Re: [PATCH v2 QEMU] virtio-balloon: Provide a interface for "bubble hinting"

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jul 24, 2019 at 10:12:10AM -0700, Alexander Duyck wrote:
> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> 
> Add support for what I am referring to as "bubble hinting". Basically the
> idea is to function very similar to how the balloon works in that we
> basically end up madvising the page as not being used. However we don't
> really need to bother with any deflate type logic since the page will be
> faulted back into the guest when it is read or written to.
> 
> This is meant to be a simplification of the existing balloon interface
> to use for providing hints to what memory needs to be freed. I am assuming
> this is safe to do as the deflate logic does not actually appear to do very
> much other than tracking what subpages have been released and which ones
> haven't.
> 
> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>

BTW I wonder about migration here.  When we migrate we lose all hints
right?  Well destination could be smarter, detect that page is full of
0s and just map a zero page. Then we don't need a hint as such - but I
don't think it's done like that ATM.


I also wonder about interaction with deflate.  ATM deflate will add
pages to the free list, then balloon will come right back and report
them as free.


> ---
>  hw/virtio/virtio-balloon.c                      |   40 +++++++++++++++++++++++
>  include/hw/virtio/virtio-balloon.h              |    2 +
>  include/standard-headers/linux/virtio_balloon.h |    1 +
>  3 files changed, 42 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> index 2112874055fb..70c0004c0f88 100644
> --- a/hw/virtio/virtio-balloon.c
> +++ b/hw/virtio/virtio-balloon.c
> @@ -328,6 +328,39 @@ static void balloon_stats_set_poll_interval(Object *obj, Visitor *v,
>      balloon_stats_change_timer(s, 0);
>  }
>  
> +static void virtio_bubble_handle_output(VirtIODevice *vdev, VirtQueue *vq)
> +{
> +    VirtQueueElement *elem;
> +
> +    while ((elem = virtqueue_pop(vq, sizeof(VirtQueueElement)))) {
> +    	unsigned int i;
> +
> +        for (i = 0; i < elem->in_num; i++) {
> +            void *addr = elem->in_sg[i].iov_base;
> +            size_t size = elem->in_sg[i].iov_len;
> +            ram_addr_t ram_offset;
> +            size_t rb_page_size;
> +            RAMBlock *rb;
> +
> +            if (qemu_balloon_is_inhibited())
> +                continue;
> +
> +            rb = qemu_ram_block_from_host(addr, false, &ram_offset);
> +            rb_page_size = qemu_ram_pagesize(rb);
> +
> +            /* For now we will simply ignore unaligned memory regions */
> +            if ((ram_offset | size) & (rb_page_size - 1))
> +                continue;
> +
> +            ram_block_discard_range(rb, ram_offset, size);
> +        }
> +
> +        virtqueue_push(vq, elem, 0);
> +        virtio_notify(vdev, vq);
> +        g_free(elem);
> +    }
> +}
> +
>  static void virtio_balloon_handle_output(VirtIODevice *vdev, VirtQueue *vq)
>  {
>      VirtIOBalloon *s = VIRTIO_BALLOON(vdev);
> @@ -782,6 +815,11 @@ static void virtio_balloon_device_realize(DeviceState *dev, Error **errp)
>      s->svq = virtio_add_queue(vdev, 128, virtio_balloon_receive_stats);
>  
>      if (virtio_has_feature(s->host_features,
> +                           VIRTIO_BALLOON_F_HINTING)) {
> +        s->hvq = virtio_add_queue(vdev, 128, virtio_bubble_handle_output);
> +    }
> +
> +    if (virtio_has_feature(s->host_features,
>                             VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
>          s->free_page_vq = virtio_add_queue(vdev, VIRTQUEUE_MAX_SIZE,
>                                             virtio_balloon_handle_free_page_vq);
> @@ -897,6 +935,8 @@ static Property virtio_balloon_properties[] = {
>                      VIRTIO_BALLOON_F_DEFLATE_ON_OOM, false),
>      DEFINE_PROP_BIT("free-page-hint", VirtIOBalloon, host_features,
>                      VIRTIO_BALLOON_F_FREE_PAGE_HINT, false),
> +    DEFINE_PROP_BIT("guest-page-hinting", VirtIOBalloon, host_features,
> +                    VIRTIO_BALLOON_F_HINTING, true),
>      DEFINE_PROP_LINK("iothread", VirtIOBalloon, iothread, TYPE_IOTHREAD,
>                       IOThread *),
>      DEFINE_PROP_END_OF_LIST(),
> diff --git a/include/hw/virtio/virtio-balloon.h b/include/hw/virtio/virtio-balloon.h
> index 1afafb12f6bc..a58b24fdf29d 100644
> --- a/include/hw/virtio/virtio-balloon.h
> +++ b/include/hw/virtio/virtio-balloon.h
> @@ -44,7 +44,7 @@ enum virtio_balloon_free_page_report_status {
>  
>  typedef struct VirtIOBalloon {
>      VirtIODevice parent_obj;
> -    VirtQueue *ivq, *dvq, *svq, *free_page_vq;
> +    VirtQueue *ivq, *dvq, *svq, *free_page_vq, *hvq;
>      uint32_t free_page_report_status;
>      uint32_t num_pages;
>      uint32_t actual;
> diff --git a/include/standard-headers/linux/virtio_balloon.h b/include/standard-headers/linux/virtio_balloon.h
> index 9375ca2a70de..f9e3e8256261 100644
> --- a/include/standard-headers/linux/virtio_balloon.h
> +++ b/include/standard-headers/linux/virtio_balloon.h
> @@ -36,6 +36,7 @@
>  #define VIRTIO_BALLOON_F_DEFLATE_ON_OOM	2 /* Deflate balloon on OOM */
>  #define VIRTIO_BALLOON_F_FREE_PAGE_HINT	3 /* VQ to report free pages */
>  #define VIRTIO_BALLOON_F_PAGE_POISON	4 /* Guest is using page poisoning */
> +#define VIRTIO_BALLOON_F_HINTING	5 /* Page hinting virtqueue */
>  
>  /* Size of a PFN in the balloon interface. */
>  #define VIRTIO_BALLOON_PFN_SHIFT 12