linux-kernel - Re: [PATCH 1/2] virtio_balloon: add work around for out of spec QEMU

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Message-ID: <e9658837-1613-4277-8b21-867f48d34cd9@redhat.com>
Date: Wed, 10 Jul 2024 05:11:02 +0200
From: David Hildenbrand <david@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>, linux-kernel@...r.kernel.org
Cc: Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Jason Wang <jasowang@...hat.com>,
 Eugenio Pérez <eperezma@...hat.com>,
 virtualization@...ts.linux.dev
Subject: Re: [PATCH 1/2] virtio_balloon: add work around for out of spec QEMU

On 05.07.24 12:08, Michael S. Tsirkin wrote:
> QEMU implemented the configuration
> 	VIRTIO_BALLOON_F_REPORTING && ! VIRTIO_BALLOON_F_FREE_PAGE_HINT
> incorrectly: it then uses vq3 for reporting, spec says it is always 4.
> 
> This is masked by a corresponding bug in driver:
> add a work around as I'm going to try and fix the driver bug.
> 
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ---
>   drivers/virtio/virtio_balloon.c | 19 +++++++++++++++++--
>   1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 9a61febbd2f7..7dc3fcd56238 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -597,8 +597,23 @@ static int init_vqs(struct virtio_balloon *vb)
>   
>   	err = virtio_find_vqs(vb->vdev, VIRTIO_BALLOON_VQ_MAX, vqs,
>   			      callbacks, names, NULL);
> -	if (err)
> -		return err;
> +	if (err) {
> +		/*
> +		 * Try to work around QEMU bug which since 2020 confused vq numbers
> +		 * when VIRTIO_BALLOON_F_REPORTING but not
> +		 * VIRTIO_BALLOON_F_FREE_PAGE_HINT are offered.
> +		 */
> +		if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING) &&
> +		    !virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
> +			names[VIRTIO_BALLOON_VQ_FREE_PAGE] = "reporting_vq";
> +			callbacks[VIRTIO_BALLOON_VQ_FREE_PAGE] = balloon_ack;
> +			err = virtio_find_vqs(vb->vdev,
> +					      VIRTIO_BALLOON_VQ_REPORTING, vqs, callbacks, names, NULL);
> +		}
> +
> +		if (err)
> +			return err;
> +	}
>   
>   	vb->inflate_vq = vqs[VIRTIO_BALLOON_VQ_INFLATE];
>   	vb->deflate_vq = vqs[VIRTIO_BALLOON_VQ_DEFLATE];

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

-- 
Cheers,

David / dhildenb