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SubjectRe: [PATCH 1/2] x86: Mark __vdso entries as asmlinkage
On Wed, Feb 26, 2014 at 9:22 PM, H. Peter Anvin <hpa@zytor.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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