arm64/mm: Document write abort detection from ESR

This patch adds an is_write_abort() wrapper and documents the detection
of the abort type on cache maintenance operations.

Cc: Will Deacon <will.deacon@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
[catalin.marinas@arm.com: only keep the is_write_abort() wrapper]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/mm/fault.c

@@ -435,6 +435,15 @@ static bool is_el0_instruction_abort(unsigned int esr)
 	return ESR_ELx_EC(esr) == ESR_ELx_EC_IABT_LOW;
 }
 
+/*
+ * Note: not valid for EL1 DC IVAC, but we never use that such that it
+ * should fault. EL0 cannot issue DC IVAC (undef).
+ */
+static bool is_write_abort(unsigned int esr)
+{
+	return (esr & ESR_ELx_WNR) && !(esr & ESR_ELx_CM);
+}
+
 static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
 				   struct pt_regs *regs)
 {
@@ -460,7 +469,7 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
 	if (is_el0_instruction_abort(esr)) {
 		vm_flags = VM_EXEC;
 		mm_flags |= FAULT_FLAG_INSTRUCTION;
-	} else if ((esr & ESR_ELx_WNR) && !(esr & ESR_ELx_CM)) {
+	} else if (is_write_abort(esr)) {
 		vm_flags = VM_WRITE;
 		mm_flags |= FAULT_FLAG_WRITE;
 	}