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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Stefano Garzarella (sgarzare@redhat.com) 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.

Be a little careful that fixing this probably causes a migration from
5.2->6.0 to fail with this machine type;  so when we do these
type of fixes we often fix an old machine type between some pair of qemu
versions and then break it between a different set.

Dave

> 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 '0000:00:03.0/virtio-vhost_vsock'
>     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 disabled")
> 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);
>  
> -- 
> 2.26.2
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK