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
