linux-kernel - Re: [PATCH 2/3] x86: Pass in pt_regs pointer for syscalls that need it

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Message-ID: <20090211150144.GD10525@elte.hu>
Date:	Wed, 11 Feb 2009 16:01:44 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] x86: Pass in pt_regs pointer for syscalls that
	need it


* Brian Gerst <brgerst@...il.com> wrote:

> On Wed, Feb 11, 2009 at 2:41 AM, Tejun Heo <tj@...nel.org> wrote:
> > Hello, Brian.
> >
> > Brian Gerst wrote:
> >> Some syscalls need to access the pt_regs structure, either to copy
> >> user register state or to modifiy it.  This patch adds stubs to load
> >> the address of the pt_regs struct into the %eax register, and changes
> >> the syscalls to regparm(1) to receive the pt_regs pointer as the
> >> first argument.
> >
> > Heh... neat.  Just one question.
> >
> >> -asmlinkage long sys_iopl(unsigned long regsp)
> >> +ptregscall long sys_iopl(struct pt_regs *regs, unsigned int level)
> >>  {
> >> -     struct pt_regs *regs = (struct pt_regs *)&regsp;
> >> -     unsigned int level = regs->bx;
> >
> > Here and at other places where the function takes more than one
> > arguments, wouldn't it be better to just take *regs and use other
> > parameters from regs?  That way we won't have to worry about gcc
> > corrupting register frame at all and I think it's cleaner that way.
> 
> Expanding the parameters is good documentation.  [...]

Well, that way we shuffle the parameter expansion into assembly code,
instead of creating it as a local variable in the C function.

The latter sure looks better documented, and less error-prone as
well, right? The compiler might also be able to optimize it some.
(and we save one instruction in any case)

	Ingo
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