Re: [PATCH RFC] paravirt: cleanup lazy mode handling

[Rusty Russell <rusty@rustcorp.com.au>]

On Mon, 2007-10-01 at 23:29 -0700, Jeremy Fitzhardinge wrote: 
> Rusty Russell wrote:
> > That's good, but this code does lose on native because we no longer
> > simply replace the entire thing with noops.
> >
> > Perhaps inverting this and having (inline) helpers is the way to go?
> > 
> > static inline void paravirt_enter_lazy(enum paravirt_lazy_mode mode)
> > {
> > 	BUG_ON(x86_read_percpu(paravirt_lazy_mode) != PARAVIRT_LAZY_NONE);
> > 	BUG_ON(preemptible());
> >
> > 	x86_write_percpu(paravirt_lazy_mode, mode);
> > }
> >
> > static inline void paravirt_exit_lazy(enum paravirt_lazy_mode mode)
> > {
> > 	BUG_ON(x86_read_percpu(paravirt_lazy_mode) != mode);
> > 	BUG_ON(preemptible());
> >
> > 	x86_write_percpu(paravirt_lazy_mode, PARAVIRT_LAZY_NONE);
> > }
> >   
> 
> Er, they should probably call something to make the switch actually
> happen, no?

No, they're helpers.  eg:

static void lguest_exit_lazy(enum paravirt_lazy_mode mode)
{
paravirt_exit_lazy(mode);
lguest_flush_hcalls();
}

> > The only trick would be that the flushes are so rarely required it's
> > probably worth putting the unlikely() in the top level:
> 
> Sure, I guess.  Would it make any difference?  (I've never personally
> noticed likely/unlikely change the generated code in any seriously
> positive way.)

Probably overkill (I was trying to avoid the branch for the case where
we don't need to flush, as that's always what happens).

So just expose a flush hook:

static inline void arch_flush_lazy_cpu_mode(void)
{
	PVOP_VCALL1(flush_lazy_mode, PARAVIRT_LAZY_CPU);
}

....

static void lguest_flush_lazy_mode(enum paravirt_lazy_mode mode)
{
	if (unlikely(x86_read_percpu(paravirt_lazy_mode) == mode)) {
		lguest_lazy_cpu_leave();
		lguest_lazy_cpu_enter();
	}
}

Cheers,
Rusty.