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

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Date:	Wed, 26 Feb 2014 22:19:15 -0700
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 10:06 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 02/26/2014 07:39 PM, Andi Kleen wrote:
>> On Wed, Feb 26, 2014 at 05:02:13PM -0800, Andy Lutomirski wrote:
>>> This makes no difference for 64-bit, bit it's critical for 32-bit code:
>>> these functions are called from outside the kernel, so they need to comply
>>> with the ABI.
>>
>> That's an odd patch. If that was wrong things couldn't have worked at all.
>> Probably hidden by inlining? If yes just make it static
>>
>> Also you would rather need notrace more often.
>>
>
> It has to support *an* ABI... the syscall vdso entry point uses the old
> int $0x80 calling convention rather than the normal ABI.  It would
> depend on the test program and eventual glibc implementation.  And sure
> enough, the test program has:
>
> int (*vdso_gettimeofday)(struct timeval *tv, struct timezone *tz)
> __attribute__ ((regparm (3)));
> int (*vdso_clock_gettime)(clockid_t clk_id, struct timespec *tp)
> __attribute__ ((regparm (3)));
> time_t (*vdso_time)(time_t *t) __attribute__ ((regparm (3)));
>
> That being said, since this code is compiled separately, the compiler
> flags there determine what actually matters.  However, there we have:
>
> KBUILD_CFLAGS_32 += -m32 -msoft-float -mregparm=3 -freg-struct-return -fpic
>
> 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).

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.

--Andy
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