pci: Implement reassignable BARs

BARs are used by the guest to configure the access to the PCI device by
writing the address to which the device will respond. The basic idea for
adding support for reassignable BARs is straightforward: deactivate
emulation for the memory region described by the old BAR value, and
activate emulation for the new region.

BAR reassignment can be done while device access is enabled and memory
regions for different devices can overlap as long as no access is made to
the overlapping memory regions. This means that it is legal for the BARs of
two distinct devices to point to an overlapping memory region, and indeed,
this is how Linux does resource assignment at boot. To account for this
situation, the simple algorithm described above is enhanced to scan for all
devices and:

- Deactivate emulation for any BARs that might overlap with the new BAR
  value.

- Enable emulation for any BARs that were overlapping with the old value
  after the BAR has been updated.

Activating/deactivating emulation of a memory region has side effects.  In
order to prevent the execution of the same callback twice we now keep track
of the state of the region emulation. For example, this can happen if we
program a BAR with an address that overlaps a second BAR, thus deactivating
emulation for the second BAR, and then we disable all region accesses to
the second BAR by writing to the command register.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Link: https://lore.kernel.org/r/1589470709-4104-11-git-send-email-alexandru.elisei@arm.com


Signed-off-by: Will Deacon <will@kernel.org>