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Subject: Re: [PATCH v3] x86/vdso: Handle clock_gettime(CLOCK_TAI) in vDSO
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <0120ea99-fcac-839a-ef8f-1daa74c17bec@redhat.com>
Date:   Thu, 13 Sep 2018 08:22:07 -0700
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        Matt Rickard <matt@softrans.com.au>,
        Stephen Boyd <sboyd@kernel.org>,
        John Stultz <john.stultz@linaro.org>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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Message-Id: <F5CE2714-B491-490D-9B03-E111A61E333F@amacapital.net>
References: <20180901015935.CCF0B18E20A9@virtux64.softrans.com.au> <CALCETrU4PzskO9pE6G4EkXNRRX3Mu7Vvjv1P8XgrfRMdgGDZSg@mail.gmail.com> <alpine.DEB.2.21.1809092159440.1402@nanos.tec.linutronix.de> <dde36e93-a21b-6223-8ffc-6dd11f687d38@redhat.com> <alpine.DEB.2.21.1809121616040.9957@nanos.tec.linutronix.de> <59403142-1073-3926-65dd-ac55ead357b9@redhat.com> <alpine.DEB.2.21.1809121623430.9957@nanos.tec.linutronix.de> <DEC1E2DC-2D89-4955-81FC-4EDCF3692214@amacapital.net> <0120ea99-fcac-839a-ef8f-1daa74c17bec@redhat.com>
To:     Florian Weimer <fweimer@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Sep 13, 2018, at 1:07 AM, Florian Weimer <fweimer@redhat.com> wrote:
>=20
> On 09/12/2018 07:11 PM, Andy Lutomirski wrote:
>>> The multiplexer interfaces need much more surgery and talking about fute=
x,
>>> we'd need to sit down with quite some people and identify the things the=
y
>>> actually care about before just splitting it up and keeping the existing=

>>> overloaded trainwreck the same.
>>>=20
>> There=E2=80=99s also the issue of how much the speedup matters. For futex=
, maybe a better interface saves 3ns, but a futex syscall is hundreds of ns.=
 clock_gettime() is called at high frequency and can be ~25ns. Saving a few n=
s is a bigger deal.
>=20
> My concern is that the userspace system call wrappers currently do not kno=
w how many arguments the individual operations take and what types the argum=
ents have (hence the =E2=80=9Ctype-polymorphic=E2=80=9D nature I mentioned).=
 This could be a problem for on-stack argument passing (where you might read=
 values beyond the end of the stack, and glibc avoids that most of the time b=
y having enough cruft on the stack), and for architectures which pass pointe=
rs and integers in different registers (like some m68k ABIs do for the retur=
n value).
>=20
>=20

Isn=E2=80=99t clock_gettime already special because of the vDSO entry point,=
 though?=