arm64: entry.S: Remove disable_dbg

enable_step_tsk is the only user of disable_dbg, which doesn't respect
our 'dai' order for exception masking. enable_step_tsk may enable
single-step, so previously needed to mask debug exceptions to prevent us
from single-stepping kernel_exit. enable_step_tsk is called at the end
of the ret_to_user loop, which has already masked all exceptions so this
is no longer needed.

Remove disable_dbg, add a comment that enable_step_tsk's caller should
have masked debug.
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/assembler.h

@@ -69,13 +69,6 @@
 	msr	daif, \flags
 	.endm
 
-/*
- * Enable and disable debug exceptions.
- */
-	.macro	disable_dbg
-	msr	daifset, #8
-	.endm
-
 	.macro	enable_dbg
 	msr	daifclr, #8
 	.endm
@@ -89,9 +82,9 @@
 9990:
 	.endm
 
+	/* call with daif masked */
 	.macro	enable_step_tsk, flgs, tmp
 	tbz	\flgs, #TIF_SINGLESTEP, 9990f
-	disable_dbg
 	mrs	\tmp, mdscr_el1
 	orr	\tmp, \tmp, #DBG_MDSCR_SS
 	msr	mdscr_el1, \tmp