On 2023/11/03 22:14, Yuri Benditovich wrote:
On Fri, Nov 3, 2023 at 11:55 AM Akihiko Odaki <akihiko.od...@daynix.com <mailto: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> > <mailto: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>> > > <mailto: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).
We do not need to ensure good performance when RSS is enabled by the guest for backends without eBPF steering program as you say. In-QEMU RSS is only useful for testing and not meant to improve the performance.
However, if you set rss=on, QEMU will advertise the availability of RSS feature. The guest will have no mean to know if it's implemented in a way not performance-wise so it may decide to use the feature to improve the performance, which can result in performance degradation. Therefore, it's better not to set rss=on for such backends.
