linux-kernel - Re: 64-syscall args on 32-bit vs syscall()

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Message-ID: <1268820825.2335.190.camel@pasglop>
Date:	Wed, 17 Mar 2010 21:13:45 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Ulrich Drepper <drepper@...hat.com>
Cc:	munroesj@...ibm.com, "H. Peter Anvin" <hpa@...or.com>,
	David Miller <davem@...emloft.net>, ralf@...ux-mips.org,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	kernel@...savvy.com, torvalds@...ux-foundation.org
Subject: Re: 64-syscall args on 32-bit vs syscall()

On Wed, 2010-03-17 at 02:14 -0700, Ulrich Drepper wrote:
> >> I believe that should cover it, at least for powerpc, possibly for
> other
> >> archs too though as I said, I may have missed something there.
> 
> How can this possibly be the case?  This will screw people who
> currently
> work around the ppc limitations of the existing syscall.

No it won't. As I said, it will work for both cases. The problem is a
register pair alignment problem. If the alignment is corrected with the
trick I proposed, 64-bit values will end up in the right pair, but
manually worked-around cases where the value is already broken up will
-also- end up in the right pair.

The problem with syscall() as it is is that it skews the arguments by 1
register, which causes the compiler to skip a register when generating
the call for a 64-bit value. By doing the trick I propose, that skew
will be gone, both 32 and 64 bit arguments will end up where expected.

Cheers,
Ben.


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