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Userspace can inspect the flags that the > kernel supports and confirm the next kernel also supports them. I'm fine with that bit, it is really just a different way to encode an ABI ID #. If ABI 5 is spelled b11111 instead it doesn't materially change how things work. It was always the case you can use the same struct extension techinque with a version number. A version can't handle 'holes' in the feature mask, but I haven't thought that was a meaningful use case. "if (id > 10)" instead "if (id & FEAT)" isn't a big coding difference. My concern is the implied position that the kernel must make full use of this to maximize compatability or bust. That's what has always concerned me about all these proposals since the start. I do not want to have arguments upstream about not changing things because we didn't do a perfect job preserving luo compatability with upgrade and downgrade for arbitary kernel versions. That has always been my position and worry. So, change the simple version to a u64 features[] tuple (be sure to use an array, we will need alot of them!!) and I'm fine. But I'm not going to promise that every upstream kernel will strictly only have an increasing string of 1's. You are going to get some that look like b1110000 and still wont work with old kernels. Sean's initial email was asking for strict never-break ABI compatability rules on top, and that is what I've been reacting to. Jason