[PATCH v2] docs: Document reserved PCI addresses for QEMU

[Michal Privoznik]

>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: Michal Privoznik <mprivozn at redhat.com>
---

diff to v1:
- All Laine's comment worked in, hopefully.

 docs/pci-addresses.rst | 50 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/docs/pci-addresses.rst b/docs/pci-addresses.rst
index 885d50517a..4f123c7eb9 100644
--- a/docs/pci-addresses.rst
+++ b/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 ``I440FX`` 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):
+
+============  ==================
+0000:00:02.0  primary video card
+============  ==================
+
+
+For ``Q35`` 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
+============  ===============
-- 
2.25.3