arm64: kexec: Add comment to explain use of __flush_icache_range()

Now that we understand the deadlock arising from flush_icache_range()
on the kexec crash kernel path, add a comment to justify the use of
__flush_icache_range() here.

Reported-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/machine_kexec.c

@@ -184,6 +184,13 @@ void machine_kexec(struct kimage *kimage)
 
 	/* Flush the reboot_code_buffer in preparation for its execution. */
 	__flush_dcache_area(reboot_code_buffer, arm64_relocate_new_kernel_size);
+
+	/*
+	 * Although we've killed off the secondary CPUs, we don't update
+	 * the online mask if we're handling a crash kernel and consequently
+	 * need to avoid flush_icache_range(), which will attempt to IPI
+	 * the offline CPUs. Therefore, we must use the __* variant here.
+	 */
 	__flush_icache_range((uintptr_t)reboot_code_buffer,
 			     arm64_relocate_new_kernel_size);