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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Feb 01, 2017 at 10:50:49AM -0800, Stefano Stabellini wrote:
> On Wed, 1 Feb 2017, Roger Pau Monné wrote:
> > On Wed, Jan 25, 2017 at 06:53:20PM +0000, Julien Grall wrote:
> > > Hi Stefano,
> > > 
> > > On 24/01/17 20:07, Stefano Stabellini wrote:
> > > > On Tue, 24 Jan 2017, Julien Grall wrote:
> > > When using ECAM like host bridge, I don't think it will be an issue to 
> > > have
> > > both DOM0 and Xen accessing configuration space at the same time. 
> > > Although,
> > > we need to define who is doing what. In general case, DOM0 should not
> > > touched an assigned PCI device. The only possible interaction would be
> > > resetting a device (see my answer below).
> > 
> > Iff Xen is really going to perform the reset of passthrough devices, then I
> > don't see any reason to expose those devices to Dom0 at all, IMHO you should
> > hide them from ACPI and ideally prevent Dom0 from interacting with them 
> > using
> > the PCI configuration space (although that would require trapping on 
> > accesses
> > to the PCI config space, which AFAIK you would like to avoid).
> 
> Right! A much cleaner solution! If we are going to have Xen handle ECAM
> and emulating PCI host bridges, then we should go all the way and have
> Xen do everything about PCI.

Replying here because this thread has become so long that's hard to find a good
place to put this information.

I've recently been told (f2f), that more complex passthrough (like Nvidia vGPU
or Intel XenGT) work in a slightly different way, which seems to be a bit
incompatible with what we are proposing. I've been told that Nvidia vGPU
passthrough requires a driver in Dom0 (closed-source Nvidia code AFAIK), and
that upon loading this driver a bunch of virtual functions appear out of the
blue in the PCI bus.

Now, if we completely hide passed-through devices from Dom0, it would be
impossible to load this driver, and thus to make the virtual functions appear.
I would like someone that's more familiar with this to comment, so I'm adding
Paul and Anshul to the conversation.

To give some context to them, we were currently discussing to completely hide
passthrough PCI devices from Dom0, and have Xen perform the reset of the
device. This would apply to PVH and ARM. Can you comment on whether such
approach would work with things like vGPU passthrough?

Roger.


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