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

[Oren Laadan <orenl@cs.columbia.edu>]

Matt Helsley wrote:
> 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@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
>> hit.
> 
> 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?

In c/r, we only need it at restart when a task calls it on itself.

However the interface itself of get_syscall_nr() can be called by
any task on another task.

(In fact, I think that for the most part, saving the syscall number
at checkpoint time may be better than figuring out at restart time).

Oren.