On 2023/11/03 18:35, Yuri Benditovich wrote:
>
>
> On Thu, Nov 2, 2023 at 4:56 PM Akihiko Odaki <akihiko.odaki@daynix.com
> <mailto:akihiko.odaki@daynix.com>> wrote:
>
> On 2023/11/02 19:20, Yuri Benditovich wrote:
> >
> >
> > On Thu, Nov 2, 2023 at 11:33 AM Michael S. Tsirkin
> <mst@redhat.com <mailto:mst@redhat.com>
> > <mailto:mst@redhat.com <mailto:mst@redhat.com>>> wrote:
> >
> > On Thu, Nov 02, 2023 at 11:09:27AM +0200, Yuri Benditovich wrote:
> > > Probably we mix two different patches in this discussion.
> > > Focusing on the patch in the e-mail header:
> > >
> > > IMO it is not acceptable to fail QEMU run for one feature
> that we
> > can't make
> > > active when we silently drop all other features in such a
> case.
> >
> > If the feature is off by default then it seems more reasonable
> > and silent masking can be seen as a bug.
> > Most virtio features are on by default this is why it's
> > reasonable to mask them.
> >
> >
> > If we are talking about RSS: setting it initially off is the
> development
> > time decision.
> > When it will be completely stable there is no reason to keep it
> off by
> > default, so this is more a question of time and of a readiness of
> libvirt.
>
> It is not ok to make "on" the default; that will enable RSS even when
> eBPF steering support is not present and can result in performance
> degradation.
>
>
> Exactly as it is today - with vhost=on the host does not suggest RSS
> without eBPF.
> I do not understand what you call "performance degradation", can you
> describe the scenario?
I was not clear, but I was talking about the case of vhost=off or peers
other than tap (e.g., user). rss=on employs in-qemu RSS, which incurs
overheads for such configurations.
So, vhost=off OR peers other than tap:
In the case of peers other than tap (IMO) we're not talking about performance at all.
Backends like "user" (without vnet_hdr) do not support _many_ performance-oriented features.
If RSS is somehow "supported" for such backends this is rather a misunderstanding (IMO again).
In the case of tap with vhost=off the RSS support does not create any performance degradation without eBPF.