[PATCH] pci: a note on non-transitional devices

["Michael S. Tsirkin" <mst@redhat.com>]

Add a note documenting handling legacy drivers for
non transitional devices.

VIRTIO-34

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 virtio-v1.0-wd01-part1-specification.txt | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/virtio-v1.0-wd01-part1-specification.txt b/virtio-v1.0-wd01-part1-specification.txt
index 8a649f2..6ab4ff8 100644
--- a/virtio-v1.0-wd01-part1-specification.txt
+++ b/virtio-v1.0-wd01-part1-specification.txt
@@ -1171,7 +1171,22 @@ space in BAR0.
 
 Transitional drivers should look for the Virtio PCI
 Capabilities on the capability list.
-If there are not present, driver should assume a legacy device.
+If these are not present, driver should assume a legacy device.
+
+Non-transitional drivers should look for the Virtio PCI
+Capabilities on the capability list.
+If these are not present, driver should assume a legacy device,
+and fail gracefully.
+
+Non-transitional devices, on a platform where a legacy driver for
+a legacy device with the same ID might have previously existed,
+must take the following steps to fail gracefully when a legacy
+driver attempts to drive them:
+
+1) Present an I/O BAR in BAR0, and
+2) Respond to a single-byte zero write to offset 18
+   (corresponding to Device Status register in the legacy layout)
+   of BAR0 by presenting zeroes on every BAR and ignoring writes.
 
 2.3.1.3.1.1. Queue Vector Configuration
 --------------------------------------
-- 
MST