Re: [PATCH RFC] vPCI: account for hidden devices in modify_bars()

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

On Mon, Aug 30, 2021 at 03:04:55PM +0200, Jan Beulich wrote:
> Hidden devices (e.g. an add-in PCI serial card used for Xen's serial
> console) are associated with DomXEN, not Dom0. This means that while
> looking for overlapping BARs such devices cannot be found on Dom0's
> list of devices; DomXEN's list also needs to be scanned.

Thanks for looking into this, I certainly didn't take hidden devices
into account for vPCI dom0.

> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> RFC: Patch intentionally mis-formatted, as the necessary re-indentation
>      would make the diff difficult to read. At this point I'd merely
>      like to gather input towards possible better approaches to solve
>      the issue (not the least because quite possibly there are further
>      places needing changing).

I have a couple of questions, AFAICT we currently hide the serial
console and/or the VGA adapter if it's in use by Xen.

I wonder whether we need to add vPCI handlers for those:
setup_one_hwdom_device will attempt to add vPCI handlers to the hidden
device because of the temporary override of pdev->domain done in
_setup_hwdom_pci_devices, but I think that for hidden devices we
shouldn't add vPCI handlers. We should instead block PCI config space
access from dom0 to the device so that it doesn't mess with Xen usage.

It's also not clear why does Xen want to have those hidden devices
partly controlled by dom0, as it would seem to me that dom0 interfering
with hidden devices in use by Xen can only lead to trouble.

Thanks, Roger.