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

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Date:	Tue, 28 Jan 2014 18:00:10 +0000
From:	Julien Grall <julien.grall@...aro.org>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
CC:	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 01/28/2014 05:46 PM, Stefano Stabellini wrote:
> 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?

We want to make sure that the vcpu is registered correctly. If not, we
can't skip it and avoid xen to have a "dead" VCPU to schedule due to BUG_ON.

I agree that now we have a BUG_ON in the middle of xen_percpu_init, but
it's possible to return an error. In this case Linux will skip this cpu
and continue to boot.

>>> 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.
>

I don't see any issue to execute xen_percpu_init for cpu1 on cpu0, all
the code is taking directly the vcpu ID to initialize.

-- 
Julien Grall
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