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

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Message-ID: <20090211101846.GH20518@elte.hu>
Date:	Wed, 11 Feb 2009 11:18:46 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Tejun Heo <tj@...nel.org>
Cc:	Brian Gerst <brgerst@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] x86: Pass in pt_regs pointer for syscalls that
	need it


* 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.

Hm, gcc cannot corrupt register arguments only on-stack arguments - but
your suggestion nevertheless makes sense as an optimization. I'd suggest
this to be done as a separate patch though, both for regression analysis
reasons (easier to bisect - the patch is large enough already) and from
a size/performance analysis POV. (so we can see the benefits in isolation)

	Ingo
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