Re: [RFC v1 1/2] vhost: eliminate duplicate dirty_bitmap sync when log shared by multiple devices

[Peter Xu <peterx@redhat.com>]

On Mon, Dec 08, 2025 at 08:09:51PM +0800, Chuang Xu wrote:
> From: xuchuangxclwt <xuchuangxclwt@bytedance.com>
> 
> Although logs can now be shared among multiple vhost devices,
> live migration still performs repeated vhost_log_sync for each
> vhost device during bitmap_sync, which increases the time required
> for bitmap_sync and makes it more difficult for dirty pages to converge.
> 
> Attempt to eliminate these duplicate sync.
> 
> Signed-off-by: Chuang Xu <xuchuangxclwt@bytedance.com>

It looks reasonable from migration POV, but I don't know the details.

Please remember to copy Jason (I added for this email) when repost.

Thanks,

> ---
>  hw/virtio/vhost.c         | 30 ++++++++++++++++++++++--------
>  include/hw/virtio/vhost.h |  1 +
>  2 files changed, 23 insertions(+), 8 deletions(-)
> 
> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> index 266a11514a..d397ca327f 100644
> --- a/hw/virtio/vhost.c
> +++ b/hw/virtio/vhost.c
> @@ -268,14 +268,6 @@ static int vhost_sync_dirty_bitmap(struct vhost_dev *dev,
>      return 0;
>  }
>  
> -static void vhost_log_sync(MemoryListener *listener,
> -                          MemoryRegionSection *section)
> -{
> -    struct vhost_dev *dev = container_of(listener, struct vhost_dev,
> -                                         memory_listener);
> -    vhost_sync_dirty_bitmap(dev, section, 0x0, ~0x0ULL);
> -}
> -
>  static void vhost_log_sync_range(struct vhost_dev *dev,
>                                   hwaddr first, hwaddr last)
>  {
> @@ -287,6 +279,27 @@ static void vhost_log_sync_range(struct vhost_dev *dev,
>      }
>  }
>  
> +static void vhost_log_sync(MemoryListener *listener,
> +                          MemoryRegionSection *section)
> +{
> +    struct vhost_dev *dev = container_of(listener, struct vhost_dev,
> +                                         memory_listener);
> +    struct vhost_log *log = dev->log;
> +
> +    if (log && log->refcnt > 1) {
> +        /*
> +         * When multiple devices use same log, we implement the logic of
> +         * vhost_log_sync just like what we do in vhost_log_put.
> +         */
> +        log->sync_cnt = (log->sync_cnt + 1) % log->refcnt;
> +        if (!log->sync_cnt) {
> +            vhost_log_sync_range(dev, 0, dev->log_size * VHOST_LOG_CHUNK - 1);
> +        }
> +    } else {
> +        vhost_sync_dirty_bitmap(dev, section, 0x0, ~0x0ULL);
> +    }
> +}
> +
>  static uint64_t vhost_get_log_size(struct vhost_dev *dev)
>  {
>      uint64_t log_size = 0;
> @@ -383,6 +396,7 @@ static struct vhost_log *vhost_log_get(VhostBackendType backend_type,
>          ++log->refcnt;
>      }
>  
> +    log->sync_cnt = 0;
>      return log;
>  }
>  
> diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
> index 08bbb4dfe9..43bf1c2150 100644
> --- a/include/hw/virtio/vhost.h
> +++ b/include/hw/virtio/vhost.h
> @@ -50,6 +50,7 @@ typedef unsigned long vhost_log_chunk_t;
>  struct vhost_log {
>      unsigned long long size;
>      int refcnt;
> +    int sync_cnt;
>      int fd;
>      vhost_log_chunk_t *log;
>  };
> -- 
> 2.20.1
> 

-- 
Peter Xu