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



Hi,

On 24/09/2019 18:56, Stefano Stabellini wrote:
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.

This is good to address my first remark. How about the others?

Cheers,

--
Julien Grall

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