Re: [patch 5/8] Immediate Values - x86 Optimization (simplified)

Mathieu Desnoyers
2007-11-20T17:12:34Z

* Rusty Russell (rusty@rustcorp.com.au) wrote:
> On Tuesday 20 November 2007 01:28:03 Mathieu Desnoyers wrote:
> > * Rusty Russell (rusty@rustcorp.com.au) wrote:
> > > I think it would be easier to just fast-path the num_online_cpus == 1
> > > case, even if you want to keep this "update_early" interface.
> >
> > Nope, that could lead to problems. I call core_immediate_update()
> > _very_ early, before boot_cpu_init() is called.
>
> Ah, I see the problem. It would in fact be clearer for us to move
> boot_cpu_init() up to just after smp_setup_processor_id() in start_kernel
> anyway, not just for this code, but in general.
>

Could be done.

> > Therefore,
> > cpu_online_map is not set yet. I am not sure the benefit of using
> > num_online_cpus outweights the added fragility wrt other boot process
> > initializations.
>
> I think it's still a win, though worth a comment that we always go via the
> non-IPI path for the early boot case.
>

Ok, another potential problem then :

If we fast-path the num_online_cpus == 1, then updates done after the
boot process will have to go through this code. Therefore, we would have
to disable interrupts in the early boot code.

However, I doubt it is safe to use the paravirtualized
local_irq_disable/enable before the paravirt code is executed ?

Mathieu

> Cheers,
> Rusty.