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Tue, 01 Sep 2026 09:03:34 -0700 (PDT) Received: from jwang-ThinkPad-T14-Gen-6.fkb.profitbricks.net ([2001:9e8:1471:a200:e095:5b15:7c12:5f73]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49cdce0b456sm87367435e9.2.2026.09.01.09.03.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 01 Sep 2026 09:03:34 -0700 (PDT) From: Jack Wang To: qemu-devel@nongnu.org Cc: Peter Xu , Fabiano Rosas , Li Zhijian , yanfei.xu@bytedance.com, Jack Wang Subject: [PATCH 0/2] migration/rdma: send small control messages inline Date: Tue, 1 Sep 2026 17:51:19 +0200 Message-ID: <20260901160333.29859-1-jinpu.wang@ionos.com> X-Mailer: git-send-email 2.43.0 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Received-SPF: permerror client-ip=2a00:1450:4864:20::335; envelope-from=jinpu.wang@ionos.com; helo=mail-wm1-x335.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 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_NONE=-0.0001, SPF_HELO_NONE=0.001, T_SPF_PERMERROR=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: qemu development 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 From: Jack Wang Control-channel sends (registration requests, ram block replies, etc.) carry only a few dozen bytes, but qemu_rdma_post_send_control() still always memcpy()s them into a pre-registered scratch buffer and lets the HCA fetch them with a separate local memory read before sending. This series asks the QP for a little inline send space at creation time (patch 1) and then, once that plumbing exists, drops the mandatory memcpy() by pointing the SGEs directly at the header/payload for any message that fits inline (patch 2). Messages too large to inline keep using the old copy-into-registered-buffer path, so there is no wire protocol change. In testing, this doesn't show an obvious improvement in total migration time or downtime -- the control channel isn't the bottleneck there. Still, it seems like the right thing to do: inline sends are supported by modern HCAs and reduce per-message latency on the control channel by skipping the extra local memory read, so it should help control-plane responsiveness (e.g. registration round trips) even where it doesn't move the overall migration numbers. Jack Wang (2): migration/rdma: send small control messages inline migration/rdma: avoid memcpy for inline control sends migration/rdma.c | 82 +++++++++++++++++++++++++++++++++++++----------- 1 file changed, 63 insertions(+), 19 deletions(-) -- 2.43.0