linux-kernel - Re: [PATCH v3] x86/vdso: Handle clock_gettime(CLOCK

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Message-ID: <0120ea99-fcac-839a-ef8f-1daa74c17bec@redhat.com>
Date:   Thu, 13 Sep 2018 10:07:14 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Andy Lutomirski <luto@...capital.net>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Matt Rickard <matt@...trans.com.au>,
        Stephen Boyd <sboyd@...nel.org>,
        John Stultz <john.stultz@...aro.org>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] x86/vdso: Handle clock_gettime(CLOCK_TAI) in vDSO

On 09/12/2018 07:11 PM, Andy Lutomirski wrote:
>> The multiplexer interfaces need much more surgery and talking about futex,
>> we'd need to sit down with quite some people and identify the things they
>> actually care about before just splitting it up and keeping the existing
>> overloaded trainwreck the same.
>>
> There’s 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 ns is a bigger deal.

My concern is that the userspace system call wrappers currently do not 
know how many arguments the individual operations take and what types 
the arguments have (hence the “type-polymorphic” 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 by having enough cruft on the stack), and for architectures 
which pass pointers and integers in different registers (like some m68k 
ABIs do for the return value).

Thanks,
Florian