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

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

[Amit Shah]

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>
---
 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

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

[Michael S. Tsirkin]

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
>