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envelope-from=steven.sistare@oracle.com; helo=mx0a-00069f02.pphosted.com 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=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, 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 10/8/2024 3:11 PM, Fabiano Rosas wrote: > Peter Xu writes: > >> On Tue, Oct 08, 2024 at 03:28:30PM -0300, Fabiano Rosas wrote: >>>>>> + /* Close cpr socket to tell source that we are listening */ >>>>>> + cpr_state_close(); >>>>> >>>>> Would it be possible to use some explicit reply message to mark this? >>>> >>>> In theory yes, but I fear that using a return channel with message parsing and >>>> dispatch adds more code than it is worth. >>> >>> I think this approach is fine for now, but I wonder whether we could >>> reuse the current return path (RP) by starting it earlier and take >>> benefit from it already having the message passing infrastructure in >>> place. I'm actually looking ahead to the migration handshake thread[1], >>> which could be thought to have some similarity with the early cpr >>> channel. So having a generic channel in place early on to handle >>> handshake, CPR, RP, etc. could be a good idea. >> >> The current design relies on CPR stage happens before device realize()s, so >> I assume migration channel (including RP) isn't easily applicable at as >> early as this stage. > > Well, what is the dependency for the RP? If we can send CPR state, we > can send QEMU_VM_COMMAND, no? The CPR state channel is (and must be) used before migration_object_init, and before the normal migration channel is opened. Thus we cannot use the normal return path. - Steve >> However I think dest qemu can directly write back to the cpr_uri channel >> instead if we want and then follow a protocol simple enough (even though >> it'll be separate from the migration stream protocol). >> >> What worries me more (besides using HUP as of now..) is cpr_state_save() is >> currently synchronous and can block the main iothread. It means if cpr >> destination is not properly setup, it can hang the main thread (including >> e.g. QMP monitor) at qio_channel_socket_connect_sync(). Ideally we >> shouldn't block the main thread. >> >> If async-mode can be done, it might be even easier, e.g. if we do >> cpr_state_save() in a thread, after qemu_put*() we can directly qemu_get*() >> in the same context with the pairing return qemufile. >> >> But maybe we can do it in two steps, merging HUP first. Then when a better >> protocol (plus async mode) ready, one can boost QEMU_CPR_FILE_VERSION. >> I'll see how Steve wants to address it. > > I agree HUP is fine at the moment. > >> >>> >>> Anyway, I'm probing on this a bit so I can start drafting something. I >>> got surprised that we don't even have the capability bits in the stream >>> in a useful way (currently, configuration_validate_capabilities() does >>> kind of nothing). >>> >>> 1- https://wiki.qemu.org/ToDo/LiveMigration#Migration_handshake >> >> Happy to know this. I was thinking whether I should work on this even >> earlier, so if you're looking at that it'll be great. > > As of half an hour ago =) We could put a feature branch up and work > together, if you have more concrete thoughts on how this would look like > let me know. > >> >> The major pain to me is the channel establishment part where we now have >> all kinds of channels, so we should really fix that sooner (e.g., we hope >> to enable multifd + postcopy very soon, that requires multifd and preempt >> channels appear in the same time). It was reasonable the vfio/multifd >> series tried to fix it. >