linux-ext4 - Re: [PATCH 1/5] fallocate() implementation in i86, x86

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Date:	Wed, 9 May 2007 14:00:19 +0200
From:	"Martin Schwidefsky" <schwidefsky@...glemail.com>
To:	"Paul Mackerras" <paulus@...ba.org>
Cc:	suparna@...ibm.com, "Andrew Morton" <akpm@...ux-foundation.org>,
	"Amit K. Arora" <aarora@...ux.vnet.ibm.com>,
	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org,
	xfs@....sgi.com, cmm@...ibm.com
Subject: Re: [PATCH 1/5] fallocate() implementation in i86, x86_64 and powerpc

On 5/9/07, Paul Mackerras <paulus@...ba.org> wrote:
> Suparna Bhattacharya writes:
>
> > > Of course the interface used by an application program would have the
> > > fd first.  Glibc can do the translation.
> >
> > I think that was understood.
>
> OK, then what does it matter what the glibc/kernel interface is, as
> long as it works?
>
> It's only a minor point; the order of arguments can vary between
> architectures if necessary, but it's nicer if they don't have to.
> 32-bit powerpc will need to have the two int arguments adjacent in
> order to avoid using more than 6 argument registers at the user/kernel
> boundary, and s390 will need to avoid having a 64-bit argument last
> (if I understand it correctly).

Ah, almost but not quite the point. But I admit it is hard to understand..
The trouble started with the futex call which has been the first
system call with 6 arguments. s390 supported only 5 arguments up to
that point (%r2 - %r6). For futex we added a wrapper to the glibc that
loaded the 6th argument to %r7. In entry.S we set up things so that
%r7 gets stored to the kernel stack where normal C code expects the
first overflow argument. This enabled us to use the standard futex
system call with 6 arguments.
fallocate now has an additional problem: the last argument is a 64 bit
integers AND registers %r2-%r5 are already used. In this case the 64
bit number would have to be split into the high part in %r6 and the
low part on the stack so that the glibc wrapper can load the low part
to %r7. But the C compiler will skip %r6 and store the 64 bit number
on the stack.
If the order of the arguments if modified so that %r6 is assigned to a
32-bit argument, then the entry.S magic with %r7 would work.

-- 
blue skies,
  Martin
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