Re: [RFC V1 00/14] precreate phase

[Steven Sistare <steven.sistare@oracle.com>]

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.

- Steve