linux-kernel - Re: [Xen-devel] [PATCH] arm/xen: Initialize event channels earlier

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Date:	Tue, 28 Jan 2014 17:46:44 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Julien Grall <julien.grall@...aro.org>
CC:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Ian Campbell <ian.campbell@...rix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Vrabel <david.vrabel@...rix.com>,
	"patches@...aro.org" <patches@...aro.org>,
	<xen-devel@...ts.xenproject.org>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Xen-devel] [PATCH] arm/xen: Initialize event channels earlier

On Tue, 28 Jan 2014, Julien Grall wrote:
> >> +static int xen_cpu_notification(struct notifier_block *self,
> >> +				unsigned long action,
> >> +				void *hcpu)
> >> +{
> >> +	int cpu = (long)hcpu;
> >> +
> >> +	switch (action) {
> >> +	case CPU_UP_PREPARE:
> >> +		xen_percpu_init(cpu);
> >> +		break;
> >> +	case CPU_STARTING:
> >> +		xen_interrupt_init();
> >> +		break;
> > 
> > Is CPU_STARTING guaranteed to be called on the new cpu only?
> 
> Yes.
> 
> > If so, why not call both xen_percpu_init and xen_interrupt_init on
> > CPU_STARTING?
> 
> Just in case that xen_vcpu is used somewhere else by a cpu notifier
> callback CPU_STARTING. We don't know which callback is called first.

Could you please elaborate a bit more on the problem you are trying to
describe?


> > As it stands I think you introduced a subtle change (that might be OK
> > but I think is unintentional): xen_percpu_init might not be called from
> > the same cpu as its target anymore.
> 
> No, xen_percpu_init and xen_interrupt_init are called on the boot cpu at
> the end of xen_guest_init.
 
Is CPU_UP_PREPARE guaranteed to be called on the target cpu? I think
not, therefore you would be executing xen_percpu_init for cpu1 on cpu0.
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