Re: MultiFD and default channel out of order mapping on receive side.

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Oct 13, 2022 at 01:23:40AM +0530, manish.mishra wrote:
> Hi Everyone,
> Hope everyone is doing great. I have seen some live migration issues with qemu-4.2 when using multiFD. Signature of issue is something like this.
> 2022-10-01T09:57:53.972864Z qemu-kvm: failed to receive packet via multifd channel 0: multifd: received packet magic 5145564d expected 11223344
> 
> Basically default live migration channel packet is received on multiFD channel. I see a older patch explaining potential reason for this behavior.
> https://lists.gnu.org/archive/html/qemu-devel/2019-10/msg05920.html
> > [PATCH 3/3] migration/multifd: fix potential wrong acception order of IO.
> 
> But i see this patch was not merged. By looking at qemu master code, i
> could not find any other patch too which can handle this issue. So as
> per my understanding this is still a potential issue even in qemu
> master. I mainly wanted to check why this patch was dropped?

See my repllies in that message - it broke compatilibity of data on
the wire, meaning old QEMU can't talk to new QEMU and vica-verca.

We need a fix for this issue, but it needs to take into account
wire compatibility.

With regards,
Daniel
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