Messages in this thread | | | Date | Wed, 21 Feb 2007 15:03:33 -0800 (PST) | From | Davide Libenzi <> | Subject | Re: [patch 08/13] syslets: x86, add move_user_context() method |
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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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