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Subject: Re: [patch 11/11] x66/vdso: Add CLOCK_TAI support
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <alpine.DEB.2.21.1809141625040.10480@nanos.tec.linutronix.de>
Date:   Fri, 14 Sep 2018 07:59:42 -0700
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@kernel.org>, 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>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        devel@linuxdriverproject.org,
        virtualization@lists.linux-foundation.org,
        Paolo Bonzini <pbonzini@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>, Juergen Gross <jgross@suse.com>
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Message-Id: <FAE37798-8CCE-44A5-9EF0-F69C2199BE60@amacapital.net>
References: <20180914125006.349747096@linutronix.de> <20180914125119.081037164@linutronix.de> <ABB53D42-2A06-41B9-8A8C-40B39CD0289D@amacapital.net> <alpine.DEB.2.21.1809141625040.10480@nanos.tec.linutronix.de>
To:     Thomas Gleixner <tglx@linutronix.de>
Sender: linux-kernel-owner@vger.kernel.org
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> On Sep 14, 2018, at 7:27 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
>=20
> On Fri, 14 Sep 2018, Andy Lutomirski wrote:
>>> On Sep 14, 2018, at 5:50 AM, Thomas Gleixner <tglx@linutronix.de> wrote:=

>>>=20
>>> 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 u=
se
>>> of the fact that:
>>>=20
>>> - 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
>>>=20
>>> Add the mask to the basetime array lookup and set up the CLOCK_TAI base
>>> time in update_vsyscall().
>>=20
>> That=E2=80=99s... horrible. In an amazing way. Can you add BUILD_BUG_ON s=
omewhere
>> to assert that this actually works?
>=20
> Sure, but changing any of the clock ids will cause more wreckage than that=
.
>=20

I=E2=80=99m more concerned that we add a new one and break the magic masking=
. Maybe two start sharing the same slot.=20

> Thanks,
>=20
>    tglx