Hi Folks, I've heard through the grapevine that there's some concern that we should not be bothering to enable XSAVES because there's not a sufficient use case for it. Maybe it's meager today, but I still think we should do it.
I'll try to lay out why. Today, on every Skylake system, this patch saves 128 bytes in each task_struct. If there were an Atom system with XSAVES it would save 384 bytes since there is no AVX support on Atom. If there were a future processor which has an xstate _past_ AVX-512, but that does not have AVX-512 itself, that savings goes up to 2048+384 bytes. I believe it is *inevitable* that the savings will become substantial. Plus, if the processors ever start supporting a supervisor state that we _need_ in Linux, we have to XSAVES support anyway. It's inevitable that we _will_ need it. Why do it today? Now that Skylake is out, we _can_ get reasonable testing of this feature from early adopters in the wild. If we turn this on today, and it breaks, we break a relatively modest number of Skylake systems (1%? 2%? 0.1%?). Let's say we wait $X years when the benefits are greater. We turn it on, and something breaks. We'll break 50% (or 40% or whatever) of the systems in production. Once we *HAVE* XSAVES support, it also opens up the possibilities for doing things like dynamic XSAVE buffer allocation. For instance, let threads that are not _using_ AVX-512 not waste the 2k of space for it. So why wait?
