Re: [Xen-devel] Design doc of adding ACPI support for arm64 on Xen - version 2

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.08.15 at 11:05, <ian.campbell@xxxxxxxxxx> wrote:
> The other reason for modification is to hide the Xen console device (i.e.
> one UART) from the dom0 kernel, since it is unusable. How does that work on
> x86? Do you just not bother and you expect the admin to arrange the
> configuration to work or is there some other trick?

When it's I/O port based we simply disallow access to the ports (i.e.
Dom0 reads return all ones, Dom0 writes get dropped). When it's
MMIO based (which iirc became an option only in the not so distant
past) we can't always do that, since there could be other devices
on the same MMIO page. In any event we then pci_hide_devices() or
pci_ro_device() the device (see ns16550_init_postirq()) to limit the
damage Dom0 can do.

> BTW, IIRC x86 does modify at least one ACPI table which is the DMAR (I
> think), to hide the IOMMU from the guest? That's another table we would
> want to frob on ARM I think (or it's equivalent, which I think is IORT).

Eliminating that hack is supposed to be on the VT-d maintainers'
TODO list(s) - Dom0 has no business looking at that table (and its
AMD counterpart already isn't being fiddled with in the same way).

Jan


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