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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 11 Sep 2019, Julien Grall wrote:
> 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".
 
What do you think of the following:

The implication of this change is that without nr_spis dom0less domains
get the same amount of SPI allocated as dom0, regardless of how many
physical devices they have assigned, and regardless of whether they have
a virtual pl011 (which also needs an emulated SPI).

When nr_spis is present, the domain gets exactly nr_spis allocated SPIs.
If the number is too low, it might not be enough for the devices
assigned it to it. If the number is less than GUEST_VPL011_SPI, the
virtual pl011 won't work.

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