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. 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 With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
