Re: [PATCH v3 QEMU 2/2] virtio-balloon: Provide a interface for unused page reporting

[Nitesh Narayan Lal <nitesh@redhat.com>]

On 8/1/19 6:43 PM, Alexander Duyck wrote:
> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>
> Add support for what I am referring to as "unused page reporting".
> 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>
> ---
>  hw/virtio/virtio-balloon.c                      |   46 ++++++++++++++++++++++-
>  include/hw/virtio/virtio-balloon.h              |    2 +
>  include/standard-headers/linux/virtio_balloon.h |    1 +
>  3 files changed, 46 insertions(+), 3 deletions(-)
>
> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> index 003b3ebcfdfb..7a30df63bc77 100644
> --- a/hw/virtio/virtio-balloon.c
> +++ b/hw/virtio/virtio-balloon.c
> @@ -320,6 +320,40 @@ static void balloon_stats_set_poll_interval(Object *obj, Visitor *v,
>      balloon_stats_change_timer(s, 0);
>  }
>  
> +static void virtio_balloon_handle_report(VirtIODevice *vdev, VirtQueue *vq)
> +{
> +    VirtIOBalloon *dev = VIRTIO_BALLOON(vdev);
> +    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() || dev->poison_val)
> +                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);
> @@ -627,7 +661,8 @@ static size_t virtio_balloon_config_size(VirtIOBalloon *s)
>          return sizeof(struct virtio_balloon_config);
>      }
>      if (virtio_has_feature(features, VIRTIO_BALLOON_F_PAGE_POISON) ||
> -        virtio_has_feature(features, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
> +        virtio_has_feature(features, VIRTIO_BALLOON_F_FREE_PAGE_HINT) ||
> +        virtio_has_feature(features, VIRTIO_BALLOON_F_REPORTING)) {
>          return sizeof(struct virtio_balloon_config);
>      }
>      return offsetof(struct virtio_balloon_config, free_page_report_cmd_id);
> @@ -715,7 +750,8 @@ static uint64_t virtio_balloon_get_features(VirtIODevice *vdev, uint64_t f,
>      VirtIOBalloon *dev = VIRTIO_BALLOON(vdev);
>      f |= dev->host_features;
>      virtio_add_feature(&f, VIRTIO_BALLOON_F_STATS_VQ);
> -    if (virtio_has_feature(f, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
> +    if (virtio_has_feature(f, VIRTIO_BALLOON_F_FREE_PAGE_HINT) ||
> +        virtio_has_feature(f, VIRTIO_BALLOON_F_REPORTING)) {
>          virtio_add_feature(&f, VIRTIO_BALLOON_F_PAGE_POISON);
>      }
>  
> @@ -805,6 +841,10 @@ static void virtio_balloon_device_realize(DeviceState *dev, Error **errp)
>      s->dvq = virtio_add_queue(vdev, 128, virtio_balloon_handle_output);
>      s->svq = virtio_add_queue(vdev, 128, virtio_balloon_receive_stats);
>  
> +    if (virtio_has_feature(s->host_features, VIRTIO_BALLOON_F_REPORTING)) {
> +        s->rvq = virtio_add_queue(vdev, 32, virtio_balloon_handle_report);
> +    }
> +
This does makes sense. I haven't seen the kernel patch yet, but I am guessing
you will use this max_vq size to define the capacity.
>      if (virtio_has_feature(s->host_features,
>                             VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
>          s->free_page_vq = virtio_add_queue(vdev, VIRTQUEUE_MAX_SIZE,
> @@ -931,6 +971,8 @@ static Property virtio_balloon_properties[] = {
>       */
>      DEFINE_PROP_BOOL("qemu-4-0-config-size", VirtIOBalloon,
>                       qemu_4_0_config_size, false),
> +    DEFINE_PROP_BIT("unused-page-reporting", VirtIOBalloon, host_features,
> +                    VIRTIO_BALLOON_F_REPORTING, 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 7fe78e5c14d7..db5bf7127112 100644
> --- a/include/hw/virtio/virtio-balloon.h
> +++ b/include/hw/virtio/virtio-balloon.h
> @@ -42,7 +42,7 @@ 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, *rvq;
>      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..1c5f6d6f2de6 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_REPORTING	5 /* Page reporting virtqueue */

Do we really need this change? or is this something which is picked from the
Linux kernel?
If we do need it, then Cornelia suggested to split off any update to this header
into a separate patch, so that it can be replaced by a proper headers update
when it is merged.

>  
>  /* Size of a PFN in the balloon interface. */
>  #define VIRTIO_BALLOON_PFN_SHIFT 12
>
-- 
Thanks
Nitesh