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(p200300cfd70866e5a5d0fe9228997179.dip0.t-ipconnect.de. [2003:cf:d708:66e5:a5d0:fe92:2899:7179]) by smtp.gmail.com with ESMTPSA id i11-20020a170906a28b00b009737b8d47b6sm6465450ejz.203.2023.10.09.02.13.34 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 09 Oct 2023 02:13:34 -0700 (PDT) Message-ID: Date: Mon, 9 Oct 2023 11:13:33 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.15.1 Subject: Re: [Virtio-fs] (no subject) Content-Language: en-US From: Hanna Czenczek To: Yajun Wu , "Michael S. Tsirkin" Cc: qemu-devel@nongnu.org, virtio-fs@redhat.com, =?UTF-8?Q?Eugenio_P=c3=a9rez?= , Anton Kuchin , parav@nvidia.com, maxime.coquelin@redhat.com, =?UTF-8?Q?Alex_Benn=c3=a9e?= References: <20231004125904.110781-1-hreitz@redhat.com> <20231004125904.110781-2-hreitz@redhat.com> <20231005170852.GB1342722@fedora> <20231005131352-mutt-send-email-mst@kernel.org> <00272da3-0a48-5544-6ba8-5dfde00be241@redhat.com> <20231006043518-mutt-send-email-mst@kernel.org> <20231006051802-mutt-send-email-mst@kernel.org> <20231006055229-mutt-send-email-mst@kernel.org> <3f28b1a4-d618-39df-57e6-6152f61cac6e@redhat.com> <8f3694c1-48d4-f34b-8f91-3bc217182ffa@redhat.com> In-Reply-To: <8f3694c1-48d4-f34b-8f91-3bc217182ffa@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.129.124; envelope-from=hreitz@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -38 X-Spam_score: -3.9 X-Spam_bar: --- X-Spam_report: (-3.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-1.818, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 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 09.10.23 11:07, Hanna Czenczek wrote: > On 09.10.23 10:21, Hanna Czenczek wrote: >> On 07.10.23 04:22, Yajun Wu wrote: > > [...] > >>> The main motivation of adding VHOST_USER_SET_STATUS is to let >>> backend DPDK know >>> when DRIVER_OK bit is valid. It's an indication of all VQ >>> configuration has sent, >>> otherwise DPDK has to rely on first queue pair is ready, then >>> receiving/applying >>> VQ configuration one by one. >>> >>> During live migration, configuring VQ one by one is very time >>> consuming. >> >> One question I have here is why it wasn’t then introduced in the live >> migration code, but in the general VM stop/cont code instead. It does >> seem time-consuming to do this every time the VM is paused and resumed. >> >>> For VIRTIO >>> net vDPA, HW needs to know how many VQs are enabled to set >>> RSS(Receive-Side Scaling). >>> >>> If you don’t want SET_STATUS message, backend can remove protocol >>> feature bit >>> VHOST_USER_PROTOCOL_F_STATUS. >> >> The problem isn’t back-ends that don’t want the message, the problem >> is that qemu uses the message wrongly, which prevents well-behaving >> back-ends from implementing the message. >> >>> DPDK is ignoring SET_STATUS 0, but using GET_VRING_BASE to do device >>> close/reset. >> >> So the right thing to do for back-ends is to announce STATUS support >> and then not implement it correctly? >> >> GET_VRING_BASE should not reset the close or reset the device, by the >> way.  It should stop that one vring, not more.  We have a >> RESET_DEVICE command for resetting. >> >>> I'm not involved in discussion about adding SET_STATUS in Vhost >>> protocol. This feature >>> is essential for vDPA(same as vhost-vdpa implements >>> VHOST_VDPA_SET_STATUS). >> >> So from what I gather from your response is that there is only a >> single use for SET_STATUS, which is the DRIVER_OK bit.  If so, >> documenting that all other bits are to be ignored by both back-end >> and front-end would be fine by me. >> >> I’m not fully serious about that suggestion, but I hear the strong >> implication that nothing but DRIVER_OK was of any concern, and this >> is really important to note when we talk about the status of the >> STATUS feature in vhost today.  It seems to me now that it was not >> intended to be the virtio-level status byte, but just a DRIVER_OK >> signalling path from front-end to back-end.  That makes it a >> vhost-level protocol feature to me. > > On second thought, it just is a pure vhost-level protocol feature, and > has nothing to do with the virtio status byte as-is.  The only stated > purpose is for the front-end to send DRIVER_OK after migration, but > migration is transparent to the guest, so the guest would never change > the status byte during migration.  Therefore, if this feature is > essential, we will never be able to have a status byte that is > transparently shared between guest and back-end device, i.e. the > virtio status byte. On third thought, scratch that.  The guest wouldn’t set it, but naturally, after migration, the front-end will need to restore the status byte from the source, so the front-end will always need to set it, even if it were otherwise used controlled only by the guest and the back-end device.  So technically, this doesn’t prevent such a use case.  (In practice, it isn’t controlled by the guest right now, but that could be fixed.) > Cc-ing Alex on this mail, because to me, this seems like an important > detail when he plans on using the byte in the future. If we need a > virtio status byte, I can’t see how we could use the existing F_STATUS > for it. > > Hanna