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.
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