Re: [Xen-devel] [PATCH v5 7/8] xen/arm: introduce nr_spis

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 9/26/19 2:25 AM, Stefano Stabellini wrote:
> On Wed, 25 Sep 2019, Julien Grall wrote:
> > Hi,
> >
> > On 25/09/2019 19:49, 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.
> > Based on the code below, this is not correct when using vpl011.
> I'll write:
>
> The property is optional. When absent, we'll use the physical number of
> GIC lines for dom0-less domains, or GUEST_VPL011_SPI+1 if vpl011 is
> requested, whichever is greater.
Sounds good to me.

> > > Remove the old setting of nr_spis based on the presence of the vpl011.
> > >
> > > 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). For instance, we
> > > could end up exposing 256 SPIs for each dom0less domain without a
> > > nr_spis property. If we have 4 dom0less domains without nr_spis, it
> > > would result in 80K of additional memory being used.
> > I don't understand what you are trying to imply with your example. Ok,
> > this tell you how much memory you are going to waste... but this does
> > still not explain why the nr_spis are increased in the default case.
> I misunderstood what you wanted me to add to the commit message.
Sorry for the confusion, my main point is you can't really say this is 
low footprint as this is very subjective. Personally, I feel it is a lot 
because if you take the example, this is roughly 8% of the current size 
of Xen (in default config).
> I'll remove the example and instead write:
>
> 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). This is done because
> the SPIs allocation needs to be done before parsing any passthrough
> information, so we have to account for any potential physical SPI
> assigned to the domain.
>
>
> Is this better?
Yes, thank you.

Cheers,

--
Julien Grall

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