Re: [Qemu-devel] [PATCH v1] virtio-net: enable configurable tx queue size

[Wei Wang <wei.w.wang@intel.com>]

On 06/05/2017 11:38 PM, Michael S. Tsirkin wrote:
> On Mon, Jun 05, 2017 at 04:57:29PM +0800, Wei Wang wrote:
>> This patch enables the virtio-net tx queue size to be configurable
>> between 256 and 1024 by the user. The queue size specified by the
>> user should be power of 2. If "tx_queue_size" is not offered by the
>> user, the default queue size, 1024, will be used.
>>
>> For the traditional QEMU backend, setting the tx queue size to be 1024
>> requires the guest virtio driver to support the VIRTIO_F_MAX_CHAIN_SIZE
>> feature. This feature restricts the guest driver from chaining 1024
>> vring descriptors, which may cause the device side implementation to
>> send more than 1024 iov to writev.
>>
>> VIRTIO_F_MAX_CHAIN_SIZE is a common transport feature added for all
>> virtio devices. However, each device has the flexibility to set the max
>> chain size to limit its driver to chain vring descriptors. Currently,
>> the max chain size of the virtio-net device is set to 1023.
>>
>> In the case that the tx queue size is set to 1024 and the
>> VIRTIO_F_MAX_CHAIN_SIZE feature is not supported by the guest driver,
>> the tx queue size will be reconfigured to be 512.
> I'd like to see the reverse. Start with the current default.
> If VIRTIO_F_MAX_CHAIN_SIZE is negotiated, increase the queue size.
>

OK, we can let the queue size start with 256, and how about
increasing it to 1024 in the following two cases:
1) VIRTIO_F_MAX_CHAIN_SIZE is negotiated; or
2) the backend is vhost.

Best,
Wei