linux-kernel - Re: [RFC PATCH] virtio_console: link vq to port with a private pointer in struct virtqueue

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Date:	Wed, 18 Apr 2012 16:34:12 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Amit Shah <amit.shah@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [RFC PATCH] virtio_console: link vq to port with a private pointer
 in struct virtqueue

Il 18/04/2012 16:21, Michael S. Tsirkin ha scritto:
>> @@ -1872,6 +1864,8 @@ static int virtcons_restore(struct virtio_device *vdev)
>>  	list_for_each_entry(port, &portdev->ports, list) {
>>  		port->in_vq = portdev->in_vqs[port->id];
>>  		port->out_vq = portdev->out_vqs[port->id];
>> +		port->in_vq->vdev_priv = port;
>> +		port->out_vq->vdev_priv = port;
>>  
>>  		fill_queue(port->in_vq, &port->inbuf_lock);
>>  
> 
> Let's add an API to set this pointer.
> Document that you must not set it after
> probe/restore returned.

Why?

> With an API we can actually have a BUG_ON that checks it's not modified
> after probe.
> 
>> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
>> index c193ccf..6b39c1a 100644
>> --- a/include/linux/virtio.h
>> +++ b/include/linux/virtio.h
>> @@ -14,6 +14,7 @@
>>   * @callback: the function to call when buffers are consumed (can be NULL).
>>   * @name: the name of this virtqueue (mainly for debugging)
>>   * @vdev: the virtio device this queue was created for.
>> + * @vdev_priv: a pointer for the virtio device to use.
> 
> It's for the driver actually.

Right.  However...

>>   * @priv: a pointer for the virtqueue implementation to use.
>>   */
>>  struct virtqueue {
>> @@ -21,6 +22,7 @@ struct virtqueue {
>>  	void (*callback)(struct virtqueue *vq);
>>  	const char *name;
>>  	struct virtio_device *vdev;
>> +	void *vdev_priv;
>>  	void *priv;
> 
> The name is confusing: it seems to imply it's a device pointer.

... it's private to the driver that owns vdev, hence the name.

> Maybe we should rename priv to something like __priv and make
> priv useful for devices?

I wanted to go for the smallest possible changes.  Right now we have 1
user for each member (virtio-ring vs. virtio-console) so neither member
is really dominating.

Paolo
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