target/arm: Add commentary for CPUARMState.exclusive_high

Document the meaning of exclusive_high in a big-endian context,
and why we can't change it now.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230530191438.411344-2-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

target/arm/cpu.h

@@ -677,8 +677,16 @@ typedef struct CPUArchState {
         uint64_t zcr_el[4];   /* ZCR_EL[1-3] */
         uint64_t smcr_el[4];  /* SMCR_EL[1-3] */
     } vfp;
+
     uint64_t exclusive_addr;
     uint64_t exclusive_val;
+    /*
+     * Contains the 'val' for the second 64-bit register of LDXP, which comes
+     * from the higher address, not the high part of a complete 128-bit value.
+     * In some ways it might be more convenient to record the exclusive value
+     * as the low and high halves of a 128 bit data value, but the current
+     * semantics of these fields are baked into the migration format.
+     */
     uint64_t exclusive_high;
 
     /* iwMMXt coprocessor state.  */