Re: [PATCH v2] virtio: skip legacy support check on machine types less than 5.1

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 15 Sep 2020 15:05:14 +0200
Stefano Garzarella <sgarzare@redhat.com> wrote:

> Commit 9b3a35ec82 ("virtio: verify that legacy support is not accidentally
> on") added a check that returns an error if legacy support is on, but the
> device is not legacy.
> 
> Unfortunately some devices were wrongly declared legacy even if they
> were not (e.g vhost-vsock).
> 
> To avoid migration issues, we disable this error for machine types < 5.1,
> but we print a warning.
> 
> Cc: qemu-stable@nongnu.org
> Fixes: 9b3a35ec82 ("virtio: verify that legacy support is not accidentally on")
> Suggested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> Suggested-by: Cornelia Huck <cohuck@redhat.com>
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> ---
> v2:
>  - fixed Cornelia's e-mail address
> ---
>  include/hw/virtio/virtio.h |  1 +
>  hw/core/machine.c          |  1 +
>  hw/virtio/virtio.c         | 18 ++++++++++++++++--
>  3 files changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> index 807280451b..ed7cee348b 100644
> --- a/include/hw/virtio/virtio.h
> +++ b/include/hw/virtio/virtio.h
> @@ -103,6 +103,7 @@ struct VirtIODevice
>      bool use_started;
>      bool started;
>      bool start_on_kick; /* when virtio 1.0 feature has not been negotiated */
> +    bool disable_legacy_check;
>      VMChangeStateEntry *vmstate;
>      char *bus_name;
>      uint8_t device_endian;
> diff --git a/hw/core/machine.c b/hw/core/machine.c
> index ea26d61237..b686eab798 100644
> --- a/hw/core/machine.c
> +++ b/hw/core/machine.c
> @@ -44,6 +44,7 @@ GlobalProperty hw_compat_5_0[] = {
>      { "vmport", "x-signal-unsupported-cmd", "off" },
>      { "vmport", "x-report-vmx-type", "off" },
>      { "vmport", "x-cmds-v2", "off" },
> +    { "virtio-device", "x-disable-legacy-check", "true" },

Hm... not sure if we actually should add a new device property for
that. Maybe we can use a flag in the base machine type instead?

>  };
>  const size_t hw_compat_5_0_len = G_N_ELEMENTS(hw_compat_5_0);
>  
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index e983025217..30ccc43b8c 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -3287,6 +3287,8 @@ void virtio_init(VirtIODevice *vdev, const char *name,
>   */
>  bool virtio_legacy_allowed(VirtIODevice *vdev)
>  {
> +    bool ret = false;
> +
>      switch (vdev->device_id) {
>      case VIRTIO_ID_NET:
>      case VIRTIO_ID_BLOCK:
> @@ -3298,10 +3300,20 @@ bool virtio_legacy_allowed(VirtIODevice *vdev)
>      case VIRTIO_ID_9P:
>      case VIRTIO_ID_RPROC_SERIAL:
>      case VIRTIO_ID_CAIF:
> +        ret = true;
> +    }
> +
> +    /*
> +     * For backward compatibility, we allow legacy mode with old machine types
> +     * to get the migration working.
> +     */
> +    if (!ret && vdev->disable_legacy_check) {
> +        warn_report("device is modern-only, but for backward compatibility "
> +                    "legacy is allowed");

I don't think we should warn in the function that returns whether
legacy is allowed or not. Future code might want to call this function
for other purposes, and returning true with a warning is not that
useful for code that wants to find out whether legacy is supported for
a type in principle.

So even though it is more work, I think we should move this warning to
the code that is actively trying to fence off misconfigured devices.

>          return true;
> -    default:
> -        return false;
>      }
> +
> +    return ret;
>  }
>  
>  hwaddr virtio_queue_get_desc_addr(VirtIODevice *vdev, int n)
> @@ -3713,6 +3725,8 @@ static Property virtio_properties[] = {
>      DEFINE_VIRTIO_COMMON_FEATURES(VirtIODevice, host_features),
>      DEFINE_PROP_BOOL("use-started", VirtIODevice, use_started, true),
>      DEFINE_PROP_BOOL("use-disabled-flag", VirtIODevice, use_disabled_flag, true),
> +    DEFINE_PROP_BOOL("x-disable-legacy-check", VirtIODevice,
> +                     disable_legacy_check, false),
>      DEFINE_PROP_END_OF_LIST(),
>  };
>