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Re: [PATCH] virtio: move 'use-disabled-flag' property to hw_compat_4_2

From: Stefano Garzarella
Subject: Re: [PATCH] virtio: move 'use-disabled-flag' property to hw_compat_4_2
Date: Mon, 18 Jan 2021 10:19:55 +0100

Ping :-)

On Fri, Jan 08, 2021 at 06:12:52PM +0100, Stefano Garzarella wrote:
Commit 9d7bd0826f introduced a new 'use-disabled-flag' property
set to true by default.
To allow the migration, we set this property to false in the hw_compat,
but in the wrong place (hw_compat_4_1).

Since commit 9d7bd0826f was released with QEMU 5.0, we move
'use-disabled-flag' property to hw_compat_4_2, so 4.2 machine types
will have the pre-patch behavior and the migration can work.

The issue was discovered with vhost-vsock device and 4.2 machine
type without running any kernel in the VM:
   $ qemu-4.2 -M pc-q35-4.2,accel=kvm \
       -device vhost-vsock-pci,guest-cid=4 \
       -monitor stdio -incoming tcp:0:3333

   $ qemu-5.2 -M pc-q35-4.2,accel=kvm \
       -device vhost-vsock-pci,guest-cid=3 \
       -monitor stdio
   (qemu) migrate -d tcp:0:3333

   # qemu-4.2 output
   qemu-system-x86_64: Failed to load virtio-vhost_vsock:virtio
   qemu-system-x86_64: error while loading state for instance 0x0 of device 
   qemu-system-x86_64: load of migration failed: No such file or directory

Reported-by: Jing Zhao <jinzhao@redhat.com>
Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1907255
Fixes: 9d7bd0826f ("virtio-pci: disable vring processing when bus-mastering is 
Cc: mdroth@linux.vnet.ibm.com
CC: qemu-stable@nongnu.org
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
hw/core/machine.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/core/machine.c b/hw/core/machine.c
index de3b8f1b31..5d6163ab70 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -70,12 +70,12 @@ GlobalProperty hw_compat_4_2[] = {
    { "qxl", "revision", "4" },
    { "qxl-vga", "revision", "4" },
    { "fw_cfg", "acpi-mr-restore", "false" },
+    { "virtio-device", "use-disabled-flag", "false" },
const size_t hw_compat_4_2_len = G_N_ELEMENTS(hw_compat_4_2);

GlobalProperty hw_compat_4_1[] = {
    { "virtio-pci", "x-pcie-flr-init", "off" },
-    { "virtio-device", "use-disabled-flag", "false" },
const size_t hw_compat_4_1_len = G_N_ELEMENTS(hw_compat_4_1);


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