Re: virtio: why no full reset on virtio_set_status 0 ?

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

On Wed, Jul 27, 2022 at 12:51:31PM +0200, Claudio Fontana wrote:
> Hi Michael and all,
> 
> I have started researching a qemu / ovs / dpdk bug:
> 
> https://inbox.dpdk.org/dev/322122fb-619d-96f6-5c3e-9eabdbf3819a@redhat.com/T/
> 
> that seems to be affecting multiple parties in the telco space,
> 
> and during this process I noticed that qemu/hw/virtio/virtio.c does not do a full virtio reset
> in virtio_set_status, when receiving a status value of 0.
> 
> It seems it has always been this way, so I am clearly missing / forgetting something basic,
> 
> I checked the virtio spec at https://docs.oasis-open.org/
> 
> and from:
> 
> "
> 4.1.4.3 Common configuration structure layout
> 
> device_status
> The driver writes the device status here (see 2.1). Writing 0 into this field resets the device.
> 
> "
> 
> and
> 
> "
> 2.4.1 Device Requirements: Device Reset
> A device MUST reinitialize device status to 0 after receiving a reset.
> "
> 
> I would conclude that in virtio.c::virtio_set_status we should unconditionally do a full virtio_reset.
> 
> Instead, we have just the check:
> 
>     if ((vdev->status & VIRTIO_CONFIG_S_DRIVER_OK) !=
>         (val & VIRTIO_CONFIG_S_DRIVER_OK)) {
>         virtio_set_started(vdev, val & VIRTIO_CONFIG_S_DRIVER_OK);
>     }
> 
> which just sets the started field,
> 
> and then we have the call to the virtio device class set_status (virtio_net...),
> but the VirtioDevice is not fully reset, as per the virtio_reset() call we are missing:
> 
> "
>     vdev->start_on_kick = false;
>     vdev->started = false;
>     vdev->broken = false;
>     vdev->guest_features = 0;
>     vdev->queue_sel = 0;
>     vdev->status = 0;
>     vdev->disabled = false;
>     qatomic_set(&vdev->isr, 0);
>     vdev->config_vector = VIRTIO_NO_VECTOR;
>     virtio_notify_vector(vdev, vdev->config_vector);
> 
>     for(i = 0; i < VIRTIO_QUEUE_MAX; i++) {
>         ... initialize vdev->vq[i] ...
>     }
> "
> 
> Doing a full reset seems to fix the problem for me, so I can send tentative patches if necessary,
> but what am I missing here?
> 
> Thanks,
> 
> Claudio

I went looking at this code and found a bug, so
I just sent a patch fixing it:
https://lore.kernel.org/r/20220727161445.21428-1-mst%40redhat.com

no idea whether it's related but can't hurt to try.


> -- 
> Claudio Fontana
> Engineering Manager Virtualization, SUSE Labs Core
> 
> SUSE Software Solutions Italy Srl