arch/arc/kernel/entry.S - linux - Git at Google

 /*
  * Common Low Level Interrupts/Traps/Exceptions(non-TLB) Handling for ARC
  * (included from entry-<isa>.S
  *
  * Copyright (C) 2014-15 Synopsys, Inc. (www.synopsys.com)
  * Copyright (C) 2004, 2007-2010, 2011-2012 Synopsys, Inc. (www.synopsys.com)
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 2 as
  * published by the Free Software Foundation.
  */

 /*------------------------------------------------------------------
  *    Function                            ABI
  *------------------------------------------------------------------
  *
  *  Arguments                           r0 - r7
  *  Caller Saved Registers              r0 - r12
  *  Callee Saved Registers              r13- r25
  *  Global Pointer (gp)                 r26
  *  Frame Pointer (fp)                  r27
  *  Stack Pointer (sp)                  r28
  *  Branch link register (blink)        r31
  *------------------------------------------------------------------
  */

 ;################### Special Sys Call Wrappers ##########################

 ENTRY(sys_clone_wrapper)
 	SAVE_CALLEE_SAVED_USER
 	bl  @sys_clone
 	DISCARD_CALLEE_SAVED_USER

 	GET_CURR_THR_INFO_FLAGS   r10
 	btst r10, TIF_SYSCALL_TRACE
 	bnz  tracesys_exit

 	b .Lret_from_system_call
 END(sys_clone_wrapper)

 ENTRY(ret_from_fork)
 	; when the forked child comes here from the __switch_to function
 	; r0 has the last task pointer.
 	; put last task in scheduler queue
 	jl   @schedule_tail

 	ld   r9, [sp, PT_status32]
 	brne r9, 0, 1f

 	jl.d [r14]		; kernel thread entry point
 	mov  r0, r13		; (see PF_KTHREAD block in copy_thread)

 1:
 	; Return to user space
 	; 1. Any forked task (Reach here via BRne above)
 	; 2. First ever init task (Reach here via return from JL above)
 	;    This is the historic "kernel_execve" use-case, to return to init
 	;    user mode, in a round about way since that is always done from
 	;    a kernel thread which is executed via JL above but always returns
 	;    out whenever kernel_execve (now inline do_fork()) is involved
 	b    ret_from_exception
 END(ret_from_fork)

 ;################### Non TLB Exception Handling #############################

 ; ---------------------------------------------
 ; Instruction Error Exception Handler
 ; ---------------------------------------------

 ENTRY(instr_service)

 	EXCEPTION_PROLOGUE

 	lr  r0, [efa]
 	mov r1, sp

 	FAKE_RET_FROM_EXCPN

 	bl  do_insterror_or_kprobe
 	b   ret_from_exception
 END(instr_service)

 ; ---------------------------------------------
 ; Machine Check Exception Handler
 ; ---------------------------------------------

 ENTRY(EV_MachineCheck)

 	EXCEPTION_PROLOGUE

 	lr  r2, [ecr]
 	lr  r0, [efa]
 	mov r1, sp

 	; hardware auto-disables MMU, re-enable it to allow kernel vaddr
 	; access for say stack unwinding of modules for crash dumps
 	lr	r3, [ARC_REG_PID]
 	or	r3, r3, MMU_ENABLE
 	sr	r3, [ARC_REG_PID]

 	lsr  	r3, r2, 8
 	bmsk 	r3, r3, 7
 	brne    r3, ECR_C_MCHK_DUP_TLB, 1f

 	bl      do_tlb_overlap_fault
 	b       ret_from_exception

 1:
 	; DEAD END: can't do much, display Regs and HALT
 	SAVE_CALLEE_SAVED_USER

 	GET_CURR_TASK_FIELD_PTR   TASK_THREAD, r10
 	st  sp, [r10, THREAD_CALLEE_REG]

 	j  do_machine_check_fault

 END(EV_MachineCheck)

 ; ---------------------------------------------
 ; Privilege Violation Exception Handler
 ; ---------------------------------------------
 ENTRY(EV_PrivilegeV)

 	EXCEPTION_PROLOGUE

 	lr  r0, [efa]
 	mov r1, sp

 	FAKE_RET_FROM_EXCPN

 	bl  do_privilege_fault
 	b   ret_from_exception
 END(EV_PrivilegeV)

 ; ---------------------------------------------
 ; Extension Instruction Exception Handler
 ; ---------------------------------------------
 ENTRY(EV_Extension)

 	EXCEPTION_PROLOGUE

 	lr  r0, [efa]
 	mov r1, sp

 	FAKE_RET_FROM_EXCPN

 	bl  do_extension_fault
 	b   ret_from_exception
 END(EV_Extension)

 ;################ Trap Handling (Syscall, Breakpoint) ##################

 ; ---------------------------------------------
 ; syscall Tracing
 ; ---------------------------------------------
 tracesys:
 	; save EFA in case tracer wants the PC of traced task
 	; using ERET won't work since next-PC has already committed
 	lr  r12, [efa]
 	GET_CURR_TASK_FIELD_PTR   TASK_THREAD, r11
 	st  r12, [r11, THREAD_FAULT_ADDR]	; thread.fault_address

 	; PRE Sys Call Ptrace hook
 	mov r0, sp			; pt_regs needed
 	bl  @syscall_trace_entry

 	; Tracing code now returns the syscall num (orig or modif)
 	mov r8, r0

 	; Do the Sys Call as we normally would.
 	; Validate the Sys Call number
 	cmp     r8,  NR_syscalls
 	mov.hi  r0, -ENOSYS
 	bhi     tracesys_exit

 	; Restore the sys-call args. Mere invocation of the hook abv could have
 	; clobbered them (since they are in scratch regs). The tracer could also
 	; have deliberately changed the syscall args: r0-r7
 	ld  r0, [sp, PT_r0]
 	ld  r1, [sp, PT_r1]
 	ld  r2, [sp, PT_r2]
 	ld  r3, [sp, PT_r3]
 	ld  r4, [sp, PT_r4]
 	ld  r5, [sp, PT_r5]
 	ld  r6, [sp, PT_r6]
 	ld  r7, [sp, PT_r7]
 	ld.as   r9, [sys_call_table, r8]
 	jl      [r9]        ; Entry into Sys Call Handler

 tracesys_exit:
 	st  r0, [sp, PT_r0]     ; sys call return value in pt_regs

 	;POST Sys Call Ptrace Hook
 	bl  @syscall_trace_exit
 	b   ret_from_exception ; NOT ret_from_system_call at is saves r0 which
 	; we'd done before calling post hook above

 ; ---------------------------------------------
 ; Breakpoint TRAP
 ; ---------------------------------------------
 trap_with_param:

 	; stop_pc info by gdb needs this info
 	lr  r0, [efa]
 	mov r1, sp

 	; Now that we have read EFA, it is safe to do "fake" rtie
 	;   and get out of CPU exception mode
 	FAKE_RET_FROM_EXCPN

 	; Save callee regs in case gdb wants to have a look
 	; SP will grow up by size of CALLEE Reg-File
 	; NOTE: clobbers r12
 	SAVE_CALLEE_SAVED_USER

 	; save location of saved Callee Regs @ thread_struct->pc
 	GET_CURR_TASK_FIELD_PTR   TASK_THREAD, r10
 	st  sp, [r10, THREAD_CALLEE_REG]

 	; Call the trap handler
 	bl  do_non_swi_trap

 	; unwind stack to discard Callee saved Regs
 	DISCARD_CALLEE_SAVED_USER

 	b   ret_from_exception

 ; ---------------------------------------------
 ; syscall TRAP
 ; ABI: (r0-r7) upto 8 args, (r8) syscall number
 ; ---------------------------------------------

 ENTRY(EV_Trap)

 	EXCEPTION_PROLOGUE

 	;============ TRAP 1   :breakpoints
 	; Check ECR for trap with arg (PROLOGUE ensures r9 has ECR)
 	bmsk.f 0, r9, 7
 	bnz    trap_with_param

 	;============ TRAP  (no param): syscall top level

 	; First return from Exception to pure K mode (Exception/IRQs renabled)
 	FAKE_RET_FROM_EXCPN

 	; If syscall tracing ongoing, invoke pre-post-hooks
 	GET_CURR_THR_INFO_FLAGS   r10
 	btst r10, TIF_SYSCALL_TRACE
 	bnz tracesys  ; this never comes back

 	;============ Normal syscall case

 	; syscall num shd not exceed the total system calls avail
 	cmp     r8,  NR_syscalls
 	mov.hi  r0, -ENOSYS
 	bhi     .Lret_from_system_call

 	; Offset into the syscall_table and call handler
 	ld.as   r9,[sys_call_table, r8]
 	jl      [r9]        ; Entry into Sys Call Handler

 .Lret_from_system_call:

 	st  r0, [sp, PT_r0]     ; sys call return value in pt_regs

 	; fall through to ret_from_exception
 END(EV_Trap)

 ;############# Return from Intr/Excp/Trap (Linux Specifics) ##############
 ;
 ; If ret to user mode do we need to handle signals, schedule() et al.

 ENTRY(ret_from_exception)

 	; Pre-{IRQ,Trap,Exception} K/U mode from pt_regs->status32
 	ld  r8, [sp, PT_status32]   ; returning to User/Kernel Mode

 	bbit0  r8, STATUS_U_BIT, resume_kernel_mode

 	; Before returning to User mode check-for-and-complete any pending work
 	; such as rescheduling/signal-delivery etc.
 resume_user_mode_begin:

 	; Disable IRQs to ensures that chk for pending work itself is atomic
 	; (and we don't end up missing a NEED_RESCHED/SIGPENDING due to an
 	; interim IRQ).
 	IRQ_DISABLE	r10

 	; Fast Path return to user mode if no pending work
 	GET_CURR_THR_INFO_FLAGS   r9
 	and.f  0,  r9, _TIF_WORK_MASK
 	bz     .Lrestore_regs

 	; --- (Slow Path #1) task preemption ---
 	bbit0  r9, TIF_NEED_RESCHED, .Lchk_pend_signals
 	mov    blink, resume_user_mode_begin  ; tail-call to U mode ret chks
 	j      @schedule 	; BTST+Bnz causes relo error in link

 .Lchk_pend_signals:
 	IRQ_ENABLE	r10

 	; --- (Slow Path #2) pending signal  ---
 	mov r0, sp	; pt_regs for arg to do_signal()/do_notify_resume()

 	GET_CURR_THR_INFO_FLAGS   r9
 	bbit0  r9, TIF_SIGPENDING, .Lchk_notify_resume

 	; Normal Trap/IRQ entry only saves Scratch (caller-saved) regs
 	; in pt_reg since the "C" ABI (kernel code) will automatically
 	; save/restore callee-saved regs.
 	;
 	; However, here we need to explicitly save callee regs because
 	; (i)  If this signal causes coredump - full regfile needed
 	; (ii) If signal is SIGTRAP/SIGSTOP, task is being traced thus
 	;      tracer might call PEEKUSR(CALLEE reg)
 	;
 	; NOTE: SP will grow up by size of CALLEE Reg-File
 	SAVE_CALLEE_SAVED_USER		; clobbers r12

 	; save location of saved Callee Regs @ thread_struct->callee
 	GET_CURR_TASK_FIELD_PTR   TASK_THREAD, r10
 	st  sp, [r10, THREAD_CALLEE_REG]

 	bl  @do_signal

 	; Ideally we want to discard the Callee reg above, however if this was
 	; a tracing signal, tracer could have done a POKEUSR(CALLEE reg)
 	RESTORE_CALLEE_SAVED_USER

 	b      resume_user_mode_begin	; loop back to start of U mode ret

 	; --- (Slow Path #3) notify_resume ---
 .Lchk_notify_resume:
 	btst   r9, TIF_NOTIFY_RESUME
 	blnz   @do_notify_resume
 	b      resume_user_mode_begin	; unconditionally back to U mode ret chks
 					; for single exit point from this block

 resume_kernel_mode:

 	; Disable Interrupts from this point on
 	; CONFIG_PREEMPT: This is a must for preempt_schedule_irq()
 	; !CONFIG_PREEMPT: To ensure restore_regs is intr safe
 	IRQ_DISABLE	r9

 #ifdef CONFIG_PREEMPT

 	; Can't preempt if preemption disabled
 	GET_CURR_THR_INFO_FROM_SP   r10
 	ld  r8, [r10, THREAD_INFO_PREEMPT_COUNT]
 	brne  r8, 0, .Lrestore_regs

 	; check if this task's NEED_RESCHED flag set
 	ld  r9, [r10, THREAD_INFO_FLAGS]
 	bbit0  r9, TIF_NEED_RESCHED, .Lrestore_regs

 	; Invoke PREEMPTION
 	jl      preempt_schedule_irq

 	; preempt_schedule_irq() always returns with IRQ disabled
 #endif

 	b	.Lrestore_regs

 ##### DONT ADD CODE HERE - .Lrestore_regs actually follows in entry-<isa>.S
	/*
	* Common Low Level Interrupts/Traps/Exceptions(non-TLB) Handling for ARC
	* (included from entry-<isa>.S
	*
	* Copyright (C) 2014-15 Synopsys, Inc. (www.synopsys.com)
	* Copyright (C) 2004, 2007-2010, 2011-2012 Synopsys, Inc. (www.synopsys.com)
	*
	* This program is free software; you can redistribute it and/or modify
	* it under the terms of the GNU General Public License version 2 as
	* published by the Free Software Foundation.
	*/

	/*------------------------------------------------------------------
	* Function ABI
	*------------------------------------------------------------------
	*
	* Arguments r0 - r7
	* Caller Saved Registers r0 - r12
	* Callee Saved Registers r13- r25
	* Global Pointer (gp) r26
	* Frame Pointer (fp) r27
	* Stack Pointer (sp) r28
	* Branch link register (blink) r31
	*------------------------------------------------------------------
	*/

	;################### Special Sys Call Wrappers ##########################

	ENTRY(sys_clone_wrapper)
	SAVE_CALLEE_SAVED_USER
	bl @sys_clone
	DISCARD_CALLEE_SAVED_USER

	GET_CURR_THR_INFO_FLAGS r10
	btst r10, TIF_SYSCALL_TRACE
	bnz tracesys_exit

	b .Lret_from_system_call
	END(sys_clone_wrapper)

	ENTRY(ret_from_fork)
	; when the forked child comes here from the __switch_to function
	; r0 has the last task pointer.
	; put last task in scheduler queue
	jl @schedule_tail

	ld r9, [sp, PT_status32]
	brne r9, 0, 1f

	jl.d [r14] ; kernel thread entry point
	mov r0, r13 ; (see PF_KTHREAD block in copy_thread)

	1:
	; Return to user space
	; 1. Any forked task (Reach here via BRne above)
	; 2. First ever init task (Reach here via return from JL above)
	; This is the historic "kernel_execve" use-case, to return to init
	; user mode, in a round about way since that is always done from
	; a kernel thread which is executed via JL above but always returns
	; out whenever kernel_execve (now inline do_fork()) is involved
	b ret_from_exception
	END(ret_from_fork)

	;################### Non TLB Exception Handling #############################

	; ---------------------------------------------
	; Instruction Error Exception Handler
	; ---------------------------------------------

	ENTRY(instr_service)

	EXCEPTION_PROLOGUE

	lr r0, [efa]
	mov r1, sp

	FAKE_RET_FROM_EXCPN

	bl do_insterror_or_kprobe
	b ret_from_exception
	END(instr_service)

	; ---------------------------------------------
	; Machine Check Exception Handler
	; ---------------------------------------------

	ENTRY(EV_MachineCheck)

	EXCEPTION_PROLOGUE

	lr r2, [ecr]
	lr r0, [efa]
	mov r1, sp

	; hardware auto-disables MMU, re-enable it to allow kernel vaddr
	; access for say stack unwinding of modules for crash dumps
	lr r3, [ARC_REG_PID]
	or r3, r3, MMU_ENABLE
	sr r3, [ARC_REG_PID]

	lsr r3, r2, 8
	bmsk r3, r3, 7
	brne r3, ECR_C_MCHK_DUP_TLB, 1f

	bl do_tlb_overlap_fault
	b ret_from_exception

	1:
	; DEAD END: can't do much, display Regs and HALT
	SAVE_CALLEE_SAVED_USER

	GET_CURR_TASK_FIELD_PTR TASK_THREAD, r10
	st sp, [r10, THREAD_CALLEE_REG]

	j do_machine_check_fault

	END(EV_MachineCheck)

	; ---------------------------------------------
	; Privilege Violation Exception Handler
	; ---------------------------------------------
	ENTRY(EV_PrivilegeV)

	EXCEPTION_PROLOGUE

	lr r0, [efa]
	mov r1, sp

	FAKE_RET_FROM_EXCPN

	bl do_privilege_fault
	b ret_from_exception
	END(EV_PrivilegeV)

	; ---------------------------------------------
	; Extension Instruction Exception Handler
	; ---------------------------------------------
	ENTRY(EV_Extension)

	EXCEPTION_PROLOGUE

	lr r0, [efa]
	mov r1, sp

	FAKE_RET_FROM_EXCPN

	bl do_extension_fault
	b ret_from_exception
	END(EV_Extension)

	;################ Trap Handling (Syscall, Breakpoint) ##################

	; ---------------------------------------------
	; syscall Tracing
	; ---------------------------------------------
	tracesys:
	; save EFA in case tracer wants the PC of traced task
	; using ERET won't work since next-PC has already committed
	lr r12, [efa]
	GET_CURR_TASK_FIELD_PTR TASK_THREAD, r11
	st r12, [r11, THREAD_FAULT_ADDR] ; thread.fault_address

	; PRE Sys Call Ptrace hook
	mov r0, sp ; pt_regs needed
	bl @syscall_trace_entry

	; Tracing code now returns the syscall num (orig or modif)
	mov r8, r0

	; Do the Sys Call as we normally would.
	; Validate the Sys Call number
	cmp r8, NR_syscalls
	mov.hi r0, -ENOSYS
	bhi tracesys_exit

	; Restore the sys-call args. Mere invocation of the hook abv could have
	; clobbered them (since they are in scratch regs). The tracer could also
	; have deliberately changed the syscall args: r0-r7
	ld r0, [sp, PT_r0]
	ld r1, [sp, PT_r1]
	ld r2, [sp, PT_r2]
	ld r3, [sp, PT_r3]
	ld r4, [sp, PT_r4]
	ld r5, [sp, PT_r5]
	ld r6, [sp, PT_r6]
	ld r7, [sp, PT_r7]
	ld.as r9, [sys_call_table, r8]
	jl [r9] ; Entry into Sys Call Handler

	tracesys_exit:
	st r0, [sp, PT_r0] ; sys call return value in pt_regs

	;POST Sys Call Ptrace Hook
	bl @syscall_trace_exit
	b ret_from_exception ; NOT ret_from_system_call at is saves r0 which
	; we'd done before calling post hook above

	; ---------------------------------------------
	; Breakpoint TRAP
	; ---------------------------------------------
	trap_with_param:

	; stop_pc info by gdb needs this info
	lr r0, [efa]
	mov r1, sp

	; Now that we have read EFA, it is safe to do "fake" rtie
	; and get out of CPU exception mode
	FAKE_RET_FROM_EXCPN

	; Save callee regs in case gdb wants to have a look
	; SP will grow up by size of CALLEE Reg-File
	; NOTE: clobbers r12
	SAVE_CALLEE_SAVED_USER

	; save location of saved Callee Regs @ thread_struct->pc
	GET_CURR_TASK_FIELD_PTR TASK_THREAD, r10
	st sp, [r10, THREAD_CALLEE_REG]

	; Call the trap handler
	bl do_non_swi_trap

	; unwind stack to discard Callee saved Regs
	DISCARD_CALLEE_SAVED_USER

	b ret_from_exception

	; ---------------------------------------------
	; syscall TRAP
	; ABI: (r0-r7) upto 8 args, (r8) syscall number
	; ---------------------------------------------

	ENTRY(EV_Trap)

	EXCEPTION_PROLOGUE

	;============ TRAP 1 :breakpoints
	; Check ECR for trap with arg (PROLOGUE ensures r9 has ECR)
	bmsk.f 0, r9, 7
	bnz trap_with_param

	;============ TRAP (no param): syscall top level

	; First return from Exception to pure K mode (Exception/IRQs renabled)
	FAKE_RET_FROM_EXCPN

	; If syscall tracing ongoing, invoke pre-post-hooks
	GET_CURR_THR_INFO_FLAGS r10
	btst r10, TIF_SYSCALL_TRACE
	bnz tracesys ; this never comes back

	;============ Normal syscall case

	; syscall num shd not exceed the total system calls avail
	cmp r8, NR_syscalls
	mov.hi r0, -ENOSYS
	bhi .Lret_from_system_call

	; Offset into the syscall_table and call handler
	ld.as r9,[sys_call_table, r8]
	jl [r9] ; Entry into Sys Call Handler

	.Lret_from_system_call:

	st r0, [sp, PT_r0] ; sys call return value in pt_regs

	; fall through to ret_from_exception
	END(EV_Trap)

	;############# Return from Intr/Excp/Trap (Linux Specifics) ##############
	;
	; If ret to user mode do we need to handle signals, schedule() et al.

	ENTRY(ret_from_exception)

	; Pre-{IRQ,Trap,Exception} K/U mode from pt_regs->status32
	ld r8, [sp, PT_status32] ; returning to User/Kernel Mode

	bbit0 r8, STATUS_U_BIT, resume_kernel_mode

	; Before returning to User mode check-for-and-complete any pending work
	; such as rescheduling/signal-delivery etc.
	resume_user_mode_begin:

	; Disable IRQs to ensures that chk for pending work itself is atomic
	; (and we don't end up missing a NEED_RESCHED/SIGPENDING due to an
	; interim IRQ).
	IRQ_DISABLE r10

	; Fast Path return to user mode if no pending work
	GET_CURR_THR_INFO_FLAGS r9
	and.f 0, r9, _TIF_WORK_MASK
	bz .Lrestore_regs

	; --- (Slow Path #1) task preemption ---
	bbit0 r9, TIF_NEED_RESCHED, .Lchk_pend_signals
	mov blink, resume_user_mode_begin ; tail-call to U mode ret chks
	j @schedule ; BTST+Bnz causes relo error in link

	.Lchk_pend_signals:
	IRQ_ENABLE r10

	; --- (Slow Path #2) pending signal ---
	mov r0, sp ; pt_regs for arg to do_signal()/do_notify_resume()

	GET_CURR_THR_INFO_FLAGS r9
	bbit0 r9, TIF_SIGPENDING, .Lchk_notify_resume

	; Normal Trap/IRQ entry only saves Scratch (caller-saved) regs
	; in pt_reg since the "C" ABI (kernel code) will automatically
	; save/restore callee-saved regs.
	;
	; However, here we need to explicitly save callee regs because
	; (i) If this signal causes coredump - full regfile needed
	; (ii) If signal is SIGTRAP/SIGSTOP, task is being traced thus
	; tracer might call PEEKUSR(CALLEE reg)
	;
	; NOTE: SP will grow up by size of CALLEE Reg-File
	SAVE_CALLEE_SAVED_USER ; clobbers r12

	; save location of saved Callee Regs @ thread_struct->callee
	GET_CURR_TASK_FIELD_PTR TASK_THREAD, r10
	st sp, [r10, THREAD_CALLEE_REG]

	bl @do_signal

	; Ideally we want to discard the Callee reg above, however if this was
	; a tracing signal, tracer could have done a POKEUSR(CALLEE reg)
	RESTORE_CALLEE_SAVED_USER

	b resume_user_mode_begin ; loop back to start of U mode ret

	; --- (Slow Path #3) notify_resume ---
	.Lchk_notify_resume:
	btst r9, TIF_NOTIFY_RESUME
	blnz @do_notify_resume
	b resume_user_mode_begin ; unconditionally back to U mode ret chks
	; for single exit point from this block

	resume_kernel_mode:

	; Disable Interrupts from this point on
	; CONFIG_PREEMPT: This is a must for preempt_schedule_irq()
	; !CONFIG_PREEMPT: To ensure restore_regs is intr safe
	IRQ_DISABLE r9

	#ifdef CONFIG_PREEMPT

	; Can't preempt if preemption disabled
	GET_CURR_THR_INFO_FROM_SP r10
	ld r8, [r10, THREAD_INFO_PREEMPT_COUNT]
	brne r8, 0, .Lrestore_regs

	; check if this task's NEED_RESCHED flag set
	ld r9, [r10, THREAD_INFO_FLAGS]
	bbit0 r9, TIF_NEED_RESCHED, .Lrestore_regs

	; Invoke PREEMPTION
	jl preempt_schedule_irq

	; preempt_schedule_irq() always returns with IRQ disabled
	#endif

	b .Lrestore_regs

	##### DONT ADD CODE HERE - .Lrestore_regs actually follows in entry-<isa>.S