Linux Virtualization - Jul 2011 - [PATCH] virtio_balloon: Notify guest only after deflating the balloon

Unless the host requires that requested pages won't be used until
he us notified (VIRTIO_BALLOON_F_MUST_TELL_HOST), only notify after
deflating the balloon.

This will avoid having to take an exit before actually using the pages.

Cc: Rusty Russell <rusty at rustcorp.com.au>
Cc: "Michael S. Tsirkin" <mst at redhat.com>
Cc: virtualization at lists.linux-foundation.org
Cc: kvm at vger.kernel.org
Signed-off-by: Sasha Levin <levinsasha928 at gmail.com>
---
 drivers/virtio/virtio_balloon.c |   16 ++++++++++------
 1 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index e058ace..055f95d 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -148,14 +148,18 @@ static void leak_balloon(struct virtio_balloon *vb, size_t
num)
 		vb->num_pages--;
 	}
 
-
 	/*
-	 * Note that if
-	 * virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST);
-	 * is true, we *have* to do it in this order
+	 * If the host doesn't require us to notify him before using
+	 * pages which belong to the balloon, update him only after
+	 * freeing those pages for guest use.
 	 */
-	tell_host(vb, vb->deflate_vq);
-	release_pages_by_pfn(vb->pfns, vb->num_pfns);
+	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST)) {
+		tell_host(vb, vb->deflate_vq);
+		release_pages_by_pfn(vb->pfns, vb->num_pfns);
+	} else {
+		release_pages_by_pfn(vb->pfns, vb->num_pfns);
+		tell_host(vb, vb->deflate_vq);
+	}
 }
 
 static inline void update_stat(struct virtio_balloon *vb, int idx,
-- 
1.7.6

Doesn't this basically just revert
bf50e69f63d21091e525185c3ae761412be0ba72?


On Sat, Jul 02, 2011 at 06:06:56AM +0300, Sasha Levin
wrote:
> Unless the host requires that requested pages won't be used until
> he us notified (VIRTIO_BALLOON_F_MUST_TELL_HOST), only notify after
> deflating the balloon.
> 
> This will avoid having to take an exit before actually using the pages.
> 
> Cc: Rusty Russell <rusty at rustcorp.com.au>
> Cc: "Michael S. Tsirkin" <mst at redhat.com>
> Cc: virtualization at lists.linux-foundation.org
> Cc: kvm at vger.kernel.org
> Signed-off-by: Sasha Levin <levinsasha928 at gmail.com>
> ---
>  drivers/virtio/virtio_balloon.c |   16 ++++++++++------
>  1 files changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c
b/drivers/virtio/virtio_balloon.c
> index e058ace..055f95d 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -148,14 +148,18 @@ static void leak_balloon(struct virtio_balloon *vb,
size_t num)
>  		vb->num_pages--;
>  	}
>  
> -
>  	/*
> -	 * Note that if
> -	 * virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST);
> -	 * is true, we *have* to do it in this order
> +	 * If the host doesn't require us to notify him before using
> +	 * pages which belong to the balloon, update him only after
> +	 * freeing those pages for guest use.
>  	 */
> -	tell_host(vb, vb->deflate_vq);
> -	release_pages_by_pfn(vb->pfns, vb->num_pfns);
> +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST)) {
> +		tell_host(vb, vb->deflate_vq);
> +		release_pages_by_pfn(vb->pfns, vb->num_pfns);
> +	} else {
> +		release_pages_by_pfn(vb->pfns, vb->num_pfns);
> +		tell_host(vb, vb->deflate_vq);
> +	}
>  }
>  
>  static inline void update_stat(struct virtio_balloon *vb, int idx,
> -- 
> 1.7.6