The variable MmSubsectionBase is not initialized.

It is necessary for the correct working of MI_MAKE_SUBSECTION_PTE() and MiSubsectionPteToSubsection() on X86 systems.
These functions are also called during OS initialization in MmArmInitSystem () during debugging. Obviously, although debugging checks do not produce errors, they do not work correctly.
On MmSubsectionBase1.png, my solution to the problem. MmSubsectionBase2.png and MmSubsectionBase3.png show where this is used.