x86/pat: Simplify the PAT programming protocol

The programming protocol for the PAT MSR follows the MTRR programming
protocol. However, this protocol is cumbersome and requires disabling
caching (CR0.CD=1), which is not possible on some platforms.

Specifically, a TDX guest is not allowed to set CR0.CD. It triggers
a #VE exception.

It turns out that the requirement to follow the MTRR programming
protocol for PAT programming is unnecessarily strict. The new Intel
Software Developer Manual (http://www.intel.com/sdm

) (December 2023)
relaxes this requirement, please refer to the section titled
"Programming the PAT" for more information.

In short, this section provides an alternative PAT update sequence which
doesn't need to disable caches around the PAT update but only to flush
those caches and TLBs.

The AMD documentation does not link PAT programming to MTRR and is there
fore, fine too.

The kernel only needs to flush the TLB after updating the PAT MSR. The
set_memory code already takes care of flushing the TLB and cache when
changing the memory type of a page.

  [ bp: Expand commit message. ]

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20240124130650.496056-1-kirill.shutemov@linux.intel.com