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From: Rusty Russell <rusty@rustcorp.com.au>
To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Subject: Re: [patch 5/8] Immediate Values - x86 Optimization (simplified)
Date: Tue, 20 Nov 2007 10:06:34 +1100
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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.

> 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.

Cheers,
Rusty.