pci: do not disable memory decoding for devices

pci: do not disable memory decoding for devices

Commit 75cc460a1b added checks to ensure the position of the BARs from
PCI devices don't overlap with regions defined on the memory map.
When there's a collision memory decoding is left disabled for the
device, assuming that dom0 will reposition the BAR if necessary and
enable memory decoding.

While this would be the case for devices being used by dom0, devices
being used by the firmware itself that have no driver would usually be
left with memory decoding disabled by dom0 if that's the state dom0
found them in, and thus firmware trying to make use of them will not
function correctly.

The initial intent of 75cc460a1b was to prevent vPCI from creating
MMIO mappings on the dom0 p2m over regions that would otherwise
already have mappings established.  It's my view now that we likely
went too far with 75cc460a1b, and Xen disabling memory decoding of
devices (as buggy as they might be) is harmful, and reduces the set of
hardware on which Xen works.

This commits reverts most of 75cc460a1b, and instead adds checks to
vPCI in order to prevent misplaced BARs from being added to the
hardware domain p2m.  Signaling on whether BARs are mapped is tracked
in the vpci structure, so that misplaced BARs are not mapped, and thus
Xen won't attempt to unmap them when memory decoding is disabled.

This restores the behavior of Xen for PV dom0 to the state it was
previous to 75cc460a1b, while also introducing a more contained fix
for the vPCI BAR mapping issues.

Fixes: 75cc460a1b ('xen/pci: detect when BARs are not suitably positioned')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>