From: Maxime Coquelin > 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 <ma...@mellanox.com>; Noa Ezra > > <n...@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.coque...@redhat.com> > > > >> Cc: Matan Azrad <ma...@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.coque...@redhat.com] > > > >>> Sent: Wednesday, June 19, 2019 12:10 PM > > > >>> To: Noa Ezra <n...@mellanox.com> > > > >>> Cc: Matan Azrad <ma...@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. Did you test small packets \ big?
> > 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. Not sure, as TSO application may decide to not do it in spite of it is configured in 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? Yes > 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. Exactly, applications (like OVS) may decide to disable features because a lot of reasons. > > 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. Performance reasons are not good enough? > Tiwei, what's your opinion on this? > > > > Regards, > > > Maxime > >
