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

Ping for comments, thanks.

On 06/09/2017 11:00 AM, Wei Wang wrote:
On 06/09/2017 03:01 AM, Michael S. Tsirkin wrote:
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
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.

I think the limitation that we are dealing with is that the virtio-net
backend implementation in QEMU is possible to pass more than
1024 iov to writev. In this case, the QEMU backend uses the
"max_chain_size" register to tell the driver the max size of the
vring_desc chain. So, I think it should be the device (backend)
sending the max size to the driver, rather than the other way

For the vhost-user and vhost-net backend cases, they don't have
such limitation as the QEMU backend, right?
If no such limitation, I think without the negotiation of
VIRTIO_F_MAX_CHAIN_SIZE, the device should be safe to use 1024
tx queue size if it is the vhost backend.


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