Re: [Xen-devel] [PATCH v4 7/8] xen/arm: introduce nr_spis

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 8/21/19 4:53 AM, Stefano Stabellini wrote:
> We don't have a clear way to know how many virtual SPIs we need for the
> dom0-less domains. Introduce a new option under xen,domain to specify
> the number of SPIs to allocate for a domain.
>
> The property is optional. When absent, we'll use the physical number of
> GIC lines for dom0-less domains, just like for dom0. Given that
> dom0-less VMs are meant for static partitioning scenarios where the
> number of VMs is very low, increased memory overhead should not be a
> problem, and it is possible to minimize it using "nr_spis".
>
> Remove the old setting of nr_spis based on the presence of the vpl011.
I am afraid this still does not explain the implications of this patch 
to current setup (with and without VPL011).
For instance, with your change, VPL011 may not work anymore. Imagine we 
decide to push the vpl011 interrupt towards the end of the Interrupt ID 
space (i.e. 1019).
I don't think we want the user to have to select nr_spis by himself for 
this case.
Regarding the change without vpl011, this is not explained why all the 
domains (even the one without SPIs routed) will have SPIs exposed. For 
instance, if you were to expose 256 interrupts for 4 domains, this will 
roughly use 80KB of memory. I don't think this is what you had in mind 
as "low footprint".
Cheers,

--
Julien Grall

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