linux-kernel - Re: [PATCH 1/2] x86: Mark _

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Message-ID: <CALCETrWXO8zxvjbT9usjghkM_MEzGOa9_18x7xXPcOOPYeXijA@mail.gmail.com>
Date:	Thu, 27 Feb 2014 12:11:52 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andi Kleen <ak@...ux.intel.com>,
	Stefani Seibold <stefani@...bold.net>, X86 ML <x86@...nel.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	John Stultz <john.stultz@...aro.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	andriy.shevchenko@...ux.intel.com, Martin.Runge@...de-schwarz.com,
	Andreas.Brief@...de-schwarz.com
Subject: Re: [PATCH 1/2] x86: Mark __vdso entries as asmlinkage

On Wed, Feb 26, 2014 at 9:22 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 02/26/2014 09:19 PM, Andy Lutomirski wrote:
>>>
>>> The normal ABI almost certainly makes more sense; as such -mregparm=3 is
>>> probably not what we want, and I suspect it makes more sense to just
>>> drop that from the CFLAGS line?
>>
>> Hmm.  What happens on a native 32-bit build?  IIRC the whole kernel is
>> build with regparm(3).
>>
>
> Well, the vdso is still built separately, so we can use different CFLAGS
> if we want to.
>
>> If we want to save a cycle or two, then regparm(3) is probably faster.
>>  But I think that these functions should either be asmlinkage or (on
>> 32 bit builds) explicitly regparm(3) to avoid confusion.
>
> I suggest using the standard ABI, but I suggest doing it via CFLAGS.

Hmm.  This sort of goes against existing x86_32 practice where,
AFAICT, things that need a particular calling convention specify
asmlinkage and everything else uses regparm(3) if config/kbuild thinks
it's appropriate.

But I'm happy to resubmit the patch if you prefer the CFLAGS approach
for the 32-bit vdso.  I don't think anything will break, since I don't
think that the 32-bit vdso has any other exported C code.

>
> It isn't any faster if the C library has to provide a wrapper just to
> marshal parameters.

Probably true, given that the glibc wrapper could, in principle, use
an optimized tail call.  Also, I see no reason why vdso functions,
alone of all userspace code, should be special.

--Andy
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