Upate a README

TODO

@@ -560,7 +560,7 @@ o SMP
                This would also be an essential part of a high priority,
                nested interrupt implementation (unrelated).
   Status:      Open
-  Priority:    Low.  There are no know issues with the current non-maskable
+  Priority:    Low.  There are no known issues with the current non-maskable
                SGI implementation.  This change would, however, lead to
                simplification in the design and permit commonality with
                other, non-GIC implementations.

configs/sabre-6quad/README.txt

@@ -169,6 +169,30 @@ Status
   But this does not completely eliminate instabilities which seem to be
   related to memory corruption -- mm_mallinfo() asserts.
 
+CORTEX-A GIC SGI INTERRUPT MASKING (From the top-level TODO list)
+-----------------------------------------------------------------
+In the ARMv7-A GICv2 architecture, the inter-processor interrupts (SGIs) are
+non maskable and will occur even if interrupts are disabled.  This adds a
+lot of complexity to the ARMV7-A critical section design.
+
+Masayuki Ishikawa has suggested the use of the GICv2 ICCMPR register to
+control SGI interrupts.  This register (much like the ARMv7-M BASEPRI
+register) can be used to mask interrupts by interrupt priority.  Since SGIs
+may be assigned priorities the ICCMPR should be able to block execution of
+SGIs as well.
+
+Such an implementation would be very similar to the BASEPRI (vs PRIMASK)
+implementation for the ARMv7-M:  (1) The up_irq_save() and up_irq_restore()
+registers would have to set/restore the ICCMPR register, (2) register setup
+logic in arch/arm/src/armv7-a for task start-up and signal dispatch would
+have to set the ICCMPR correctly, and (3) the 'xcp' structure would have to
+be extended to hold the ICCMPR register;  logic would have to added be
+save/restore the ICCMPR register in the 'xcp' structure on each interrupt
+and context switch.
+
+This would also be an essential part of a high priority, nested interrupt
+implementation (unrelated feature).
+
 Platform Features
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