Re: [patch 09/14] syslets: x86, add move_user_context() method

[Davide Libenzi]

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?



- Davide

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