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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jan 11, 2011 at 09:47:44PM +0530, Amit Shah wrote:
> 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>
> ---

So looking at this more closely, this reduces but does
not eliminate the chance of notification event drops.

I think I liked v1 better.


>  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 */
> -- 
> 1.7.3.4
>