[C/R ARM][PATCH 1/3] ARM: Rudimentary syscall interfaces
matthltc at us.ibm.com
Wed Mar 24 21:11:32 EDT 2010
On Wed, Mar 24, 2010 at 08:36:39PM +0100, Christoffer Dall wrote:
> On Wed, Mar 24, 2010 at 4:53 PM, Oren Laadan <orenl at cs.columbia.edu> wrote:
> > Matt Helsley wrote:
> >> On Wed, Mar 24, 2010 at 12:57:46AM -0400, Oren Laadan wrote:
> >>> On Tue, 23 Mar 2010, Matt Helsley wrote:
> >>>> On Tue, Mar 23, 2010 at 08:53:42PM +0000, Russell King - ARM Linux
> >>>> wrote:
> >>>>> On Sun, Mar 21, 2010 at 09:06:03PM -0400, Christoffer Dall wrote:
> >>>>>> This small commit introduces a global state of system calls for ARM
> >>>>>> making it possible for a debugger or checkpointing to gain information
> >>>>>> about another process' state with respect to system calls.
> >>>>> I don't particularly like the idea that we always store the syscall
> >>>>> number to memory for every system call, whether the stored version is
> >>>>> used or not.
> >>>>> Since ARM caches are generally not write allocate, this means mostly
> >>>>> write-only variables can have a higher than expected expense.
> >>>>> Is there not some thread flag which can be checked to see if we need to
> >>>>> store the syscall number?
> >>>> Perhaps before we freeze the task we can save the syscall number on ARM.
> >>>> The patches suggest that the signal delivery path -- which the freezer
> >>>> utilizes -- has the syscall number already.
> > Actually, the signal path doesn't have the syscall number, it has
> > a binary "in syscall" value.
Argh. I read too much into the name :(.
> Well, this could be changed to pass the syscall number through
> registers along to try_to_freeze without any mentionable performance
Yes, that's possible. I was thinking we could still use your thread info
field but only store to it when we know it will be useful for c/r rather
than for each syscall. Personally, I'd rather avoid passing the extra
parameter into try_to_freeze(). Your idea below seems better to me.
> 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
> + sizeof(unsigned long));
> + kunmap_atomic(ptr, KM_USER1);
> + page_cache_release(page);
> + return 0;
(again, not familiar with ARM so my understanding is:
I guess swi is "syscall word immediate".
The syscall nr is embedded in the instruction as an immediate
value and you're getting a copy of that instruction using the value of
the pc register just after the syscall instruction was executed.)
Perhaps I am missing or forgetting something. Why isn't this as simple
as calling get_user() or even copy_from_user() using instr_addr?
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