Re: [RFC 1/8] virtio/virtio-pci: Handle extra notification data

On Fri, Mar 1, 2024 at 2:44 PM Jonah Palmer <[email protected]> wrote:
>
> Add support to virtio-pci devices for handling the extra data sent
> from the driver to the device when the VIRTIO_F_NOTIFICATION_DATA
> transport feature has been negotiated.
>
> The extra data that's passed to the virtio-pci device when this
> feature is enabled varies depending on the device's virtqueue
> layout.
>
> In a split virtqueue layout, this data includes:
>  - upper 16 bits: last_avail_idx
>  - lower 16 bits: virtqueue index
>
> In a packed virtqueue layout, this data includes:
>  - upper 16 bits: 1-bit wrap counter & 15-bit last_avail_idx
>  - lower 16 bits: virtqueue index
>
> Signed-off-by: Jonah Palmer <[email protected]>
> ---
>  hw/virtio/virtio-pci.c     | 13 ++++++++++---
>  hw/virtio/virtio.c         | 13 +++++++++++++
>  include/hw/virtio/virtio.h |  1 +
>  3 files changed, 24 insertions(+), 3 deletions(-)
>
> diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> index 1a7039fb0c..c7c577b177 100644
> --- a/hw/virtio/virtio-pci.c
> +++ b/hw/virtio/virtio-pci.c
> @@ -384,7 +384,7 @@ static void virtio_ioport_write(void *opaque, uint32_t addr, uint32_t val)
>  {
>      VirtIOPCIProxy *proxy = opaque;
>      VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
> -    uint16_t vector;
> +    uint16_t vector, vq_idx;
>      hwaddr pa;
>
>      switch (addr) {
> @@ -408,8 +408,15 @@ static void virtio_ioport_write(void *opaque, uint32_t addr, uint32_t val)
>              vdev->queue_sel = val;
>          break;
>      case VIRTIO_PCI_QUEUE_NOTIFY:
> -        if (val < VIRTIO_QUEUE_MAX) {
> -            virtio_queue_notify(vdev, val);
> +        if (virtio_vdev_has_feature(vdev, VIRTIO_F_NOTIFICATION_DATA)) {
> +            vq_idx = val & 0xFFFF;

Nitpick, but since vq_idx is already a uint16_t the & 0xFFFF is not
needed. I think it's cleaner just to call virtio_set_notification data
in the has_feature(...) condition, but I'm happy with this too.

> +            virtio_set_notification_data(vdev, vq_idx, val);
> +        } else {
> +            vq_idx = val;
> +        }
> +
> +        if (vq_idx < VIRTIO_QUEUE_MAX) {
> +            virtio_queue_notify(vdev, vq_idx);
>          }
>          break;
>      case VIRTIO_PCI_STATUS:
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index d229755eae..a61f69b7fd 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -2052,6 +2052,19 @@ int virtio_set_status(VirtIODevice *vdev, uint8_t val)
>      return 0;
>  }
>
> +void virtio_set_notification_data(VirtIODevice *vdev, uint16_t i, uint32_t data)
> +{
> +    VirtQueue *vq = &vdev->vq[i];
> +
> +    if (virtio_vdev_has_feature(vdev, VIRTIO_F_RING_PACKED)) {
> +        vq->last_avail_wrap_counter = (data >> 31) & 0x1;
> +        vq->last_avail_idx = (data >> 16) & 0x7FFF;
> +    } else {
> +        vq->last_avail_idx = (data >> 16) & 0xFFFF;
> +    }

It should not set last_avail_idx, only shadow_avail_idx. Otherwise,
QEMU can only see the descriptors placed after the notification.

Or am I missing something?

In that regard, I would call this function
"virtqueue_set_shadow_avail_idx". But I'm very bad at naming :).

The rest looks good to me.

Thanks!

> +    vq->shadow_avail_idx = vq->last_avail_idx;
> +}
> +
>  static enum virtio_device_endian virtio_default_endian(void)
>  {
>      if (target_words_bigendian()) {
> diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> index c8f72850bc..c92d8afc42 100644
> --- a/include/hw/virtio/virtio.h
> +++ b/include/hw/virtio/virtio.h
> @@ -345,6 +345,7 @@ void virtio_queue_reset(VirtIODevice *vdev, uint32_t queue_index);
>  void virtio_queue_enable(VirtIODevice *vdev, uint32_t queue_index);
>  void virtio_update_irq(VirtIODevice *vdev);
>  int virtio_set_features(VirtIODevice *vdev, uint64_t val);
> +void virtio_set_notification_data(VirtIODevice *vdev, uint16_t i, uint32_t data);
>
>  /* Base devices.  */
>  typedef struct VirtIOBlkConf VirtIOBlkConf;
> --
> 2.39.3
>