLOCK_TAI, make use
> > of the fact that:
> > 
> > - CLOCK ids are set in stone
> > - CLOCK_THREAD_CPUTIME is never going to be supported in the VDSO so
> >   the array slot 3 is unused
> > - CLOCK_TAI is id 11 which results in 3 when masked with 0x3
> > 
> > Add the mask to the basetime array lookup and set up the CLOCK_TAI base
> > time in update_vsyscall().
> 
> That’s... horrible. In an amazing way. Can you add BUILD_BUG_ON somewhere
> to assert that this actually works?

Sure, but changing any of the clock ids will cause more wreckage than that.

Thanks,

	tglx