Re: [Xen-devel] [RFC] ARM PCI Passthrough design document

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 30 May 2017, Julien Grall wrote:
> > > In this design document, we are considering that the host bridge driver
> > > can
> > > be ported in Xen. In the case it is not possible, a interface to forward
> > > configuration space access would need to be defined. The interface details
> > > is out of scope.
> > 
> > I think that you have to state that the driver is ported to Xen or the
> > bridge will not be supported. I don't think it's feasible to forward
> > PCI config space access from Xen to Dom0 at all.

Easy to say, but in practice there might be boards that we want to
support which require complex configurations.

Obviously having to send PCI config space read/write requests from Xen
to Dom0 is ugly and slow and doesn't match the Xen architecture, but it
might be the only solution in these cases. This is ARM: the ecosystem
has a lot more variety compared to x86: a single approach might simply
not be possible.

Another ugly (and fragile) idea to solve this problem would be to
initialize those difficult PCI host bridges in Dom0, then cede control
of them from Dom0 to Xen: I expect that once they are initialized, Xen
might be able to drive them more easily, without getting entangled with
clocks and regulators. 


> Rather than arguing on the code is not ready for that. I would have
> appreciated if you gave technical details on why it is not feasible.
> 
> I already gave quite a few times insights on why it might be difficult to port
> an host bridges in Xen.
>       - How do you configure the clock? What if they are shared?
>       - How about host bridges using indirect access (e.g cf8 like)? What
> you expose to DOM0?
>       - ....
> 
> Such host bridges will end up to pull a lot of code in Xen and require more
> design than finding about a way to forward configuration space in Xen. Those
> boards exists and people are looking at using Xen + PCI passthrough. So saying
> they are not supported is not the right solution here.

I agree


> Anyway, I mentioned it in the design document to open a discussion and not
> something I am going to focus for a first version of PCI pass-through.

Indeed: we'll cross that bridge when we get to it.

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