Re: [PATCH V7 00/24] Live update: cpr-transfer

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

On 4/9/2025 12:22 PM, Vladimir Sementsov-Ogievskiy wrote:
> 
> On 15.01.25 22:00, Steve Sistare wrote:
>> This patch series implements a minimal version of cpr-transfer.  Additional
>> series are ready to be posted to deliver the complete vision described
>> above, including
>>    * vfio
>>    * chardev
>>    * vhost and tap
>>    * blockers
>>    * cpr-exec mode
> 
> Hi Steve. First, great congratulations with finally landed cpr-transfer! I saw the history of Live Update series was started overly five years ago.

Thanks!  It's been a marathon, not a sprint.

> I've some questions, hope it's not much trouble for you.
> 
> 1. We consider porting cpr-transver + vfio part of your "Live update: vfio and iommufd" to our downstream QEMU, based on v7.2. What do you think? I mean, may be you may quickly answer "don't try, you'll have to bring more than 100 commits from different series", or visa-versa "we have downstream based on 7.2 too, so it's possible" (OK, seems the latter answer is not possible, as iommufd code just absent in v7.2).

I have not tried it, but I think this is feasible if you omit the iommufd patches.
You will also need some of the cpr-reboot patches (like mode-specific migration
blockers) which did not appear until qemu 8.2.

> 2. About cpr-exec. Do you plan resending it in future? The solution is interesting for us, as it simplifies management a lot. 

I agree!  I made that argument when I submitted it.  Perhaps your +1 will add
enough critical mass to get it accepted next time.  I do plan to resubmit it later.

> I read the discussion on cpr-exec, seems the main problem was the security constraint, that we don't want to allow exec call in seccomp profile. Didn't you consider a variant with loding the library instead of exec?
> 
> I mean:
> 
> - turn the whole QEMU into library, which may be dynamically loaded. Recently there was a question how to do it, and the answer contained an example patch: https://github.com/pbo-linaro/qemu/commit/fbb39cc64f77d4bf1e5e50795c75b62735bf5c5f
> 
> - and make a simple wrapper process for that library, which also is a container for migration state (including file descriptors), during live update.
> 
> Benefits:
> 
> - no execve, and we just need to add pattern for "qemu library" paths to apparmor profile
> 
> - probably, we can load new library _before_ starting the migration, reducing freeze-time of migration - more like migration with two processes

I have not considered that.  A colleague suggested something similar -- loading the
new qemu binary in memory and implementing exec in userland.   No doubt either
method would be a non-trivial amount of work, versus cpr-exec which already works :)

Personally I don't think that requiring exec is a show stopper. If qemu is deployed in
a container environment, then the potential targets of an exec can be limited by the
container walls.

- Steve