Re: [PATCH v2 2/2] virtio: fix vq # for balloon

[Daniel Verkamp]

On Wed, Jul 10, 2024 at 4:43 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> virtio balloon communicates to the core that in some
> configurations vq #s are non-contiguous by setting name
> pointer to NULL.
>
> Unfortunately, core then turned around and just made them
> contiguous again. Result is that driver is out of spec.

Thanks for fixing this - I think the overall approach of the patch looks good.

> Implement what the API was supposed to do
> in the 1st place. Compatibility with buggy hypervisors
> is handled inside virtio-balloon, which is the only driver
> making use of this facility, so far.

In addition to virtio-balloon, I believe the same problem also affects
the virtio-fs device, since queue 1 is only supposed to be present if
VIRTIO_FS_F_NOTIFICATION is negotiated, and the request queues are
meant to be queue indexes 2 and up. From a look at the Linux driver
(virtio_fs.c), it appears like it never acks VIRTIO_FS_F_NOTIFICATION
and assumes that request queues start at index 1 rather than 2, which
looks out of spec to me, but the current device implementations (that
I am aware of, anyway) are also broken in the same way, so it ends up
working today. Queue numbering in a spec-compliant device and the
current Linux driver would mismatch; what the driver considers to be
the first request queue (index 1) would be ignored by the device since
queue index 1 has no function if F_NOTIFICATION isn't negotiated.

[...]
> diff --git a/drivers/virtio/virtio_pci_common.c b/drivers/virtio/virtio_pci_common.c
> index 7d82facafd75..fa606e7321ad 100644
> --- a/drivers/virtio/virtio_pci_common.c
> +++ b/drivers/virtio/virtio_pci_common.c
> @@ -293,7 +293,7 @@ static int vp_find_vqs_msix(struct virtio_device *vdev, unsigned int nvqs,
>         struct virtio_pci_device *vp_dev = to_vp_device(vdev);
>         struct virtqueue_info *vqi;
>         u16 msix_vec;
> -       int i, err, nvectors, allocated_vectors, queue_idx = 0;
> +       int i, err, nvectors, allocated_vectors;
>
>         vp_dev->vqs = kcalloc(nvqs, sizeof(*vp_dev->vqs), GFP_KERNEL);
>         if (!vp_dev->vqs)
> @@ -332,7 +332,7 @@ static int vp_find_vqs_msix(struct virtio_device *vdev, unsigned int nvqs,
>                         msix_vec = allocated_vectors++;
>                 else
>                         msix_vec = VP_MSIX_VQ_VECTOR;
> -               vqs[i] = vp_setup_vq(vdev, queue_idx++, vqi->callback,
> +               vqs[i] = vp_setup_vq(vdev, i, vqi->callback,
>                                      vqi->name, vqi->ctx, msix_vec);
>                 if (IS_ERR(vqs[i])) {
>                         err = PTR_ERR(vqs[i]);
> @@ -368,7 +368,7 @@ static int vp_find_vqs_intx(struct virtio_device *vdev, unsigned int nvqs,
>                             struct virtqueue_info vqs_info[])
>  {
>         struct virtio_pci_device *vp_dev = to_vp_device(vdev);
> -       int i, err, queue_idx = 0;
> +       int i, err;
>
>         vp_dev->vqs = kcalloc(nvqs, sizeof(*vp_dev->vqs), GFP_KERNEL);
>         if (!vp_dev->vqs)
> @@ -388,8 +388,13 @@ static int vp_find_vqs_intx(struct virtio_device *vdev, unsigned int nvqs,
>                         vqs[i] = NULL;
>                         continue;
>                 }
> +<<<<<<< HEAD
>                 vqs[i] = vp_setup_vq(vdev, queue_idx++, vqi->callback,
>                                      vqi->name, vqi->ctx,
> +=======
> +               vqs[i] = vp_setup_vq(vdev, i, callbacks[i], names[i],
> +                                    ctx ? ctx[i] : false,
> +>>>>>>> f814759f80b7... virtio: fix vq # for balloon

This still has merge markers in it.

Thanks,
-- Daniel