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Again, I strongly disagree with this MUST. This is not a variant of "we don't break userspace". It is internal kabi and the kernel gets to decide which versions it supports. There are no promises here. It is not a breaking change to say we won't support A->B. Community needs to agree on what these parameters are. I've been clear my stake in the ground is same stable version only as a starting point. This is typical in the industry, nobody does live update where you can jump ahead multiple years of versions. Sequential releases only is very normal. > QEMU has been doing backwards live migration forever, and QEMU is a > gnarly C program that has grown by accretion as we were learning all > this stuff, so it's not impossible at all. I agree it is not impossible, you can always make things harder and more complicated to keep it working. I don't want the kernel to turn into a "gnarly C program" because we over engineered this. > It's not easy, but you aren't even trying to do it right in the kernel. You Right, I've been pretty clear about this, I want to start with same-stable-version only and if that works out then consider if we want to do more. I am not doing this for any sort of technical reason. Not because it is impossible, but because it is HARD. I do not want to force anyone to take on hard and difficult projects like this for a niche feature only a few CSPs are likely to use. If we take this approach LUO will fail because maintainers will be scared to take on this extra work and simply won't merge LUO patches. There needs to be some balance that we can try this out and see if a broader use case develops before we commit to doing the hardest possible version of it. A niche feature only for CSPs like live update must not become a boat anchor on kernel progress! > fact! It's just that you're solving for the wrong complexity, and only > partially so because (if I'm not wrong) there's still the question of how to > bring the manifest of kernel B into kernel A. There is a proposal to encode information into ELF sections. Feature flags should flow through there just fine, I think. I have not seen a proposal to re-inject this information back into the A kernel as a 'restriction' so it fails luo instead of producing something unsupported. Jason