Re: [PATCH 3/3] migration/multifd: fix potential wrong acception order of IOChannel

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

On Wed, Oct 23, 2019 at 11:32:14AM +0800, cenjiahui wrote:
> From: Jiahui Cen <cenjiahui@huawei.com>
> 
> Multifd assumes the migration thread IOChannel is always established before
> the multifd IOChannels, but this assumption will be broken in many situations
> like network packet loss.
> 
> For example:
> Step1: Source (migration thread IOChannel)  --SYN-->  Destination
> Step2: Source (migration thread IOChannel)  <--SYNACK  Destination
> Step3: Source (migration thread IOChannel, lost) --ACK-->X  Destination
> Step4: Source (multifd IOChannel) --SYN-->    Destination
> Step5: Source (multifd IOChannel) <--SYNACK   Destination
> Step6: Source (multifd IOChannel, ESTABLISHED) --ACK-->  Destination
> Step7: Destination accepts multifd IOChannel
> Step8: Source (migration thread IOChannel, ESTABLISHED) -ACK,DATA->  Destination
> Step9: Destination accepts migration thread IOChannel
> 
> The above situation can be reproduced by creating a weak network environment,
> such as "tc qdisc add dev eth0 root netem loss 50%". The wrong acception order
> will cause magic check failure and thus lead to migration failure.
> 
> This patch fixes this issue by sending a migration IOChannel initial packet with
> a unique id when using multifd migration. Since the multifd IOChannels will also
> send initial packets, the destination can judge whether the processing IOChannel
> belongs to multifd by checking the id in the initial packet. This mechanism can
> ensure that different IOChannels will go to correct branches in our test.

Isn't this going to break back compatibility when new QEMU talks to old
QEMU with multifd enabled ? New QEMU will be sending a packet that old
QEMU isn't expecting IIUC.

> Signed-off-by: Jiahui Cen <cenjiahui@huawei.com>
> Signed-off-by: Ying Fang <fangying1@huawei.com>

Regards,
Daniel
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