[Qemu-devel] [PATCH v2] virtio-serial-bus: Bump up control vq descriptors to min. required

[Amit Shah <amit.shah@redhat.com>]

The current default of 16 buffers for the control vq is too small for
the default max_nr_ports of 32.  We can get more entries in there,
example when asking the guest to add max. allowed ports.

Default to using the minimum required (next power of 2) of the
max_nr_ports in use.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
---
v2:
 Make the descriptor entries depend on the max_nr_ports being used to
prevent wastage.  This also copes up with any future enhancing of
max_supported_ports to higher values.

 hw/virtio-serial-bus.c |   13 ++++++++++---
 1 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/hw/virtio-serial-bus.c b/hw/virtio-serial-bus.c
index 74ba5ec..eb7b362 100644
--- a/hw/virtio-serial-bus.c
+++ b/hw/virtio-serial-bus.c
@@ -736,7 +736,7 @@ VirtIODevice *virtio_serial_init(DeviceState *dev, uint32_t max_nr_ports)
 {
     VirtIOSerial *vser;
     VirtIODevice *vdev;
-    uint32_t i, max_supported_ports;
+    uint32_t i, max_supported_ports, cvq_len;
 
     if (!max_nr_ports)
         return NULL;
@@ -769,10 +769,17 @@ VirtIODevice *virtio_serial_init(DeviceState *dev, uint32_t max_nr_ports)
     /* Add a queue for guest to host transfers for port 0 (backward compat) */
     vser->ovqs[0] = virtio_add_queue(vdev, 128, handle_output);
 
+    /*
+     * The number of descriptors to use should always be a power of
+     * two.  Use the next power of 2 of max_nr_ports to keep the
+     * entries to a minimum.
+     */
+    cvq_len = 1 << qemu_fls(max_nr_ports - 1);
+
     /* control queue: host to guest */
-    vser->c_ivq = virtio_add_queue(vdev, 16, control_in);
+    vser->c_ivq = virtio_add_queue(vdev, cvq_len, control_in);
     /* control queue: guest to host */
-    vser->c_ovq = virtio_add_queue(vdev, 16, control_out);
+    vser->c_ovq = virtio_add_queue(vdev, cvq_len, control_out);
 
     for (i = 1; i < vser->bus->max_nr_ports; i++) {
         /* Add a per-port queue for host to guest transfers */
-- 
1.7.3.2