From e4cf85bd3ad44aed28cf3e8d3bfd67fec38ebdab Mon Sep 17 00:00:00 2001
From: Laszlo Ersek <lersek@redhat.com>
Date: Thu, 6 Jan 2022 15:09:05 +0100
Subject: [PATCH] output/create_libvirt_xml: pick "virtio-transitional" models
 when needed

In the domain XML we generate, we do not assign PCI B/D/F addresses to
devices; that job is left to libvirtd. When using the Q35 machine type,
libvirtd places the virtio devices into PCI Express Root Ports. As a
consequence, QEMU disables virtio-0.9.5 support on these devices, and so
guest OSes without virtio-1.0 drivers cannot drive them.

Prevent QEMU from turning off the virtio-0.9.5 ("legacy") protocol by
specifying the "virtio-transitional" (not "virtio") model for our virtio
devices:

- For non-disk devices, simply change the value of the existent "model"
  attribute.

- For disk devices, add the "model" attribute as a new one.

(In fact, libvirtd doesn't (only) add the "disable_legacy=off" QEMU device
property for the "virtio-transitional" devices -- libvirtd even moves
these devices from PCI Express Root Ports to a dedicated
"pcie-to-pci-bridge". This has the same effect on QEMU.)

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1942325
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20220106140910.13695-5-lersek@redhat.com>
Acked-by: Richard W.M. Jones <rjones@redhat.com>
---
 output/create_libvirt_xml.ml | 28 ++++++++++++++++++----------
 1 file changed, 18 insertions(+), 10 deletions(-)

diff --git a/output/create_libvirt_xml.ml b/output/create_libvirt_xml.ml
index 9413cc0b..87bfab17 100644
--- a/output/create_libvirt_xml.ml
+++ b/output/create_libvirt_xml.ml
@@ -312,18 +312,26 @@ let create_libvirt_xml ?pool source inspect
   (* The devices. *)
   let devices = ref [] in
 
+  (* This will affect all of the virtio devices (if any). *)
+  let virtio_transitional =
+    guestcaps.gcaps_machine = Q35 && not guestcaps.gcaps_virtio_1_0 in
+  let virtio_model =
+    if virtio_transitional then "virtio-transitional" else "virtio" in
+
   (* Fixed and removable disks. *)
   let () =
-    let make_disk bus_name drive_prefix i = function
+    let make_disk bus_name ?(viotrans = false) drive_prefix i = function
     | BusSlotEmpty -> Comment (sprintf "%s slot %d is empty" bus_name i)
 
     | BusSlotDisk d ->
        let outdisk = outdisk_name d.s_disk_id in
 
-        e "disk" [
-          "type", if pool = None then "file" else "volume";
-          "device", "disk"
-        ] [
+        e "disk" (
+          [
+            "type", if pool = None then "file" else "volume";
+            "device", "disk"
+          ] @ if (viotrans) then [ "model", "virtio-transitional" ] else []
+        ) [
           e "driver" [
             "name", "qemu";
             "type", output_format;
@@ -364,7 +372,7 @@ let create_libvirt_xml ?pool source inspect
     in
 
     List.push_back_list devices
-      (List.mapi (make_disk "virtio" "vd")
+      (List.mapi (make_disk "virtio" ~viotrans:virtio_transitional "vd")
                  (Array.to_list target_buses.target_virtio_blk_bus));
     let ide_disks =
       match guestcaps.gcaps_machine with
@@ -392,7 +400,7 @@ let create_libvirt_xml ?pool source inspect
   let nics =
     let net_model =
       match guestcaps.gcaps_net_bus with
-      | Virtio_net -> "virtio" | E1000 -> "e1000" | RTL8139 -> "rtl8139" in
+      | Virtio_net -> virtio_model | E1000 -> "e1000" | RTL8139 -> "rtl8139" in
     List.map (
       fun { s_mac = mac; s_vnet_type = vnet_type; s_vnet = vnet } ->
         let vnet_type_str =
@@ -483,7 +491,7 @@ let create_libvirt_xml ?pool source inspect
   (* Miscellaneous KVM devices. *)
   if guestcaps.gcaps_virtio_rng then
     List.push_back devices (
-      e "rng" ["model", "virtio"] [
+      e "rng" ["model", virtio_model] [
         (* XXX Using /dev/urandom requires libvirt >= 1.3.4.  Libvirt
          * was broken before that.
          *)
@@ -496,7 +504,7 @@ let create_libvirt_xml ?pool source inspect
   List.push_back devices (
     e "memballoon"
       ["model",
-       if guestcaps.gcaps_virtio_balloon then "virtio" else "none"]
+       if guestcaps.gcaps_virtio_balloon then virtio_model else "none"]
       []
   );
   if guestcaps.gcaps_isa_pvpanic then
@@ -508,7 +516,7 @@ let create_libvirt_xml ?pool source inspect
   List.push_back devices (
     e "viosock"
       ["model",
-        if guestcaps.gcaps_virtio_socket then "virtio" else "none"]
+        if guestcaps.gcaps_virtio_socket then virtio_model else "none"]
        []
   );
 
-- 
2.31.1