x86, mpx: context-switch new MPX address size MSR

As mentioned in previous patches, larger address spaces mean
larger MPX tables.  But, the entire system is either entirely
using 5-level paging, or not.  We do not mix pagetable formats.

If the size of the MPX tables depended soley on the paging mode,
old binaries would break because the format of the tables changed
underneath them.  So, since CR4 never changes, but we need some
way to change the MPX table format, a new MSR is introduced:

If we are in 5-level paging mode *and* the enable bit in this MSR
is set, the CPU will use the new, larger MPX bounds table format.
If 5-level paging is disabled, or the enable bit is clear, then
the legacy-style smaller tables will be used.

But, we might mix legacy and non-legacy binaries on the same
system, so this MSR needs to be context-switched.  Add code to
do this, along with some simple optimizations to skip the MSR
writes if the MSR does not need to be updated.
2 files changed