Re: RFC: unlazy fpu for frequent fpu users

[Arjan van de Ven <arjan@infradead.org>]

> You can do better that that.  FXSR doesn't destroy the FPU contents; if
> you track the context carefully you can completely avoid the restore.
> This requires keeping a per-cpu variable that holds a pointer to the


to be honest, while I like the idea, it does scare me from a security
point of view, both in terms of leaks and in terms of injecting bad
stuff. 

> > --- linux-2.6.17-sleazyfpu.orig/arch/x86_64/kernel/process.c
> > +++ linux-2.6.17-sleazyfpu/arch/x86_64/kernel/process.c
> > @@ -515,6 +515,9 @@ __switch_to(struct task_struct *prev_p, 
> >       int cpu = smp_processor_id();  
> >       struct tss_struct *tss = &per_cpu(init_tss, cpu);
> >  
> > +     /* prefetch the fxsave area into the cache */
> > +     prefetch(&next->i387.fxsave);
> > +
> >       /*
> >        * Reload esp0, LDT and the page table pointer:
> >        */
> 
> This prefetch is probably a bad idea.  I ported your patch to i386 and it was
> actually slower until I changed it:
> 
> +       if (next_p->fpu_counter > 5)
> +               /* prefetch the fxsave area into the cache */
> +               prefetch(&next->i387.fxsave);
> +
> 
> Now it's ~.4% faster.  The test was an FP program doing a simple benchmark
> while a non-FP program ran in a tight loop.

nice! I sort of am not a big fan of if .. prefetch() but if it shows
gain... then you convinced me. 0.4% is roughly the same order I saw.
It's not gigantic but it's almost free to do so it may be worth it
anyway... 
Can you send me your patch so that I can integrate it (and I'll port
your if() to the prefetch)... 

Greetings,
    Arjan van de Ven

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