docs: Document reserved PCI addresses for QEMU

From time to time we are asked which PCI addresses are reserved
in QEMU. Let's document them in one place, it's easier than
reconstructing the list from the code each time.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NLaine Stump <laine@redhat.com>

docs/pci-addresses.rst

@@ -235,3 +235,53 @@ guest OS rather than as ``0001:08:00.1``, which is the address of the
 device on the host.
 
 Of course, all the rules and behaviors described above still apply.
+
+
+Reserved addresses
+==================
+
+Due to some historical reasons hypervisors might expect some PCI
+devices to appear at certain addresses instead of 'random' ones.
+For QEMU this is machine type and guest architecture dependant.
+But to give you at least a gist here is list of reserved PCI
+addresses:
+
+For the x86_64 architecture's ``I440FX``-based machine types the following
+devices are hard coded into QEMU and can't be moved or eliminated:
+
+============  ======================
+0000:00:00.0  Host bridge
+0000:00:01.0  ISA bridge
+0000:00:01.1  primary IDE controller
+0000:00:01.2  PIIX3 USB controller
+0000:00:01.3  ACPI (power management) and SMBus controller
+============  ======================
+
+The following addresses will be used as default ones for the corresponding
+devices (if the address is free or a different address wasn't provided for the
+device). It is okay to use this address for any other device.
+
+============  ==================
+0000:00:02.0  primary video card
+============  ==================
+
+
+For the x86_64 architecture's ``Q35``-based machine types the following
+devices are hard coded into QEMU and can't be moved or eliminated:
+
+============  =======================
+0000:00:00.0  Host bridge
+0000:00:1f.2  primary SATA controller
+0000:00:1f.0  ISA bridge
+0000:00:1f.3  SMBus
+============  =======================
+
+The following addresses will be used as default ones for the corresponding
+devices (if the address is free or a different address wasn't provided for the
+device) because that's how real ``Q35`` would do it:
+
+============  ===============
+0000:00:1a.0  USB2 controller
+0000:00:1b.0  ICH9 sound chip
+0000:00:1d.0  USB2 controller
+============  ===============