Re: [PATCH v12 8/8] virtio-balloon: VIRTIO_BALLOON_F_CMD_VQ

[Wei Wang]

On 07/13/2017 08:22 AM, Michael S. Tsirkin wrote:
> On Wed, Jul 12, 2017 at 08:40:21PM +0800, Wei Wang wrote:
> > Add a new vq, cmdq, to handle requests between the device and driver.
> >
> > This patch implements two commands sent from the device and handled in
> > the driver.
> > 1) VIRTIO_BALLOON_CMDQ_REPORT_STATS: this command is used to report
> > the guest memory statistics to the host. The stats_vq mechanism is not
> > used when the cmdq mechanism is enabled.
> > 2) VIRTIO_BALLOON_CMDQ_REPORT_UNUSED_PAGES: this command is used to
> > report the guest unused pages to the host.
> >
> > Since now we have a vq to handle multiple commands, we need to keep only
> > one vq operation at a time. Here, we change the existing START_USE()
> > and END_USE() to lock on each vq operation.
> >
> > Signed-off-by: Wei Wang <wei.w.wang@xxxxxxxxx>
> > Signed-off-by: Liang Li <liang.z.li@xxxxxxxxx>
> > ---
> >   drivers/virtio/virtio_balloon.c     | 245 ++++++++++++++++++++++++++++++++++--
> >   drivers/virtio/virtio_ring.c        |  25 +++-
> >   include/linux/virtio.h              |   2 +
> >   include/uapi/linux/virtio_balloon.h |  10 ++
> >   4 files changed, 265 insertions(+), 17 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> > index aa4e7ec..ae91fbf 100644
> > --- a/drivers/virtio/virtio_balloon.c
> > +++ b/drivers/virtio/virtio_balloon.c
> > @@ -54,11 +54,12 @@ static struct vfsmount *balloon_mnt;
> >   
> >   struct virtio_balloon {
> >   	struct virtio_device *vdev;
> > -	struct virtqueue *inflate_vq, *deflate_vq, *stats_vq;
> > +	struct virtqueue *inflate_vq, *deflate_vq, *stats_vq, *cmd_vq;
> >   
> >   	/* The balloon servicing is delegated to a freezable workqueue. */
> >   	struct work_struct update_balloon_stats_work;
> >   	struct work_struct update_balloon_size_work;
> > +	struct work_struct cmdq_handle_work;
> >   
> >   	/* Prevent updating balloon when it is being canceled. */
> >   	spinlock_t stop_update_lock;
> > @@ -90,6 +91,12 @@ struct virtio_balloon {
> >   	/* Memory statistics */
> >   	struct virtio_balloon_stat stats[VIRTIO_BALLOON_S_NR];
> >   
> > +	/* Cmdq msg buffer for memory statistics */
> > +	struct virtio_balloon_cmdq_hdr cmdq_stats_hdr;
> > +
> > +	/* Cmdq msg buffer for reporting ununsed pages */
> > +	struct virtio_balloon_cmdq_hdr cmdq_unused_page_hdr;
> > +
> >   	/* To register callback in oom notifier call chain */
> >   	struct notifier_block nb;
> >   };
> > @@ -485,25 +492,214 @@ static void update_balloon_size_func(struct work_struct *work)
> >   		queue_work(system_freezable_wq, work);
> >   }
> >   
> > +static unsigned int cmdq_hdr_add(struct virtqueue *vq,
> > +				 struct virtio_balloon_cmdq_hdr *hdr,
> > +				 bool in)
> > +{
> > +	unsigned int id = VIRTQUEUE_DESC_ID_INIT;
> > +	uint64_t hdr_pa = (uint64_t)virt_to_phys((void *)hdr);
> > +
> > +	virtqueue_add_chain_desc(vq, hdr_pa, sizeof(*hdr), &id, &id, in);
> > +
> > +	/* Deliver the hdr for the host to send commands. */
> > +	if (in) {
> > +		hdr->flags = 0;
> > +		virtqueue_add_chain(vq, id, 0, NULL, hdr, NULL);
> > +		virtqueue_kick(vq);
> > +	}
> > +
> > +	return id;
> > +}
> > +
> > +static void cmdq_add_chain_desc(struct virtio_balloon *vb,
> > +				struct virtio_balloon_cmdq_hdr *hdr,
> > +				uint64_t addr,
> > +				uint32_t len,
> > +				unsigned int *head_id,
> > +				unsigned int *prev_id)
> > +{
> > +retry:
> > +	if (*head_id == VIRTQUEUE_DESC_ID_INIT) {
> > +		*head_id = cmdq_hdr_add(vb->cmd_vq, hdr, 0);
> > +		*prev_id = *head_id;
> > +	}
> > +
> > +	virtqueue_add_chain_desc(vb->cmd_vq, addr, len, head_id, prev_id, 0);
> > +	if (*head_id == *prev_id) {
> That's an ugly way to detect ring full.

It's actually not detecting ring full. I will call it tail_id, instead 
of prev_id.
So, *head_id == *tail_id is the case that the first desc was just added by
 virtqueue_add_chain_desc().

Best,
Wei