Re: [PATCH V2 13/13] migration: cpr-transfer mode

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

On 10/8/2024 3:48 PM, Peter Xu wrote:
> On Tue, Oct 08, 2024 at 04:11:38PM -0300, Fabiano Rosas wrote:
>> As of half an hour ago =) We could put a feature branch up and work
>> together, if you have more concrete thoughts on how this would look like
>> let me know.
> 
> [I'll hijack this thread with one more email, as this is not cpr-relevant]
> 
> I think I listed all the things I can think of in the wiki, so please go
> ahead.
> 
> One trivial suggestion is we can start from the very simple, which is the
> handshake itself, with a self-bootstrap protocol, probably feature-bit
> based or whatever you prefer.  Then we set bit 0 saying "this QEMU knows
> how to handshake".
> 
> Comparing to the rest requirement, IMHO we can make the channel
> establishment the 1st feature, then it's already good for merging, having
> feature bit 1 saying "this qemu understands named channel establishment".
> 
> Then we add new feature bits on top of the handshake feature, by adding
> more feature bits.  Both QEMUs should first handshake on the feature bits
> they support and enable only the subset that all support.
> 
> Or instead of bit, feature strings, etc. would all work which you
> prefer. Just to say we don't need to impl all the ideas there, as some of
> them might take more time (e.g. device tree check), and that list is
> probably not complete anyway.

While writing a qtest for cpr-transfer, I discovered a problem that could be
solved with an early migration handshake, prior to cpr_save_state / cpr_load_state.

There is currently no way to set migration caps on dest qemu before starting
cpr-transfer, because dest qemu blocks in cpr_state_load before creating any
devices or monitors. It is unblocked after the user sends the migrate command
to source qemu, but then the migration starts and it is too late to set migration
capabilities or parameters on the dest.

Are you OK with that restriction (for now, until a handshake is implemented)?
If not, I have a problem.

I can hack the qtest to make it work with the restriction.

- Steve