Re: [Xen-devel] [PATCH v4 2/2] arm: prefer PSCI for SMP bringup

[Rob Herring <robherring2@xxxxxxxxx>]

On 03/29/2013 12:53 PM, Nicolas Pitre wrote:
> On Fri, 29 Mar 2013, Stefano Stabellini wrote:
> 
>> On Fri, 29 Mar 2013, Nicolas Pitre wrote:
>>> On Fri, 29 Mar 2013, Stefano Stabellini wrote:
>>>
>>>> If PSCI initializes correctly and PSCI SMP operations are available, use 
>>>> them.
>>>> This is required for SMP support in Dom0 on Xen.
>>>>
>>>> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
>>>> CC: will.deacon@xxxxxxx
>>>> CC: arnd@xxxxxxxx
>>>> CC: marc.zyngier@xxxxxxx
>>>> CC: linux@xxxxxxxxxxxxxxxx
>>>> CC: nico@xxxxxxxxxx
>>>
>>> I'd suggest you also include in your series the patch I posted earlier 
>>> providing a runtime mdesc->smp_init method as well.
>>
>> OK.
>>
>>
>>> This way the 
>>> priority order would be:
>>>
>>> - If mdesc->smp_init is non null then use that.
>>>
>>> - Otherwise, if PSCI is available then use that.
>>>
>>> - Otherwise use mdesc->smp.
>>>
>>> This way, if the PSCI default has to be overriden (like in the MCPM case 
>>> because it needs to wrap PSCI itself, or to cover Rob's concern) then 
>>> this can be achieved at run time on a per mdesc basis.
>>
>> Actually that's not a bad idea, it could make everybody happy.
>> What about the following, in this precise order:
>>
>> - if a xen hypervisor node is present on device tree, use PSCI;
>> - otherwise if mdesc->smp_init is non null then use it;
>> - otherwise if PSCI is available then use it;
>> - otherwise use mdesc->smp.
>>
>> It's the most practical solution to satisfy everybody's needs.
> 
> Maybe I'm missing something obvious, but why can't xen declare a mdesc 
> of its own?  Given it is going to tweak the DT passed to the kernel 
> anyway that shouldn't be a problem.

Xen does have it's own mdesc. It is (or will be) mach-virt, but that is
only for DomU guests. For Dom0, you still need all the platform specific
code except smp_ops. However, I'm doubtful this would work without other
changes on more complicated platforms like OMAP.

I would say wait to add this until you have platforms that actually need
the first case.

Rob

> 
> That would be more eleguant than adding xen exception hooks in generic 
> code.
> 
> 
> Nicolas
> 
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> 


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