xen-tools -- Unmediated PCI register access in qemu

The Xen Project reports:

Qemu allows guests to not only read, but also write all parts of
the PCI config space (but not extended config space) of passed
through PCI devices not explicitly dealt with for (partial)
emulation purposes.
Since the effect depends on the specific purpose of the the config
space field, it’s not possible to give a general statement about the
exact impact on the host or other guests. Privilege escalation,
host crash (Denial of Service), and leaked information all cannot be
excluded.