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

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Message-ID: <7d08b87d1003250329t589ba41er2f1127190be078cf@mail.gmail.com>
Date:	Thu, 25 Mar 2010 11:29:45 +0100
From:	Christoffer Dall <christofferdall@...istofferdall.dk>
To:	Matt Helsley <matthltc@...ibm.com>
Cc:	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 Thu, Mar 25, 2010 at 2:17 AM, Matt Helsley <matthltc@...ibm.com> wrote:
> 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)
>

Yes. Thanks for pointing it out.
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