Re: [PATCH v2 00/21] xen/arm: Add support for non-pci passthrough

[Ian Campbell <Ian.Campbell@citrix.com>]

On Thu, 2014-07-31 at 16:00 +0100, Julien Grall wrote:
>     - Only common device properties (interrupts, regs) are written to
>     the guest device tree. Device that needs other properties may not work.

So I've glanced through the later (more toolstack oriented) bits from
towards the end but I think there's a question of the target users which
needs thinking about before I can have a sensible opinion on those.

As I see it the main purpose of this series is to get the underlying
plumbing in place (wiring up iommus, routing IRQs etc) to support guests
with passthrough devices, for embedded folks to use and to provide a
basis for eventual PCI passthrough functionality. I really want to see
this stuff in 4.5

What I'm concerned about is the toolstack side. TBH I'm not very keen on
the thing with exposing very DT specific stuff like compatible strings
down from the hypervisor via domctls.

It's not really clear how best to expose this functionality, I have a
feeling that this series either goes too far or not far enough and ends
up not really satisfying anyone.

My suspicion is that regular folks won't really be using passthrough
until it is via PCI and that in the meantime this functionality is only
going to be used by e.g. people building embedded system and superkeen
early adopters both of whom know what they are doing and can tolerate
some hacks etc to get things working (and I think that's fine, it's
still a worthwhile set of things to get into 4.5 and those folks are
worth supporting).

I'm also worried that we may be committing ourselves to a libxl API
already without really working through all the issues (e.g. other
properties).

Given that I wonder if we wouldn't be better off for 4.5 supporting
something much simpler at the toolstack level, namely allowing users to
use iomem= and irq= in their domain config to map platform devices
through (already works with your series today?) and perhaps a back door
to allow the injection a blob of DT into the guest's DT to describe
them. i.e. enough to actually get stuff done but not pretending to be
too finely integrated.

Then we can revisit the "proper" toolstack side for 4.6. Otherwise I
fear that by the time we get the toolstack side sorted out to our
satisfaction the basic functionality (which seems to be largely done)
will have missed 4.5.

Ian.