This PR adds some extras to the three control registers CR0, CR4, and XCR0. It also adds some checks for XCR0. (We could extend this to other registers as well.)

Bits added

• CR0:
  • Extension Type (ET, bit 4 of CR0)
• CR4:
  • Key-Locker-Enable Bit (KL, bit 19 of CR4)
  • Control-Flow Enforcement Technology (CET) enable (CET, bit 23 of CR4)
  • Enable protection keys for supervisor-mode pages (PKS, bit 24 of CR4)
• XCR0:
  • Enable XSAVE to manage the bound registers (BNDREG, bit 3 of XCR0)
  • Enable XSAVE to manage the BCFGU and BNDSTATUS registers (BNDCSR, bit 4 of XCR0)
  • Enable AVX-512 and allow XSAVE to manage the mask registers K0-K7 (OPMASK, bit 5 of XCR0)
  • Enable AVX-512 and allow XSAVE to manage the upper halves of the lower ZMM registers (ZMM_HI256, bit 6 of XCR0)
  • Enable AVX-512 and allow XSAVE to manage the upper ZMM registers (HI16_ZMM, bit 7 of XCR0)

Checks

This PR adds invariant checks that cannot be disabled via the assert! macro when using the write() function on the XCr0 type. Primarily, these invariants enforce those listed on pg. 2-20 of volume 3A of the intel SDMs; they disallow attempts to:

• clear XCR0.x87;
• clear XCR0.SSE and set XCR0.AVX;
• clear XCR0.AVX and set any of XCR0.opmask, XCR0.ZMM_Hi256, and XCR0.Hi16_ZMM;
• set either XCR0.BNDREG and XCR0.BNDCSR while not setting the other; and
• set any of XCR0.opmask, XCR0.ZMM_Hi256, and XCR0.Hi16_ZMM while not setting all of them.

I missed the initial invariant (that setting any reserved bits causes a general-protection fault) but I will add that if you guys would like me to (somebody else can do so as well). We should probably consider enforcing all invariants on all control registers to catch problems like this before they arise and generate exceptions, since these APIs are supposed to be "safe". This PR unfortunately has a few breaking changes as well.

Breaking changes

In order for CR4.PKS to coexist with CR4.PKU and for the documentation to make sense, I was forced to rename CR4.PKU to PROTECTION_KEY_USER. I also renamed XCr0Flags::YMM to XCr0Flags::AVX to make the names match with the documentation. I hope you guys don't mind.