linux-kernel - Re: [PATCH net] vsock/virtio: enable VQs early on probe

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Date:   Tue, 22 Mar 2022 09:36:14 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        Asias He <asias@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Stefan Hajnoczi <stefanha@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net] vsock/virtio: enable VQs early on probe

On Tue, Mar 22, 2022 at 11:38:23AM +0100, Stefano Garzarella wrote:
> virtio spec requires drivers to set DRIVER_OK before using VQs.
> This is set automatically after probe returns, but virtio-vsock
> driver uses VQs in the probe function to fill rx and event VQs
> with new buffers.


So this is a spec violation. absolutely.

> Let's fix this, calling virtio_device_ready() before using VQs
> in the probe function.
> 
> Fixes: 0ea9e1d3a9e3 ("VSOCK: Introduce virtio_transport.ko")
> Signed-off-by: Stefano Garzarella <sgarzare@...hat.com>
> ---
>  net/vmw_vsock/virtio_transport.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
> index 5afc194a58bb..b1962f8cd502 100644
> --- a/net/vmw_vsock/virtio_transport.c
> +++ b/net/vmw_vsock/virtio_transport.c
> @@ -622,6 +622,8 @@ static int virtio_vsock_probe(struct virtio_device *vdev)
>  	INIT_WORK(&vsock->event_work, virtio_transport_event_work);
>  	INIT_WORK(&vsock->send_pkt_work, virtio_transport_send_pkt_work);
>  
> +	virtio_device_ready(vdev);
> +
>  	mutex_lock(&vsock->tx_lock);
>  	vsock->tx_run = true;
>  	mutex_unlock(&vsock->tx_lock);

Here's the whole code snippet:


        mutex_lock(&vsock->tx_lock);
        vsock->tx_run = true;
        mutex_unlock(&vsock->tx_lock);

        mutex_lock(&vsock->rx_lock);
        virtio_vsock_rx_fill(vsock);
        vsock->rx_run = true;
        mutex_unlock(&vsock->rx_lock);

        mutex_lock(&vsock->event_lock);
        virtio_vsock_event_fill(vsock);
        vsock->event_run = true;
        mutex_unlock(&vsock->event_lock);

        if (virtio_has_feature(vdev, VIRTIO_VSOCK_F_SEQPACKET))
                vsock->seqpacket_allow = true;

        vdev->priv = vsock;
        rcu_assign_pointer(the_virtio_vsock, vsock);

        mutex_unlock(&the_virtio_vsock_mutex);


I worry that this is not the only problem here:
seqpacket_allow and setting of vdev->priv at least after
device is active look suspicious.
E.g.:

static void virtio_vsock_event_done(struct virtqueue *vq)
{
        struct virtio_vsock *vsock = vq->vdev->priv;

        if (!vsock)
                return;
        queue_work(virtio_vsock_workqueue, &vsock->event_work);
}

looks like it will miss events now they will be reported earlier.
One might say that since vq has been kicked it might send
interrupts earlier too so not a new problem, but
there's a chance device actually waits until DRIVER_OK
to start operating.


> -- 
> 2.35.1