linux-kernel - Re: [C/R ARM][PATCH 1/3] ARM: Rudimentary syscall interfaces

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Message-ID: <20100325011753.GF5704@count0.beaverton.ibm.com>
Date:	Wed, 24 Mar 2010 18:17:53 -0700
From:	Matt Helsley <matthltc@...ibm.com>
To:	Matt Helsley <matthltc@...ibm.com>
Cc:	Christoffer Dall <christofferdall@...istofferdall.dk>,
	Oren Laadan <orenl@...columbia.edu>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	containers <containers@...ts.linux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Roland McGrath <roland@...hat.com>
Subject: Re: [C/R ARM][PATCH 1/3] ARM: Rudimentary syscall interfaces

On Wed, Mar 24, 2010 at 06:11:32PM -0700, Matt Helsley wrote:
> On Wed, Mar 24, 2010 at 08:36:39PM +0100, Christoffer Dall wrote:
<snip>
> > Re-using the assembly code or factoring it out so that it can be used
> > from multiple places doesn't seem very pleasing to me, as the assembly
> > code is in the critical path and written specifically for the context
> > of a process entering the kernel. Please correct me if I'm wrong.
> > 
> > I imagine simply a function in C, more or less re-implementing the
> > logic that's already in entry-common.S, might do the trick. I wouldn't
> > worry much about the performance in this case as it will not be used
> > often. The following _untested_ snippet illustrates my idea:
> > 
> > ---
> >  arch/arm/include/asm/syscall.h |   93 +++++++++++++++++++++++++++++++++++++++-
> >  1 files changed, 92 insertions(+), 1 deletions(-)
> > 
> > diff --git a/arch/arm/include/asm/syscall.h b/arch/arm/include/asm/syscall.h
> > index 3b3248f..a7f2615 100644
> > --- a/arch/arm/include/asm/syscall.h
> > +++ b/arch/arm/include/asm/syscall.h
> > @@ -10,10 +10,101 @@
> >  #ifndef _ASM_ARM_SYSCALLS_H
> >  #define _ASM_ARM_SYSCALLS_H
> > 
> > +static inline int get_swi_instruction(struct task_struct *task,
> > +				      struct pt_regs *regs,
> > +				      unsigned long *instr)
> > +{
> > +	struct page *page = NULL;
> > +	unsigned long instr_addr;
> > +	unsigned long *ptr;
> > +	int ret;
> > +
> > +	instr_addr = regs->ARM_pc - 4;
> > +
> > +	down_read(&task->mm->mmap_sem);
> > +	ret = get_user_pages(task, task->mm, instr_addr,
> > +			     1, 0, 0, &page, NULL);
> > +	up_read(&task->mm->mmap_sem);
> > +
> > +	if (ret < 0)
> > +		return ret;
> > +
> > +	ptr = (unsigned long *)kmap_atomic(page, KM_USER1);
> > +	memcpy(instr,
> > +	       ptr + (instr_addr >> PAGE_SHIFT),
> 			^shouldn't this be:
> 		      instr_addr & PAGE_MASK

Oops, made my own mistake. I think the address of the kmap'd instruction
would be:

	ptr + (instr_addr & ~PAGE_MASK)

Cheers,
	-Matt Helsley
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