Re: [PATCH RFC v2] vPCI: account for hidden devices

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 24 May 2023, Jan Beulich wrote:
> >> RFC: _setup_hwdom_pci_devices()' loop may want splitting: For
> >>      modify_bars() to consistently respect BARs of hidden devices while
> >>      setting up "normal" ones (i.e. to avoid as much as possible the
> >>      "continue" path introduced here), setting up of the former may want
> >>      doing first.
> > 
> > But BARs of hidden devices should be mapped into dom0 physmap?
> 
> Yes.

The BARs would be mapped read-only (not read-write), right? Otherwise we
let dom0 access devices that belong to Xen, which doesn't seem like a
good idea.

But even if we map the BARs read-only, what is the benefit of mapping
them to Dom0? If Dom0 loads a driver for it and the driver wants to
initialize the device, the driver will crash because the MMIO region is
read-only instead of read-write, right?

How does this device hiding work for dom0? How does dom0 know not to
access a device that is present on the PCI bus but is used by Xen?

The reason why I was suggesting to hide the device completely in the
past was that I assumed we had to hide the device (make it "disappear"
on the PCI bus) otherwise Dom0 would access it. Is there another way to
mark a PCI devices as "inaccessible" or "disabled"?