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

[Ingo Molnar <mingo@elte.hu>]

* Tejun Heo <tj@kernel.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