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

[manish.mishra]

On 13/10/22 1:45 pm, Daniel P. Berrangé wrote:
> 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://urldefense.proofpoint.com/v2/url?u=https-3A__lists.gnu.org_archive_html_qemu-2Ddevel_2019-2D10_msg05920.html&d=DwIBaQ&c=s883GpUCOChKOHiocYtGcg&r=c4KON2DiMd-szjwjggQcuUvTsPWblztAL0gVzaHnNmc&m=LZBcU_C3HMbpUCFZgqxkS-pV8C2mHOjqUTzt45LlLwa26DA0pCAjJVDoamnX8vnC&s=B-b_HMnn_ee6JeA87-PVNBrBqxzdWYgo5PpaP91dqT8&e=
> > > [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

ok got it, thank you so much Daniel, in that case i will try to create some 
patch considering backward compatibility and send for review. Mainly i wanted 
to understand if it is handled somehow differently in upstream master, but 
manually looking code it did not look like that, so just wanted to confirm.

Thanks

Manish Mishra