Messages in this thread | | | Date | Thu, 15 Feb 2007 19:15:24 -0800 (PST) | From | Davide Libenzi <> | Subject | Re: [patch 09/14] syslets: x86, add move_user_context() method |
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On Thu, 15 Feb 2007, Davide Libenzi wrote:
> On Thu, 15 Feb 2007, Ingo Molnar wrote: > > > /* > > + * 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; > > +} > > Let's say that old_task ("prev" at the incoming schedule) has TS_USEDFPU > set. Its context gets moved to the new_task (the one returning to > userspace) *before* the __unlazy_fpu() done in __switch_to(). The > __unlazy_fpu() at the following schedule will save the state to the old > new_task context, and that fine as far as the going-to-sleep task goes. > The next fault happening in new_task (return to userspace one) will reload > a non up2date context (the one we got from old_task, but never hit by > the __unlazy_fpu() flush). Right?
Yeah. Given TS_USEDFPU set, 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.
- Davide
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