On 05/09/2016 01:46 PM, Yu-cheng Yu wrote: > When XSAVES is used, xsave.header.xcomp_bv[63] must be set.
So, that's not strictly correct. XSAVES can write to a completely empty (0'd) memory buffer. It's only XRSTORS that _needs_ bit 63 set. The instruction reference is pretty clear on this point. Oh, and if you decided to do this for some reason, please have mercy and go make a macro for 1<<63. Also, I don't think the kernel ever checks for this bit. So are we really calling XRSTORS on otherwise uninitialized xsave buffers?
