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Berrange" Subject: Re: [PATCH V1 20/26] migration: cpr-exec mode In-Reply-To: <39b6e4b1-1910-4411-a3f0-d96214bcd6d6@oracle.com> (Steven Sistare's message of "Thu, 2 May 2024 12:00:23 -0400") References: <1714406135-451286-1-git-send-email-steven.sistare@oracle.com> <1714406135-451286-21-git-send-email-steven.sistare@oracle.com> <87bk5o7679.fsf@pond.sub.org> <39b6e4b1-1910-4411-a3f0-d96214bcd6d6@oracle.com> Date: Fri, 03 May 2024 08:26:50 +0200 Message-ID: <87edaj2yxh.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.2 Received-SPF: pass client-ip=170.10.129.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -35 X-Spam_score: -3.6 X-Spam_bar: --- X-Spam_report: (-3.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.476, 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_H4=0.001, RCVD_IN_MSPIKE_WL=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 Steven Sistare writes: > On 5/2/2024 8:23 AM, Markus Armbruster wrote: >> Steve Sistare writes: >> >>> Add the cpr-exec migration mode. Usage: >>> qemu-system-$arch -machine memfd-alloc=on ... >>> migrate_set_parameter mode cpr-exec >>> migrate_set_parameter cpr-exec-args \ >>> ... -incoming >>> migrate -d >>> >>> The migrate command stops the VM, saves state to the URI, >>> directly exec's a new version of QEMU on the same host, >>> replacing the original process while retaining its PID, and >>> loads state from the URI. Guest RAM is preserved in place, >>> albeit with new virtual addresses. >>> >>> Arguments for the new QEMU process are taken from the >>> @cpr-exec-args parameter. The first argument should be the >>> path of a new QEMU binary, or a prefix command that exec's the >>> new QEMU binary. >>> >>> Because old QEMU terminates when new QEMU starts, one cannot >>> stream data between the two, so the URI must be a type, such as >>> a file, that reads all data before old QEMU exits. >>> >>> Memory backend objects must have the share=on attribute, and >>> must be mmap'able in the new QEMU process. For example, >>> memory-backend-file is acceptable, but memory-backend-ram is >>> not. >>> >>> The VM must be started with the '-machine memfd-alloc=on' >>> option. This causes implicit ram blocks (those not explicitly >>> described by a memory-backend object) to be allocated by >>> mmap'ing a memfd. Examples include VGA, ROM, and even guest >>> RAM when it is specified without a memory-backend object. >>> >>> The implementation saves precreate vmstate at the end of normal >>> migration in migrate_fd_cleanup, and tells the main loop to call >>> cpr_exec. Incoming qemu loads preceate state early, before objects >>> are created. The memfds are kept open across exec by clearing the >>> close-on-exec flag, their values are saved in precreate vmstate, >>> and they are mmap'd in new qemu. >>> >>> Note that the memfd-alloc option is not related to memory-backend-memfd. >>> Later patches add support for memory-backend-memfd, and for additional >>> devices, including vfio, chardev, and more. >>> >>> Signed-off-by: Steve Sistare >> >> [...] >> >>> diff --git a/qapi/migration.json b/qapi/migration.json >>> index 49710e7..7c5f45f 100644 >>> --- a/qapi/migration.json >>> +++ b/qapi/migration.json >>> @@ -665,9 +665,37 @@ >>> # or COLO. >>> # >>> # (since 8.2) >>> +# >>> +# @cpr-exec: The migrate command stops the VM, saves state to the URI, What URI? I know you mean the migration URI, but will readers know? Elsewhere, we use "migration URI". Hmm. That's no good, either: we may not *have* a migration URI since commit 074dbce5fcce (migration: New migrate and migrate-incoming argument 'channels') and its fixup commit 57fd4b4e1075 made command migrate argument @uri optional and mutually exclusive with @channels. I think we better use more generic terminology here. Let's have a look at migrate's documentation for inspiration: ## # @migrate: # # Migrates the current running guest to another Virtual Machine. # # @uri: the Uniform Resource Identifier of the destination VM # # @channels: list of migration stream channels with each stream in the # list connected to a destination interface endpoint. # [...] # Notes: [...] # 4. The uri argument should have the Uniform Resource Identifier # of default destination VM. This connection will be bound to # default network. # # 5. For now, number of migration streams is restricted to one, # i.e. number of items in 'channels' list is just 1. # # 6. The 'uri' and 'channels' arguments are mutually exclusive; # exactly one of the two should be present. Perhaps "saves the state to the migration destination"? >>> +# directly exec's a new version of QEMU on the same host, >>> +# replacing the original process while retaining its PID, and >>> +# loads state from the URI. Guest RAM is preserved in place, "loads the state from the migration destination"? We should also fix up existing uses of "migration URI": @mapped-ram, @cpr-reboot, @tls-hostname. Not this series' job. I'll report it separately. >>> +# albeit with new virtual addresses. >> >> Do you mean the virtual addresses of guest RAM may differ betwen old and >> new QEMU process? > > The VA at which a guest RAM segment is mapped in the QEMU process > changes. The end user would not notice or care, so I'll drop that > detail here. > >>> +# >>> +# Arguments for the new QEMU process are taken from the >>> +# @cpr-exec-args parameter. The first argument should be the >>> +# path of a new QEMU binary, or a prefix command that exec's the >>> +# new QEMU binary. >> >> What's a "prefix command"? A wrapper script, perhaps? > > A prefix command is any command of the form: > command1 command1-args command2 command2-args > where command1 performs some set up before exec'ing command2. > However, I will drop the word "prefix", it adds no meaning here. Maybe "the command to start the new QEMU process"? Hmm. @cpr-exec-args is documented like this: # @cpr-exec-args: Arguments passed to new QEMU for @cpr-exec mode. # See @cpr-exec for details. (Since 9.1) Is it a good idea to keep the details with @cpr-exec? Let me try not to. Replace the "Arguments for the new QEMU process..." paragraph by # The new QEMU process is started according to migration parameter # @cpr-exec-args. Then document cpr-exec-args like # @cpr-exec-args: Command to start the new QEMU process for MigMode # @cpr-exec. The first list element is the program's filename, the # remainder its arguments. What do you think? Naming the thing "-args" feels questionable. It's program and arguments. For what it's worth, QGA command guest-exec has them separate: # @path: path or executable name to execute # # @arg: argument list to pass to executable The name @path is poorly chosen. qmp_guest_exec() then prepends @path to @arg to make the argv[] for the execve() wrapper it uses. I figure you'd rather not have them separate, to keep migration parameters simpler. Name it @cpr-exec-command? >>> +# >>> +# Because old QEMU terminates when new QEMU starts, one cannot >>> +# stream data between the two, so the URI must be a type, such as >>> +# a file, that reads all data before old QEMU exits. >> >> What happens when you specify a URI that doesn't? > > Old QEMU will quietly block indefinitely writing to the URI. Worth spelling that out in the doc comment? >>> +# >>> +# Memory backend objects must have the share=on attribute, and >>> +# must be mmap'able in the new QEMU process. For example, >>> +# memory-backend-file is acceptable, but memory-backend-ram is >>> +# not. >>> +# >>> +# The VM must be started with the '-machine memfd-alloc=on' >> >> What happens when you don't? > > If '-only-migratable-modes cpr-exec' is specified, then QEMU will fail > to start, and print a clear error message. > > Otherwise, a blocker is registered and any attempt to cpr-exec will fail > with a clear error message. With clear errors, no further documentation is needed. Good :) > - Steve > >>> +# option. This causes implicit ram blocks -- those not explicitly >>> +# described by a memory-backend object -- to be allocated by >>> +# mmap'ing a memfd. Examples include VGA, ROM, and even guest >>> +# RAM when it is specified without a memory-backend object. >>> +# >>> +# (since 9.1) >>> ## >>> { 'enum': 'MigMode', >>> - 'data': [ 'normal', 'cpr-reboot' ] } >>> + 'data': [ 'normal', 'cpr-reboot', 'cpr-exec' ] } >>> >>> ## >>> # @ZeroPageDetection: >> >> [...] >>