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[99.254.114.190]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-6d17973d8bdsm36497896d6.18.2024.10.28.14.56.11 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 28 Oct 2024 14:56:12 -0700 (PDT) Date: Mon, 28 Oct 2024 17:56:10 -0400 From: Peter Xu To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: Steven Sistare , Paolo Bonzini , qemu-devel@nongnu.org, Fabiano Rosas , David Hildenbrand , Marcel Apfelbaum , Eduardo Habkost , Philippe Mathieu-Daude , Markus Armbruster Subject: Re: [RFC V1 00/14] precreate phase Message-ID: References: <1729178055-207271-1-git-send-email-steven.sistare@oracle.com> <922177b7-216f-4176-a57a-a86f32252664@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Received-SPF: pass client-ip=170.10.129.124; envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -24 X-Spam_score: -2.5 X-Spam_bar: -- X-Spam_report: (-2.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.373, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-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 Fri, Oct 25, 2024 at 02:43:06PM +0100, Daniel P. Berrangé wrote: > On Fri, Oct 25, 2024 at 09:33:51AM -0400, Steven Sistare wrote: > > On 10/25/2024 4:46 AM, Daniel P. Berrangé wrote: > > > On Thu, Oct 24, 2024 at 05:16:14PM -0400, Steven Sistare wrote: > > > > > > > > Regarding: "what you want is effectively to execute monitor commands > > > > from the migration stream" > > > > > > > > That is not the goal of this series. It could be someone else's goal, when > > > > fully developing a precreate phase, and in that context I understand and > > > > agree with your comments. I have a narrower immediate problem to solve, > > > > however. > > > > > > > > For CPR, src qemu sends file descriptors to dst qemu using SCM_RIGHTS over > > > > a dedicated channel, then src qemu sends migration state over the normal > > > > migration channel. > > > > > > > > Dst qemu reads the fds early, then calls the backend and device creation > > > > functions which use them. Dst qemu then accepts and reads the migration > > > > channel. > > > > > > > > We need a way to send monitor commands that set dst migration capabilities, > > > > before src qemu starts the migration. Hence the dst cannot proceed to > > > > backend and device creation because the src has not sent fd's yet. Hence > > > > we need a dst monitor before device creation. The precreate phase does that. > > > > > > Sigh, what we obviously need here, is what we've always talked about as our > > > long term design goal: > > > > > > A way to launch QEMU with the CLI only specifying the QMP socket, and every > > > other config aspect done by issuing QMP commands, which are processed in the > > > order the mgmt app sends them, so QEMU hasn't have to hardcode processing > > > of different pieces in different phases. > > > > > > Anything that isn't that, is piling more hacks on top of our existing > > > mountain of hacks. That's OK if it does something useful as a side effect > > > that moves us incrementally closer towards that desired end goal. > > > > > > > Regarding: "This series makes this much more complex." > > > > > > > > I could simplify it if I abandon CPR for chardevs. Then qemu_create_early_backends > > > > and other early dependencies can remain as is. I would drop the notion of > > > > a precreate phase, and instead leverage the preconfig phase. I would move > > > > qemu_create_late_backends, and a small part at the end of qemu_init, to > > > > qmp_x_exit_preconfig. > > > > > > Is CPR still going to useful enough in the real world if you drop chardev > > > support ? Every VM has at least one chardev for a serial device doesn't > > > it, and often more since we wire chardevs into all kinds of places. > > > > CPR for chardev is not as useful for cpr-transfer mode because the mgmt layer already > > knows how to create and manage new connections to dest qemu, as it would for normal > > migration. > > > > CPR for chardev is very useful for cpr-exec mode. And cpr-exec mode does not need any > > of these monitor patches, because old qemu exec's new qemu, and they are never active > > at the same time. One must completely specify the migration using src qemu before > > initiating the exec. I mourn cpr-exec mode. > > > > Which begs the question, do we really need to allow migration parameters to be set > > in the dest monitor when using cpr? CPR is a very restricted mode of migration. > > Let me discuss this with Peter. > > The migration QAPI design has always felt rather odd to me, in that we > have perfectly good commands "migrate" & "migrate-incoming" that are able > to accept an arbitrary list of parameters when invoked. Instead of passing > parameters to them though, we instead require apps use the separate > migreate-set-parameters/capabiltiies commands many times over to set > global variables which the later 'migrate' command then uses. Just to mention, we will still need some special parameters that can change during migration, like max-bandwidth, max-downtime etc. So not all of them can be made into "migrate"/"migrate-incoming" arguments. > > The reason for this is essentially a historical mistake - we copied the > way we did it from HMP, which was this way because HMP was bad at supporting > arbitrary customizable paramters to commands. I wish we hadn't copied this > design over to QMP. > > To bring it back on topic, we need QMP on the dest to set parameters, > because -incoming was limited to only take the URI. > > If the "migrate-incoming" command accepted all parameters directly, > then we could use QAPI visitor to usupport a "-incoming ..." command > that took an arbitrary JSON document and turned it into a call to > "migrate-incoming". > > With that we would never need QMP on the target for cpr-exec, avoiding > this ordering poblem you're facing....assuming we put processing of > -incoming at the right point in the code flow > > Can we fix this design and expose the full configurability on the > CLI using QAPI schema & inline JSON, like we do for other QAPI-ified > CLI args. > > It seems entirely practical to me to add parameters to 'migrate-incoming' > in a backwards compatible manner and deprecate set-parameters/capabilities Fabiano started working on handshake recently. After that is ready, we should logically also achieve similar goal at least for dest qemu so no cap/parameter is ever needed there. Thanks, -- Peter Xu