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Tsirkin" , qemu-devel@nongnu.org Subject: vhost-user protocol feature negotiation Date: Wed, 05 Aug 2020 15:13:06 +0000 Message-ID: <87sgd1ktx9.fsf@alyssa.is> MIME-Version: 1.0 Content-Type: text/plain Received-SPF: pass client-ip=66.111.4.28; envelope-from=hi@alyssa.is; helo=out4-smtp.messagingengine.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/08/05 11:13:08 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 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" Quoting from the definition of VHOST_USER_SET_PROTOCOL_FEATURES in vhost-user.rst: > Only legal if feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` is present in > ``VHOST_USER_GET_FEATURES``. > > .. Note:: > Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must support > this message even before ``VHOST_USER_SET_FEATURES`` was called. To me, this could mean either of two things: (1) If VHOST_USER_F_PROTOCOL_FEATURES hasn't been set, upon receiving VHOST_USER_SET_PROTOCOL_FEATURES, a backend should enable the protocol features immediately. (2) If VHOST_USER_F_PROTOCOL_FEATURES hasn't been set, upon receiving VHOST_USER_SET_PROTOCOL_FEATURES, a backend should store those feature bits, but not actually consider them to be enabled until after VHOST_USER_SET_FEATURES has been received (presumably containing VHOST_USER_F_PROTOCOL_FEATURES). The reason I bring this up is that QEMU appears to interpret it as (1), while the vhost-user-net backend in Intel's cloud-hypervisor[1] interprets it as (2). So I'm looking for a clarification. [1]: https://github.com/cloud-hypervisor/cloud-hypervisor Thanks in advance.