linux-kernel - Re: [PATCH] virtio_balloon: Notify guest only after deflating the balloon

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Date:	Sun, 3 Jul 2011 11:27:48 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	dave@...ux.vnet.ibm.com, Amit Shah <amit.shah@...hat.com>
Subject: Re: [PATCH] virtio_balloon: Notify guest only after deflating the
 balloon


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@...tcorp.com.au>
> Cc: "Michael S. Tsirkin" <mst@...hat.com>
> Cc: virtualization@...ts.linux-foundation.org
> Cc: kvm@...r.kernel.org
> Signed-off-by: Sasha Levin <levinsasha928@...il.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
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