Re: [patch 08/13] syslets: x86, add move_user_context() method

[Davide Libenzi <davidel@xmailserver.org>]

On Wed, 21 Feb 2007, Ingo Molnar wrote:

> From: Ingo Molnar <mingo@elte.hu>
> 
> add the move_user_context() method to move the user-space
> context of one kernel thread to another kernel thread.
> User-space might notice the changed TID, but execution,
> stack and register contents (general purpose and FPU) are
> still the same.

Also signal handling should/must be maintained, on top of TID.
You don't want the user to be presented with a different signal handling 
after an sys_async_exec call.




> Signed-off-by: Ingo Molnar <mingo@elte.hu>
> Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
> ---
>  arch/i386/kernel/process.c |   21 +++++++++++++++++++++
>  include/asm-i386/system.h  |    7 +++++++
>  2 files changed, 28 insertions(+)
> 
> Index: linux/arch/i386/kernel/process.c
> ===================================================================
> --- linux.orig/arch/i386/kernel/process.c
> +++ linux/arch/i386/kernel/process.c
> @@ -820,6 +820,27 @@ unsigned long get_wchan(struct task_stru
>  }
>  
>  /*
> + * Move user-space context from one kernel thread to another.
> + * This includes registers and FPU state. Callers must make
> + * sure that neither task is running user context at the moment:
> + */
> +void
> +move_user_context(struct task_struct *new_task, struct task_struct *old_task)
> +{
> +	struct pt_regs *old_regs = task_pt_regs(old_task);
> +	struct pt_regs *new_regs = task_pt_regs(new_task);
> +	union i387_union *tmp;
> +
> +	*new_regs = *old_regs;
> +	/*
> +	 * Flip around the FPU state too:
> +	 */
> +	tmp = new_task->thread.i387;
> +	new_task->thread.i387 = old_task->thread.i387;
> +	old_task->thread.i387 = tmp;
> +}

This is not going to work in this case (already posted twice in other 
emails):

---
Given TS_USEDFPU set (NTSK == new_task, OTSK == old_task), before 
move_user_context():

CPU  => FPUc
NTSK => FPUn
OTSK => FPUo

After move_user_context():

CPU  => FPUc
NTSK => FPUo
OTSK => FPUn

After the incoming __unlazy_fpu() in __switch_to():

CPU  => FPUc
NTSK => FPUo
OTSK => FPUc

After the first fault in NTSK:

CPU  => FPUo
NTSK => FPUo
OTSK => FPUc

So NTSK loads a non up2date FPUo, instead of the FPUc that was the "dirty" 
context to migrate (since TS_USEDFPU was set). I think you need an early 
__unlazy_fpu() in that case, that would turn the above into:

Before move_user_context():

CPU  => FPUc
NTSK => FPUn
OTSK => FPUo

After an early __unlazy_fpu() before FPU member swap:

CPU  => FPUc
NTSK => FPUn
OTSK => FPUc

After move_user_context():

CPU  => FPUc
NTSK => FPUc
OTSK => FPUn

After the first fault in NTSK:

CPU  => FPUc
NTSK => FPUc
OTSK => FPUn

So, NTSK (the return-to-userspace task) will get the correct FPUc after a 
fault. But the OTSK (now becoming service thread) will load FPUn after a 
fault, that is not what expected. You may need a copy in that case.
I think correct FPU context handling is not going to be as easy as 
swapping FPU pointers.



- Davide


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