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

[Julien Grall <julien.grall@xxxxxxx>]

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

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel