On Fri, Nov 3, 2023 at 11:55 AM Akihiko Odaki <akihiko.od...@daynix.com> wrote:
> On 2023/11/03 18:35, Yuri Benditovich wrote: > > > > > > On Thu, Nov 2, 2023 at 4:56 PM Akihiko Odaki <akihiko.od...@daynix.com > > <mailto:akihiko.od...@daynix.com>> wrote: > > > > On 2023/11/02 19:20, Yuri Benditovich wrote: > > > > > > > > > On Thu, Nov 2, 2023 at 11:33 AM Michael S. Tsirkin > > <m...@redhat.com <mailto:m...@redhat.com> > > > <mailto:m...@redhat.com <mailto:m...@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.
