Re: [RFC v1 5/5] xen/arm: add SCI mediator support for DomUs

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 19 Jan 2022, Oleksii Moisieiev wrote:
> On Fri, Dec 24, 2021 at 02:30:50PM +0100, Julien Grall wrote:
> > Hi,
> > 
> > On 23/12/2021 20:06, Stefano Stabellini wrote:
> > > On Wed, 22 Dec 2021, Stefano Stabellini wrote:
> > > > # Future Ideas
> > > > 
> > > > A great suggestion by Julien is to start supporting the dom0less partial
> > > > device tree format in xl/libxl as well so that we can have a single
> > > > "device_tree" option in dom.cfg instead of 4 (device_tree, iomem, irqs,
> > > > dtdev).
> > > > 
> > > > Even with that implemented, the user has still to provide a partial dtb,
> > > > xen,reg and xen,path. I think this is a great step forward and we should
> > > > do that, if nothing else to make it easier to switch between dom0less
> > > > and normal domU configurations. But the number of options and
> > > > information that the user has to provide is still similar to what we
> > > > have today.
> > > 
> > > I have just realized that if we start to parse the partial DTB in
> > > xl/libxl the same way that we do for dom0less guests (parse "xen,path",
> > > "xen,reg", and "interrupts", making dtdev, irqs and iomem optional)
> > > actually we can achieve the goal below thanks to the combination:
> > > "xen,path" + "xen,force-assign-without-iommu".
> > > 
> > > In other words, with dom0less we already have a way to specify the link
> > > to the host node even if the device is not a DMA master. We can do that
> > > by specifying both xen,path and xen,force-assign-without-iommu for a
> > > device.
> > > 
> > > This is just FYI. I am not suggesting we should introduce dom0less-style
> > > partial DTBs in order to get SCMI support in guests (although it would
> > > be great to have). I think the best way forward for this series is one
> > > of the combinations below, like a) + d), or a) + c)
> > 
> > I strongly prefer a) + c) because a warning is easy to miss/ignore. At least
> > with the extra property the user made an action to think about it and agree
> > that this is the way do it.
> > 
> > It is also easier to spot if we ask the user to provide the configuration
> > file.
> > 
> 
> Let me share my thoughts about c), which is:
> c) require force-assign-without-iommu="true" in dom.cfg
> 
> Adding this parameter to domain config means removing
> xen,force-assign-without-iommu param from partial DTB.

Why? No I don't think so.


> This will affect dom0less configuration, which I can't test for now
> without extra effort.

We are just talking about adding:

force-assign-without-iommu="true"

to the xl config file. For instance:

dtdev = [ "/amba/serial@ff000000" ]
iomem = ["0xff000,1"]
force-assign-without-iommu="true"

It is unrelated to the dom0less partial DTB. There is no need to test
dom0less to do this.