On Mon, Oct 13, 2025 at 12:45 PM H. Peter Anvin <[email protected]> wrote: > > On 2025-10-13 10:14, Andy Lutomirski wrote: > > > > I don't actually remember whether the kernel ever used this. It's > > possible that there are ancient kernels where passing a wild, non-null > > pointer would blow up. But it's certainly safe to pass null, and it's > > certainly safe for the kernel to ignore the parameter. > > > > One could imagine an architecture which would have to execute an actual system > call wanting to use this, but on x86 it is pointless -- even the LSL trick is > much faster than a system call, and once you account for whatever hassle you > would have to deal with do make the cache make sense (probably having a global > generation number and/or a timestamp to expire it) it well and truly makes no > sense.
The global timestamp would just be some field in the vvar area, which we have plenty of anyway. But I agree, accelerating getcpu is pointless. In any case, anything that historically thought it really really wanted accelerated getcpu can, and probably does, use rseq these days. --Andy
