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

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.


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