Re: [PATCH 1/4] x86/fpu: document user_fpu_begin()

Oleg Nesterov
2015-03-13T14:37:01Z

On 03/13, Borislav Petkov wrote:
>
> On Wed, Mar 11, 2015 at 06:34:09PM +0100, Oleg Nesterov wrote:
> > @@ -464,7 +464,9 @@ static inline int restore_xstate_sig(void __user *buf, int ia32_frame)
> > * Need to be preemption-safe.
> > *
> > * NOTE! user_fpu_begin() must be used only immediately before restoring
> > - * it. This function does not do any save/restore on their own.
> > + * it. This function does not do any save/restore on its own. In a lazy
> > + * fpu mode this is just optimization to avoid a dna fault, the task can
> > + * lose FPU right after preempt_enable().
> > */
>
> I cleaned it up a bit more, if you don't mind:
...

> /*
> - * Need to be preemption-safe.
> + * Needs to be preemption-safe.
> *
> * NOTE! user_fpu_begin() must be used only immediately before restoring
> - * it. This function does not do any save/restore on their own.
> + * the save state. It does not do any saving/restoring on its own. In
> + * lazy FPU mode, it is just an optimization to avoid a #NM exception,
> + * the task can lose the FPU right after preempt_enable().
> */

Thank!

Oleg.