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Re: [PATCH v1] virtio-pci: store virtqueue size directly to a device
From: |
Michael S. Tsirkin |
Subject: |
Re: [PATCH v1] virtio-pci: store virtqueue size directly to a device |
Date: |
Mon, 23 Dec 2019 09:31:45 -0500 |
On Mon, Dec 23, 2019 at 02:37:58PM +0300, Denis Plotnikov wrote:
> Currenly, the virtqueue size is saved to the proxy on pci writing and
> is read from the device pci reading.
> The virtqueue size is propagated later on form the proxy to the device
> on virqueue enabling stage.
>
> This could be a problem, if a guest, on the virtqueue configuration, sets
> the size and then re-read it immediatly before the queue enabling
> in order to check if the desiged size has been set.
>
> This happens in seabios: (sebios snippet)
>
> vp_find_vq()
> {
> ...
> /* check if the queue is available */
> if (vp->use_modern) {
> num = vp_read(&vp->common, virtio_pci_common_cfg, queue_size);
> if (num > MAX_QUEUE_NUM) {
> vp_write(&vp->common, virtio_pci_common_cfg, queue_size,
> MAX_QUEUE_NUM);
> num = vp_read(&vp->common, virtio_pci_common_cfg, queue_size);
> }
> } else {
> num = vp_read(&vp->legacy, virtio_pci_legacy, queue_num);
> }
> if (!num) {
> dprintf(1, "ERROR: queue size is 0\n");
> goto fail;
> }
> if (num > MAX_QUEUE_NUM) {
> dprintf(1, "ERROR: queue size %d > %d\n", num, MAX_QUEUE_NUM);
> goto fail;
> }
> ...
> }
>
> If the device queue num is greater then the max queue size supported by
> seabios,
> seabios tries to reduce the queue size, then re-read it again, I suppose to
> check if the setting actually happens, and then checks the virtqueue size
> again,
> to deside whether it is satisfied with the vaule.
> In this case, if device's virtqueue size is 512 and seabios max supported
> queue
> size is 256, seabios tries to set 256 but than read 512 again and can't
> proceed
> with that vaule, preventing the guest from successful booting.
> The root case was investigated by Roman Kagan <address@hidden>
>
> The patch fixes the problem, by propagating the queue size to the device right
> away, so the written value could be read on the next step, if the value was
> ok for the device.
>
> Suggested-by: Roman Kagan <address@hidden>
> Suggested-by: Michael S. Tsirkin <address@hidden>
> Signed-off-by: Denis Plotnikov <address@hidden>
Thanks, I already have this queued as:
commit 8aabbbd9d04f95d5581d2275362996ecb5516dd9
Author: Michael S. Tsirkin <address@hidden>
Date: Fri Dec 13 09:22:48 2019 -0500
virtio: update queue size on guest write
Some guests read back queue size after writing it.
Update the size immediatly upon write otherwise
they get confused.
Signed-off-by: Michael S. Tsirkin <address@hidden>
I would appreciate checking other transports, they likely
need the same fix.
> ---
> hw/virtio/virtio-pci.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> index c6b47a9c73..e5c759e19e 100644
> --- a/hw/virtio/virtio-pci.c
> +++ b/hw/virtio/virtio-pci.c
> @@ -1256,6 +1256,8 @@ static void virtio_pci_common_write(void *opaque,
> hwaddr addr,
> break;
> case VIRTIO_PCI_COMMON_Q_SIZE:
> proxy->vqs[vdev->queue_sel].num = val;
> + virtio_queue_set_num(vdev, vdev->queue_sel,
> + proxy->vqs[vdev->queue_sel].num);
> break;
> case VIRTIO_PCI_COMMON_Q_MSIX:
> msix_vector_unuse(&proxy->pci_dev,
> --
> 2.17.0