Re: [RFC PATCH] x86/arch_prctl: Add ARCH_SET_XCR0 to mask XCR0 per-thread

[Keno Fischer]

>> 4) Catch the fault thrown by xsaves/xrestors in this situation, update
>>     XCR0, redo the xsaves/restores, put XCR0 back and continue
>>     execution after the faulting instruction.
>
> I'm worried about the kernel pieces that go digging in the XSAVE data
> getting confused more than the hardware getting confused.

So you prefer this option? If so, I can try to have a go at implementing it
this way and seeing if I run into any trouble.

>> At least currently, it is my understanding that `xfeatures_mask` only has
>> user features, am I mistaken about that?
>
> We're slowing adding supervisor support.  I think accounting for
> supervisor features is a requirement for any new XSAVE code.

Sure, I don't think this is in any way incompatible with that (though
probably also informs that we want to keep the memory layout the
same if possible).