v4:
The v3 posting didn't do what it promised to do, so do it now for real.
V3 changes:
change to mov instruction (non locking instuction) instead
of xchg to simplify design.
V2 changes:
Add xchg 16 bit instructions to handle sgdt and sidt base
63:48 bits and 47:32 bits.
Add comment to explain why xchg 64bit isnt being used
Split lock happens when a locking instruction is used on mis-aligned data
that crosses two cachelines. If close source platform enables Alignment
Check Exception(#AC), They can hit a double fault due to split lock being
in CpuExceptionHandlerLib.
sigt and sgdt saves 10 bytes to memory, 8 bytes is base and 2 bytes is limit.
The data is mis-aligned, can cross two cacheline, and a xchg
instruction(locking instuction) is being utilize.
Signed-off-by: John E Lofgren <john.e.lofgren@intel.com> Reviewed-by: Eric Dong <eric.dong@intel.com> Reviewed-by: Laszlo Ersek <lersek@redhat.com>