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

[Michael S. Tsirkin]

On Wed, Jun 07, 2017 at 09:04:29AM +0800, Wei Wang wrote:
> 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:

I think it should be
1) VIRTIO_F_MAX_CHAIN_SIZE is negotiated
and
2) user requested large size

> 1) VIRTIO_F_MAX_CHAIN_SIZE is negotiated; or
> 2) the backend is vhost.

For vhost we also need vhost backend to support VIRTIO_F_MAX_CHAIN_SIZE.
We also need to send the max chain size to backend.

> Best,
> Wei