Re: [RFC for QEMU] virtio-balloon: Add option thp-order to set VIRTIO_BALLOON_F_THP_ORDER

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

On Thu, Mar 12, 2020 at 03:49:55PM +0800, Hui Zhu wrote:
> If the guest kernel has many fragmentation pages, use virtio_balloon
> will split THP of QEMU when it calls MADV_DONTNEED madvise to release
> the balloon pages.
> Set option thp-order to on will open flags VIRTIO_BALLOON_F_THP_ORDER.
> It will set balloon size to THP size to handle the THP split issue.
> 
> Signed-off-by: Hui Zhu <teawaterz@linux.alibaba.com>

What's wrong with just using the PartiallyBalloonedPage machinery
instead? That would make it guest transparent.

> ---
>  hw/virtio/virtio-balloon.c                      | 67 ++++++++++++++++---------
>  include/standard-headers/linux/virtio_balloon.h |  4 ++
>  2 files changed, 47 insertions(+), 24 deletions(-)
> 
> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> index a4729f7..cfe86b0 100644
> --- a/hw/virtio/virtio-balloon.c
> +++ b/hw/virtio/virtio-balloon.c
> @@ -340,37 +340,49 @@ static void virtio_balloon_handle_output(VirtIODevice *vdev, VirtQueue *vq)
>          while (iov_to_buf(elem->out_sg, elem->out_num, offset, &pfn, 4) == 4) {
>              unsigned int p = virtio_ldl_p(vdev, &pfn);
>              hwaddr pa;
> +            size_t handle_size = BALLOON_PAGE_SIZE;
>  
>              pa = (hwaddr) p << VIRTIO_BALLOON_PFN_SHIFT;
>              offset += 4;
>  
> -            section = memory_region_find(get_system_memory(), pa,
> -                                         BALLOON_PAGE_SIZE);
> -            if (!section.mr) {
> -                trace_virtio_balloon_bad_addr(pa);
> -                continue;
> -            }
> -            if (!memory_region_is_ram(section.mr) ||
> -                memory_region_is_rom(section.mr) ||
> -                memory_region_is_romd(section.mr)) {
> -                trace_virtio_balloon_bad_addr(pa);
> -                memory_region_unref(section.mr);
> -                continue;
> -            }
> +            if (virtio_has_feature(s->host_features,
> +                                   VIRTIO_BALLOON_F_THP_ORDER))
> +                handle_size = BALLOON_PAGE_SIZE << VIRTIO_BALLOON_THP_ORDER;
> +
> +            while (handle_size > 0) {
> +                section = memory_region_find(get_system_memory(), pa,
> +                                             BALLOON_PAGE_SIZE);
> +                if (!section.mr) {
> +                    trace_virtio_balloon_bad_addr(pa);
> +                    continue;
> +                }
> +                if (!memory_region_is_ram(section.mr) ||
> +                    memory_region_is_rom(section.mr) ||
> +                    memory_region_is_romd(section.mr)) {
> +                    trace_virtio_balloon_bad_addr(pa);
> +                    memory_region_unref(section.mr);
> +                    continue;
> +                }
>  
> -            trace_virtio_balloon_handle_output(memory_region_name(section.mr),
> -                                               pa);
> -            if (!qemu_balloon_is_inhibited()) {
> -                if (vq == s->ivq) {
> -                    balloon_inflate_page(s, section.mr,
> -                                         section.offset_within_region, &pbp);
> -                } else if (vq == s->dvq) {
> -                    balloon_deflate_page(s, section.mr, section.offset_within_region);
> -                } else {
> -                    g_assert_not_reached();
> +                trace_virtio_balloon_handle_output(memory_region_name(section.mr),
> +                                                   pa);
> +                if (!qemu_balloon_is_inhibited()) {
> +                    if (vq == s->ivq) {
> +                        balloon_inflate_page(s, section.mr,
> +                                             section.offset_within_region,
> +                                             &pbp);
> +                    } else if (vq == s->dvq) {
> +                        balloon_deflate_page(s, section.mr,
> +                                             section.offset_within_region);
> +                    } else {
> +                        g_assert_not_reached();
> +                    }
>                  }
> +                memory_region_unref(section.mr);
> +
> +                pa += BALLOON_PAGE_SIZE;
> +                handle_size -= BALLOON_PAGE_SIZE;
>              }
> -            memory_region_unref(section.mr);
>          }
>  
>          virtqueue_push(vq, elem, offset);
> @@ -693,6 +705,8 @@ static void virtio_balloon_set_config(VirtIODevice *vdev,
>  
>      memcpy(&config, config_data, virtio_balloon_config_size(dev));
>      dev->actual = le32_to_cpu(config.actual);
> +    if (virtio_has_feature(vdev->host_features, VIRTIO_BALLOON_F_THP_ORDER))
> +        dev->actual <<= VIRTIO_BALLOON_THP_ORDER;
>      if (dev->actual != oldactual) {
>          qapi_event_send_balloon_change(vm_ram_size -
>                          ((ram_addr_t) dev->actual << VIRTIO_BALLOON_PFN_SHIFT));
> @@ -728,6 +742,9 @@ static void virtio_balloon_to_target(void *opaque, ram_addr_t target)
>      }
>      if (target) {
>          dev->num_pages = (vm_ram_size - target) >> VIRTIO_BALLOON_PFN_SHIFT;
> +        if (virtio_has_feature(dev->host_features,
> +                               VIRTIO_BALLOON_F_THP_ORDER))
> +            dev->num_pages >>= VIRTIO_BALLOON_THP_ORDER;
>          virtio_notify_config(vdev);
>      }
>      trace_virtio_balloon_to_target(target, dev->num_pages);
> @@ -916,6 +933,8 @@ 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("thp-order", VirtIOBalloon, host_features,
> +                    VIRTIO_BALLOON_F_THP_ORDER, false),
>      /* QEMU 4.0 accidentally changed the config size even when free-page-hint
>       * is disabled, resulting in QEMU 3.1 migration incompatibility.  This
>       * property retains this quirk for QEMU 4.1 machine types.
> diff --git a/include/standard-headers/linux/virtio_balloon.h b/include/standard-headers/linux/virtio_balloon.h
> index 9375ca2..f54d613 100644
> --- a/include/standard-headers/linux/virtio_balloon.h
> +++ b/include/standard-headers/linux/virtio_balloon.h
> @@ -36,10 +36,14 @@
>  #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_THP_ORDER	5 /* Set balloon page order to thp order */
>  
>  /* Size of a PFN in the balloon interface. */
>  #define VIRTIO_BALLOON_PFN_SHIFT 12
>  
> +/* The order of the balloon page */
> +#define VIRTIO_BALLOON_THP_ORDER 9
> +
>  #define VIRTIO_BALLOON_CMD_ID_STOP	0
>  #define VIRTIO_BALLOON_CMD_ID_DONE	1
>  struct virtio_balloon_config {
> -- 
> 2.7.4