[dpdk-dev] [PATCH 2/2] net/vhost: support mrg-rxbuf disabling

[Maxime Coquelin]

On 6/20/19 8:52 AM, Matan Azrad wrote:
> Hi all
> 
>> -----Original Message-----
>> From: Noa Ezra
>> Sent: Thursday, June 20, 2019 8:58 AM
>> To: Maxime Coquelin <maxime.coquelin at redhat.com>
>> Cc: Matan Azrad <matan at mellanox.com>; dev at dpdk.org
>> Subject: RE: [PATCH 2/2] net/vhost: support mrg-rxbuf disabling
>>
>> Hi Maxime,
>> Thanks for your comment, please see below.
>>
>>> -----Original Message-----
>>> From: Maxime Coquelin [mailto:maxime.coquelin at redhat.com]
>>> Sent: Wednesday, June 19, 2019 12:10 PM
>>> To: Noa Ezra <noae at mellanox.com>
>>> Cc: Matan Azrad <matan at mellanox.com>; dev at dpdk.org
>>> Subject: Re: [PATCH 2/2] net/vhost: support mrg-rxbuf disabling
>>>
>>> Hi Noa,
>>>
>>> On 6/19/19 8:13 AM, Noa Ezra wrote:
>>>> Rx mergeable buffers is a virtio feature that allows chaining of
>>>> multiple virtio descriptors to handle large packet size.
>>>> This behavior is supported and enabled by default, however in case
>>>> the user knows that rx mergeable buffers are not needed, he can
>>>> disable the feature.
>>>> The user should also set mrg_rxbuf=off in virtual machine's xml.
>>>
>>> I'm not sure to understand why it is needed, as the vhost-user library
>>> supports the feature, it's better to let it being advertised.
>>>
>>> As you say, it is up to the user to disable it in the VM's XML.
>>> Done this way, the feature won't be negotiated.
>>>
>> I agree with you, I'll remove this patch from the series.
> 
> Are you sure that no performance impact exists for redundant merg-rx-buf configuration here?

I'm not sure to understand what you mean, could you please elaborate?

> What if the second side want it and the current side no?

The feature won't be negotiated, assuming it has been disabled in QEMU
cmdline (or via libvirt).

> It may be that the vhost PMD user may want to disable it to save performance from some reasons, no?
> 

Then this user should disable it at QEMU level.

Regards,
Maxime