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

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 6/26/19 9:50 AM, Matan Azrad wrote:
> Hi Maxim
> 
> Any response here?
> 
> Besides that,
> 
> Regarding the TSO and this patch:
> I think we shouldn't be so strict to not take them for this version:
> 1. The later time was a technical issue with the mailer - a mistake.
> 2. The patches don't change any default and makes sense - will not hurt anyone.
> 
> So I think we can do it beyond the letter of the law.
> 
>   From: Maxime Coquelin
>   > Sent: Thursday, June 20, 2019 10:19 AM
>   > To: Matan Azrad <matan@mellanox.com>; Noa Ezra
>   <noae@mellanox.com>
>   > Cc: dev@dpdk.org
>   > Subject: Re: [PATCH 2/2] net/vhost: support mrg-rxbuf disabling
>   >
>   >
>   >
>   > 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@redhat.com>
>   > >> Cc: Matan Azrad <matan@mellanox.com>; dev@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@redhat.com]
>   > >>> Sent: Wednesday, June 19, 2019 12:10 PM
>   > >>> To: Noa Ezra <noae@mellanox.com>
>   > >>> Cc: Matan Azrad <matan@mellanox.com>; dev@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?
>   >
>   I guess that if this feature is enabled and the feature actually are not used
>   (no packets are scattered or merged) it will hurt the performance.

Well, latest performance measurements does not show a big impact now on 
enabling mergeable buffers feature unconditionaly.


>   So if one of the sides doesn't want to use it because of performance, it may
>   want to disable it.

And even if there is an impact, the way to disable it is through
Libvirt/Qemu.

>   > > 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.
>   >
>   So the vhost PMD is not one of the sides to decide?
>   If so, why do we need the APIs to configure the features?

Are you talking about the rte_vhost_driver_set_features() and related
APIs?

This is used for example by the external backends that support features
specific to the backend type (e.g. crypto), or also used by OVS-DPDK, to
disable TSO. So these usages are for functional reasons, not tuning.

>   Looks like also the qemu is configured with the feature the VM\host sides
>   may decide in some cases to disable it.

For functional reasons, I agree. So I that's why I agree with your tso
patch as the application has to support it, but that's not the case of
the mergeable buffers features.

Tiwei, what's your opinion on this?

>   > Regards,
>   > Maxime
>