linux-kernel - Re: [PATCH v12 8/8] virtio-balloon: VIRTIO_BALLOON_F_CMD

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Date:   Thu, 13 Jul 2017 16:46:29 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
CC:     linux-kernel@...r.kernel.org, qemu-devel@...gnu.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, david@...hat.com, cornelia.huck@...ibm.com,
        akpm@...ux-foundation.org, mgorman@...hsingularity.net,
        aarcange@...hat.com, amit.shah@...hat.com, pbonzini@...hat.com,
        liliang.opensource@...il.com, virtio-dev@...ts.oasis-open.org,
        yang.zhang.wz@...il.com, quan.xu@...yun.com
Subject: Re: [PATCH v12 8/8] virtio-balloon: VIRTIO_BALLOON_F_CMD_VQ

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@...el.com>
>> Signed-off-by: Liang Li <liang.z.li@...el.com>
>> ---
>>   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