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([2400:4050:a840:1e00:78d2:b862:10a7:d486]) by smtp.gmail.com with ESMTPSA id l4-20020a170902eb0400b001c61901ed2esm1401381plb.219.2023.11.11.07.28.45 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 11 Nov 2023 07:28:47 -0800 (PST) Message-ID: <8439be4e-a739-4cbd-a569-89b6c7f68ab9@daynix.com> Date: Sun, 12 Nov 2023 00:28:44 +0900 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 11/21] virtio-net: Return an error when vhost cannot enable RSS Content-Language: en-US To: Yuri Benditovich Cc: "Michael S. Tsirkin" , Jason Wang , qemu-devel@nongnu.org, Andrew Melnychenko References: <58fb3b75-dd69-4715-a8ec-4c3df3b7e4c5@daynix.com> <8880b6f9-f556-46f7-a191-eeec0fe208b0@daynix.com> <20231101023805-mutt-send-email-mst@kernel.org> <39a02a4c-f8fa-437c-892f-caca84b8d85d@daynix.com> <20231101050838-mutt-send-email-mst@kernel.org> <20231102053202-mutt-send-email-mst@kernel.org> <2fbdee21-60f4-49ff-b61b-923c895f90ba@daynix.com> From: Akihiko Odaki In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: none client-ip=2607:f8b0:4864:20::62f; envelope-from=akihiko.odaki@daynix.com; helo=mail-pl1-x62f.google.com X-Spam_score_int: -18 X-Spam_score: -1.9 X-Spam_bar: - X-Spam_report: (-1.9 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_NONE=0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 2023/11/03 22:14, Yuri Benditovich wrote: > > > On Fri, Nov 3, 2023 at 11:55 AM Akihiko Odaki > wrote: > > On 2023/11/03 18:35, Yuri Benditovich wrote: > > > > > > On Thu, Nov 2, 2023 at 4:56 PM Akihiko Odaki > > > >> wrote: > > > >     On 2023/11/02 19:20, Yuri Benditovich wrote: > >      > > >      > > >      > On Thu, Nov 2, 2023 at 11:33 AM Michael S. Tsirkin > >      > > > >      > > >>> 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.