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

[Julien Grall <julien.grall@xxxxxxxxxx>]

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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