9pfs/virtio: implement response_buffer_size callback

Add and implement the response_buffer_size callback for the virtio
transport.

Returns the actual current virtio response buffer size for the supplied
PDU, which will be used as safety guard for limiting the response size
when generating a 9p response.

Link: https://lore.kernel.org/qemu-devel/5bbed2768f7a0da8fa2be183e75928c5d1ef691d.1781287774.git.qemu_oss@crudebyte.com
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>

hw/9pfs/virtio-9p-device.c

diff --git a/hw/9pfs/virtio-9p-device.c b/hw/9pfs/virtio-9p-device.c
index 8c5d86c..50dc930 100644
--- a/hw/9pfs/virtio-9p-device.c
+++ b/hw/9pfs/virtio-9p-device.c
@@ -198,6 +198,15 @@
     return (VIRTQUEUE_MAX_SIZE - 2) * guestPageSize;
 }
 
+static size_t virtio_9p_response_buffer_size(V9fsPDU *pdu)
+{
+    V9fsState *s = pdu->s;
+    V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
+    VirtQueueElement *elem = v->elems[pdu->idx];
+
+    return iov_size(elem->in_sg, elem->in_num);
+}
+
 static const V9fsTransport virtio_9p_transport = {
     .pdu_vmarshal = virtio_pdu_vmarshal,
     .pdu_vunmarshal = virtio_pdu_vunmarshal,
@@ -205,6 +214,7 @@
     .init_out_iov_from_pdu = virtio_init_out_iov_from_pdu,
     .push_and_notify = virtio_9p_push_and_notify,
     .msize_limit = virtio_9p_msize_limit,
+    .response_buffer_size = virtio_9p_response_buffer_size,
 };
 
 static void virtio_9p_device_realize(DeviceState *dev, Error **errp)