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

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

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
>> 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.
>>
> 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
> around.
>
> 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.
>
> Best,
> Wei
>
>
>
>
>
>
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