arm64: entry: move bulk of ret_to_user to C

In `ret_to_user` we perform some conditional work depending on the
thread flags, then perform some IRQ/context tracking which is intended
to balance with the IRQ/context tracking performed in the entry C code.

For simplicity and consistency, it would be preferable to move this all
to C. As a step towards that, this patch moves the conditional work and
IRQ/context tracking into a C helper function. To aid bisectability,
this is called from the `ret_to_user` assembly, and a subsequent patch
will move the call to C code.

As local_daif_mask() handles all necessary tracing and PMR manipulation,
we no longer need to handle this explicitly. As we call
exit_to_user_mode() directly, the `user_enter_irqoff` macro is no longer
used, and can be removed. As enter_from_user_mode() and
exit_to_user_mode() are no longer called from assembly, these can be
made static, and as these are typically very small, they are marked
__always_inline to avoid the overhead of a function call.

For now, enablement of single-step is left in entry.S, and for this we
still need to read the flags in ret_to_user(). It is safe to read this
separately as TIF_SINGLESTEP is not part of _TIF_WORK_MASK.

There should be no functional change as a result of this patch.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Joey Gouly <joey.gouly@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Link: https://lore.kernel.org/r/20210802140733.52716-4-mark.rutland@arm.com
[catalin.marinas@arm.com: removed unused gic_prio_kentry_setup macro]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/exception.h

@@ -55,8 +55,8 @@ asmlinkage void el0t_32_error_handler(struct pt_regs *regs);
 
 asmlinkage void call_on_irq_stack(struct pt_regs *regs,
 				  void (*func)(struct pt_regs *));
-asmlinkage void enter_from_user_mode(void);
-asmlinkage void exit_to_user_mode(void);
+asmlinkage void asm_exit_to_user_mode(struct pt_regs *regs);
+
 void do_mem_abort(unsigned long far, unsigned int esr, struct pt_regs *regs);
 void do_undefinstr(struct pt_regs *regs);
 void do_bti(struct pt_regs *regs);
@@ -73,6 +73,7 @@ void do_el0_svc(struct pt_regs *regs);
 void do_el0_svc_compat(struct pt_regs *regs);
 void do_ptrauth_fault(struct pt_regs *regs, unsigned int esr);
 void do_serror(struct pt_regs *regs, unsigned int esr);
+void do_notify_resume(struct pt_regs *regs, unsigned long thread_flags);
 
 void panic_bad_stack(struct pt_regs *regs, unsigned int esr, unsigned long far);
 #endif	/* __ASM_EXCEPTION_H */

arch/arm64/kernel/entry-common.c

@@ -104,7 +104,7 @@ static __always_inline void __enter_from_user_mode(void)
 	trace_hardirqs_off_finish();
 }
 
-asmlinkage void noinstr enter_from_user_mode(void)
+static __always_inline void enter_from_user_mode(void)
 {
 	__enter_from_user_mode();
 }
@@ -123,12 +123,29 @@ static __always_inline void __exit_to_user_mode(void)
 	lockdep_hardirqs_on(CALLER_ADDR0);
 }
 
-asmlinkage void noinstr exit_to_user_mode(void)
+static __always_inline void exit_to_user_mode(void)
 {
 	mte_check_tfsr_exit();
 	__exit_to_user_mode();
 }
 
+static __always_inline void prepare_exit_to_user_mode(struct pt_regs *regs)
+{
+	unsigned long flags;
+
+	local_daif_mask();
+
+	flags = READ_ONCE(current_thread_info()->flags);
+	if (unlikely(flags & _TIF_WORK_MASK))
+		do_notify_resume(regs, flags);
+}
+
+asmlinkage void noinstr asm_exit_to_user_mode(struct pt_regs *regs)
+{
+	prepare_exit_to_user_mode(regs);
+	exit_to_user_mode();
+}
+
 /*
  * Handle IRQ/context state management when entering an NMI from user/kernel
  * mode. Before this function is called it is not safe to call regular kernel

arch/arm64/kernel/entry.S

@@ -29,16 +29,6 @@
 #include <asm/asm-uaccess.h>
 #include <asm/unistd.h>
 
-/*
- * Context tracking and irqflag tracing need to instrument transitions between
- * user and kernel mode.
- */
-	.macro user_enter_irqoff
-#if defined(CONFIG_CONTEXT_TRACKING) || defined(CONFIG_TRACE_IRQFLAGS)
-	bl	exit_to_user_mode
-#endif
-	.endm
-
 	.macro	clear_gp_regs
 	.irp	n,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29
 	mov	x\n, xzr
@@ -474,18 +464,6 @@ SYM_CODE_END(__swpan_exit_el0)
 /* GPRs used by entry code */
 tsk	.req	x28		// current thread_info
 
-/*
- * Interrupt handling.
- */
-	.macro	gic_prio_kentry_setup, tmp:req
-#ifdef CONFIG_ARM64_PSEUDO_NMI
-	alternative_if ARM64_HAS_IRQ_PRIO_MASKING
-	mov	\tmp, #(GIC_PRIO_PSR_I_SET | GIC_PRIO_IRQON)
-	msr_s	SYS_ICC_PMR_EL1, \tmp
-	alternative_else_nop_endif
-#endif
-	.endm
-
 	.text
 
 /*
@@ -585,37 +563,17 @@ SYM_CODE_START_LOCAL(ret_to_kernel)
 	kernel_exit 1
 SYM_CODE_END(ret_to_kernel)
 
-/*
- * "slow" syscall return path.
- */
 SYM_CODE_START_LOCAL(ret_to_user)
-	disable_daif
-	gic_prio_kentry_setup tmp=x3
-#ifdef CONFIG_TRACE_IRQFLAGS
-	bl	trace_hardirqs_off
-#endif
-	ldr	x19, [tsk, #TSK_TI_FLAGS]
-	and	x2, x19, #_TIF_WORK_MASK
-	cbnz	x2, work_pending
-finish_ret_to_user:
-	user_enter_irqoff
+	mov	x0, sp
+	bl	asm_exit_to_user_mode
 	/* Ignore asynchronous tag check faults in the uaccess routines */
 	clear_mte_async_tcf
+	ldr	x19, [tsk, #TSK_TI_FLAGS]	// re-check for single-step
 	enable_step_tsk x19, x2
 #ifdef CONFIG_GCC_PLUGIN_STACKLEAK
 	bl	stackleak_erase
 #endif
 	kernel_exit 0
-
-/*
- * Ok, we need to do extra processing, enter the slow path.
- */
-work_pending:
-	mov	x0, sp				// 'regs'
-	mov	x1, x19
-	bl	do_notify_resume
-	ldr	x19, [tsk, #TSK_TI_FLAGS]	// re-check for single-step
-	b	finish_ret_to_user
 SYM_CODE_END(ret_to_user)
 
 	.popsection				// .entry.text

arch/arm64/kernel/signal.c

@@ -924,8 +924,7 @@ static bool cpu_affinity_invalid(struct pt_regs *regs)
 				 system_32bit_el0_cpumask());
 }
 
-asmlinkage void do_notify_resume(struct pt_regs *regs,
-				 unsigned long thread_flags)
+void do_notify_resume(struct pt_regs *regs, unsigned long thread_flags)
 {
 	do {
 		if (thread_flags & _TIF_NEED_RESCHED) {