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Re: [Qemu-devel] [PATCH v14 5/5] virtio-balloon: VIRTIO_BALLOON_F_FREE_P

From: Wei Wang
Subject: Re: [Qemu-devel] [PATCH v14 5/5] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ
Date: Fri, 18 Aug 2017 16:36:06 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

On 08/18/2017 10:28 AM, Michael S. Tsirkin wrote:
On Thu, Aug 17, 2017 at 11:26:56AM +0800, Wei Wang wrote:
Add a new vq to report hints of guest free pages to the host.

Signed-off-by: Wei Wang <address@hidden>
Signed-off-by: Liang Li <address@hidden>
  drivers/virtio/virtio_balloon.c     | 167 +++++++++++++++++++++++++++++++-----
  include/uapi/linux/virtio_balloon.h |   1 +
  2 files changed, 147 insertions(+), 21 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 72041b4..e6755bc 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, *free_page_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 report_free_page_work;
/* Prevent updating balloon when it is being canceled. */
        spinlock_t stop_update_lock;
@@ -90,6 +91,13 @@ struct virtio_balloon {
        /* Memory statistics */
        struct virtio_balloon_stat stats[VIRTIO_BALLOON_S_NR];
+ /*
+        * Used by the device and driver to signal each other.
+        * device->driver: start the free page report.
+        * driver->device: end the free page report.
+        */
+       __virtio32 report_free_page_signal;
        /* To register callback in oom notifier call chain */
        struct notifier_block nb;
@@ -174,6 +182,17 @@ static void send_balloon_page_sg(struct virtio_balloon *vb,
        } while (unlikely(ret == -ENOSPC));
+static void send_free_page_sg(struct virtqueue *vq, void *addr, uint32_t size)
+       unsigned int len;
+       add_one_sg(vq, addr, size);
+       virtqueue_kick(vq);
+       /* Release entries if there are */
+       while (virtqueue_get_buf(vq, &len))
+               ;
   * Send balloon pages in sgs to host. The balloon pages are recorded in the
   * page xbitmap. Each bit in the bitmap corresponds to a page of PAGE_SIZE.
@@ -511,42 +530,143 @@ static void update_balloon_size_func(struct work_struct 
                queue_work(system_freezable_wq, work);
+static void virtio_balloon_send_free_pages(void *opaque, unsigned long pfn,
+                                          unsigned long nr_pages)
+       struct virtio_balloon *vb = (struct virtio_balloon *)opaque;
+       void *addr = (void *)pfn_to_kaddr(pfn);
+       uint32_t len = nr_pages << PAGE_SHIFT;
+       send_free_page_sg(vb->free_page_vq, addr, len);
+static void report_free_page_completion(struct virtio_balloon *vb)
+       struct virtqueue *vq = vb->free_page_vq;
+       struct scatterlist sg;
+       unsigned int len;
+       int ret;
+       sg_init_one(&sg, &vb->report_free_page_signal, sizeof(__virtio32));
+       ret = virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
+       virtqueue_kick(vq);
+       if (unlikely(ret == -ENOSPC)) {
+               wait_event(vb->acked, virtqueue_get_buf(vq, &len));
+               goto retry;
+       }
So the annoying thing here is that once this starts going,
it will keep sending free pages from the list even if
host is no longer interested. There should be a way
for host to tell guest "stop" or "start from the beginning".

This can be achieved via two output signal buf here:
signal_buf_start: filled with VIRTIO_BALLOON_F_FREE_PAGE_REPORT_START
signal_buf_end: filled with VIRTIO_BALLOON_F_FREE_PAGE_REPORT_END

The device holds both, and can put one of them to the vq and notify.

It's the result of using same vq for guest to host and
host to guest communication, and I think it's not a great idea.
I'd reuse stats vq for host to guest requests maybe.

As we discussed before, we can't have a vq interleave the report of stats and free pages. The vq will be locked when one command is in use. So, when live migration starts, the periodically reported stats will be delayed. Would this be OK? Or would you like to have
one host to guest vq, and multiple host to guest vqs? That is,

- host to guest:

- guest to host:


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