linux-kernel - Re: [PATCH] virtio_balloon: clear modern features under legacy

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Date:   Fri, 10 Jul 2020 13:32:13 +0200
From:   David Hildenbrand <david@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>, linux-kernel@...r.kernel.org
Cc:     stable@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [PATCH] virtio_balloon: clear modern features under legacy

On 10.07.20 13:31, Michael S. Tsirkin wrote:
> Page reporting features were never supported by legacy hypervisors.
> Supporting them poses a problem: should we use native endian-ness (like
> current code assumes)? Or little endian-ness like the virtio spec says?
> Rather than try to figure out, and since results of
> incorrect endian-ness are dire, let's just block this configuration.
> 
> Cc: stable@...r.kernel.org
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ---
>  drivers/virtio/virtio_balloon.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 5d4b891bf84f..b9bc03345157 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -1107,6 +1107,15 @@ static int virtballoon_restore(struct virtio_device *vdev)
>  
>  static int virtballoon_validate(struct virtio_device *vdev)
>  {
> +	/*
> +	 * Legacy devices never specified how modern features should behave.
> +	 * E.g. which endian-ness to use? Better not to assume anything.
> +	 */
> +	if (!virtio_has_feature(vdev, VIRTIO_F_VERSION_1)) {
> +		__virtio_clear_bit(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT);
> +		__virtio_clear_bit(vdev, VIRTIO_BALLOON_F_PAGE_POISON);
> +		__virtio_clear_bit(vdev, VIRTIO_BALLOON_F_REPORTING);
> +	}
>  	/*
>  	 * Inform the hypervisor that our pages are poisoned or
>  	 * initialized. If we cannot do that then we should disable
> 

Very right

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Thanks,

David / dhildenb