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Does it ever make sense to call these functions if XSAVE(S) are not supported?
Would it be better to convert the checks into KASSERTs?

'else' is not needed

hm, calling these routines without xsave support does not make sense indeed, but I wonder what's the best way to handle this.
I guess we could convert these into KASSERTs and document that the caller must check for XSAVE support before calling these routines, maybe even add cpu_xsave_supported into fpu.h. What do you think?

Why cannot you check for use_xsave instead?

Typically the low level functions are only called by the higher-level when the hw has required features because high-level simply refuses to work if features are absent. This makes the re-check of the features in the low-level redundant.

But asserts are useful at least as a form of the documentation for assumptions.

I was not aware of use_xsave, thanks!

We already have xsave_area_desc.
Can we extend it instead of introducing a parallel structure?

Also look how we malloc large enough xsave_area_desc instead of hardcoding the max known extension.

Same as with`use_xsave`, I managed to miss xsave_area_desc.
I've extended the struct with a flag field, but I had to keep an additional variable for tracking XSAVE extensions.

Still I want to get rid of the max xfeature number hardcoded.

Is this ilog2()?

ilog2()?