arm64: introduce an order for exceptions

Currently SError is always masked in the kernel. To support RAS exceptions
using SError on hardware with the v8.2 RAS Extensions we need to unmask
SError as much as possible.

Let's define an order for masking and unmasking exceptions. 'dai' is
memorable and effectively what we have today.

Disabling debug exceptions should cause all other exceptions to be masked.
Masking SError should mask irq, but not disable debug exceptions.
Masking irqs has no side effects for other flags. Keeping to this order
makes it easier for entry.S to know which exceptions should be unmasked.

FIQ is never expected, but we mask it when we mask debug exceptions, and
unmask it at all other times.

Given masking debug exceptions masks everything, we don't need macros
to save/restore that bit independently. Remove them and switch the last
caller over to use the daif calls.

Signed-off-by: James Morse <james.morse@arm.com>
Reviewed-by: Julien Thierry <julien.thierry@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/irqflags.h

@@ -20,6 +20,19 @@
 
 #include <asm/ptrace.h>
 
+/*
+ * Aarch64 has flags for masking: Debug, Asynchronous (serror), Interrupts and
+ * FIQ exceptions, in the 'daif' register. We mask and unmask them in 'dai'
+ * order:
+ * Masking debug exceptions causes all other exceptions to be masked too/
+ * Masking SError masks irq, but not debug exceptions. Masking irqs has no
+ * side effects for other flags. Keeping to this order makes it easier for
+ * entry.S to know which exceptions should be unmasked.
+ *
+ * FIQ is never expected, but we mask it when we disable debug exceptions, and
+ * unmask it at all other times.
+ */
+
 /*
  * CPU interrupt mask handling.
  */
@@ -89,26 +102,5 @@ static inline int arch_irqs_disabled_flags(unsigned long flags)
 {
 	return flags & PSR_I_BIT;
 }
-
-/*
- * save and restore debug state
- */
-#define local_dbg_save(flags)						\
-	do {								\
-		typecheck(unsigned long, flags);			\
-		asm volatile(						\
-		"mrs    %0, daif		// local_dbg_save\n"	\
-		"msr    daifset, #8"					\
-		: "=r" (flags) : : "memory");				\
-	} while (0)
-
-#define local_dbg_restore(flags)					\
-	do {								\
-		typecheck(unsigned long, flags);			\
-		asm volatile(						\
-		"msr    daif, %0		// local_dbg_restore\n"	\
-		: : "r" (flags) : "memory");				\
-	} while (0)
-
 #endif
 #endif

arch/arm64/kernel/debug-monitors.c

@@ -30,6 +30,7 @@
 
 #include <asm/cpufeature.h>
 #include <asm/cputype.h>
+#include <asm/daifflags.h>
 #include <asm/debug-monitors.h>
 #include <asm/system_misc.h>
 
@@ -46,9 +47,9 @@ u8 debug_monitors_arch(void)
 static void mdscr_write(u32 mdscr)
 {
 	unsigned long flags;
-	local_dbg_save(flags);
+	flags = local_daif_save();
 	write_sysreg(mdscr, mdscr_el1);
-	local_dbg_restore(flags);
+	local_daif_restore(flags);
 }
 NOKPROBE_SYMBOL(mdscr_write);