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

[Julien Grall <julien.grall@linaro.org>]

(Adding Christoffer)

Hi Ian,

On 09/09/14 07:34, Ian Campbell wrote:
> 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.

I don't see many other solutions to get the compatible strings. There is 
no easy way to get the property from DOM0, unless we introduce a new 
driver in Linux.

> 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?)

This would need a bit a plumbing for irq part to allow the user choosing 
the VIRQ (like Arianna did for MMIO range).

 > 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.

I would be fine with this solution for Xen 4.5. I will give a look to 
see what could be done.

> 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.

Depending of the use cases, your solution based on "iomem", "irq" and DT 
blob might be enough for device platform passthrough.

Regards,

-- 
Julien Grall