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

 /* SPDX-License-Identifier: GPL-2.0-only */
 /*
  * Copyright (C) 2012 Regents of the University of California
  * Copyright (C) 2017 SiFive
  */

 #include <linux/init.h>
 #include <linux/linkage.h>

 #include <asm/asm.h>
 #include <asm/csr.h>
 #include <asm/unistd.h>
 #include <asm/thread_info.h>
 #include <asm/asm-offsets.h>
 #include <asm/errata_list.h>

 #if !IS_ENABLED(CONFIG_PREEMPTION)
 .set resume_kernel, restore_all
 #endif

 ENTRY(handle_exception)
 	/*
 	 * If coming from userspace, preserve the user thread pointer and load
 	 * the kernel thread pointer.  If we came from the kernel, the scratch
 	 * register will contain 0, and we should continue on the current TP.
 	 */
 	csrrw tp, CSR_SCRATCH, tp
 	bnez tp, _save_context

 _restore_kernel_tpsp:
 	csrr tp, CSR_SCRATCH
 	REG_S sp, TASK_TI_KERNEL_SP(tp)

 #ifdef CONFIG_VMAP_STACK
 	addi sp, sp, -(PT_SIZE_ON_STACK)
 	srli sp, sp, THREAD_SHIFT
 	andi sp, sp, 0x1
 	bnez sp, handle_kernel_stack_overflow
 	REG_L sp, TASK_TI_KERNEL_SP(tp)
 #endif

 _save_context:
 	REG_S sp, TASK_TI_USER_SP(tp)
 	REG_L sp, TASK_TI_KERNEL_SP(tp)
 	addi sp, sp, -(PT_SIZE_ON_STACK)
 	REG_S x1,  PT_RA(sp)
 	REG_S x3,  PT_GP(sp)
 	REG_S x5,  PT_T0(sp)
 	REG_S x6,  PT_T1(sp)
 	REG_S x7,  PT_T2(sp)
 	REG_S x8,  PT_S0(sp)
 	REG_S x9,  PT_S1(sp)
 	REG_S x10, PT_A0(sp)
 	REG_S x11, PT_A1(sp)
 	REG_S x12, PT_A2(sp)
 	REG_S x13, PT_A3(sp)
 	REG_S x14, PT_A4(sp)
 	REG_S x15, PT_A5(sp)
 	REG_S x16, PT_A6(sp)
 	REG_S x17, PT_A7(sp)
 	REG_S x18, PT_S2(sp)
 	REG_S x19, PT_S3(sp)
 	REG_S x20, PT_S4(sp)
 	REG_S x21, PT_S5(sp)
 	REG_S x22, PT_S6(sp)
 	REG_S x23, PT_S7(sp)
 	REG_S x24, PT_S8(sp)
 	REG_S x25, PT_S9(sp)
 	REG_S x26, PT_S10(sp)
 	REG_S x27, PT_S11(sp)
 	REG_S x28, PT_T3(sp)
 	REG_S x29, PT_T4(sp)
 	REG_S x30, PT_T5(sp)
 	REG_S x31, PT_T6(sp)

 	/*
 	 * Disable user-mode memory access as it should only be set in the
 	 * actual user copy routines.
 	 *
 	 * Disable the FPU to detect illegal usage of floating point in kernel
 	 * space.
 	 */
 	li t0, SR_SUM | SR_FS

 	REG_L s0, TASK_TI_USER_SP(tp)
 	csrrc s1, CSR_STATUS, t0
 	csrr s2, CSR_EPC
 	csrr s3, CSR_TVAL
 	csrr s4, CSR_CAUSE
 	csrr s5, CSR_SCRATCH
 	REG_S s0, PT_SP(sp)
 	REG_S s1, PT_STATUS(sp)
 	REG_S s2, PT_EPC(sp)
 	REG_S s3, PT_BADADDR(sp)
 	REG_S s4, PT_CAUSE(sp)
 	REG_S s5, PT_TP(sp)

 	/*
 	 * Set the scratch register to 0, so that if a recursive exception
 	 * occurs, the exception vector knows it came from the kernel
 	 */
 	csrw CSR_SCRATCH, x0

 	/* Load the global pointer */
 .option push
 .option norelax
 	la gp, __global_pointer$
 .option pop

 #ifdef CONFIG_TRACE_IRQFLAGS
 	call __trace_hardirqs_off
 #endif

 #ifdef CONFIG_CONTEXT_TRACKING_USER
 	/* If previous state is in user mode, call user_exit_callable(). */
 	li   a0, SR_PP
 	and a0, s1, a0
 	bnez a0, skip_context_tracking
 	call user_exit_callable
 skip_context_tracking:
 #endif

 	/*
 	 * MSB of cause differentiates between
 	 * interrupts and exceptions
 	 */
 	bge s4, zero, 1f

 	la ra, ret_from_exception

 	/* Handle interrupts */
 	move a0, sp /* pt_regs */
 	la a1, generic_handle_arch_irq
 	jr a1
 1:
 	/*
 	 * Exceptions run with interrupts enabled or disabled depending on the
 	 * state of SR_PIE in m/sstatus.
 	 */
 	andi t0, s1, SR_PIE
 	beqz t0, 1f
 	/* kprobes, entered via ebreak, must have interrupts disabled. */
 	li t0, EXC_BREAKPOINT
 	beq s4, t0, 1f
 #ifdef CONFIG_TRACE_IRQFLAGS
 	call __trace_hardirqs_on
 #endif
 	csrs CSR_STATUS, SR_IE

 1:
 	la ra, ret_from_exception
 	/* Handle syscalls */
 	li t0, EXC_SYSCALL
 	beq s4, t0, handle_syscall

 	/* Handle other exceptions */
 	slli t0, s4, RISCV_LGPTR
 	la t1, excp_vect_table
 	la t2, excp_vect_table_end
 	move a0, sp /* pt_regs */
 	add t0, t1, t0
 	/* Check if exception code lies within bounds */
 	bgeu t0, t2, 1f
 	REG_L t0, 0(t0)
 	jr t0
 1:
 	tail do_trap_unknown

 handle_syscall:
 #ifdef CONFIG_RISCV_M_MODE
 	/*
 	 * When running is M-Mode (no MMU config), MPIE does not get set.
 	 * As a result, we need to force enable interrupts here because
 	 * handle_exception did not do set SR_IE as it always sees SR_PIE
 	 * being cleared.
 	 */
 	csrs CSR_STATUS, SR_IE
 #endif
 #if defined(CONFIG_TRACE_IRQFLAGS) || defined(CONFIG_CONTEXT_TRACKING_USER)
 	/* Recover a0 - a7 for system calls */
 	REG_L a0, PT_A0(sp)
 	REG_L a1, PT_A1(sp)
 	REG_L a2, PT_A2(sp)
 	REG_L a3, PT_A3(sp)
 	REG_L a4, PT_A4(sp)
 	REG_L a5, PT_A5(sp)
 	REG_L a6, PT_A6(sp)
 	REG_L a7, PT_A7(sp)
 #endif
 	 /* save the initial A0 value (needed in signal handlers) */
 	REG_S a0, PT_ORIG_A0(sp)
 	/*
 	 * Advance SEPC to avoid executing the original
 	 * scall instruction on sret
 	 */
 	addi s2, s2, 0x4
 	REG_S s2, PT_EPC(sp)
 	/* Trace syscalls, but only if requested by the user. */
 	REG_L t0, TASK_TI_FLAGS(tp)
 	andi t0, t0, _TIF_SYSCALL_WORK
 	bnez t0, handle_syscall_trace_enter
 check_syscall_nr:
 	/* Check to make sure we don't jump to a bogus syscall number. */
 	li t0, __NR_syscalls
 	la s0, sys_ni_syscall
 	/*
 	 * Syscall number held in a7.
 	 * If syscall number is above allowed value, redirect to ni_syscall.
 	 */
 	bgeu a7, t0, 3f
 #ifdef CONFIG_COMPAT
 	REG_L s0, PT_STATUS(sp)
 	srli s0, s0, SR_UXL_SHIFT
 	andi s0, s0, (SR_UXL >> SR_UXL_SHIFT)
 	li t0, (SR_UXL_32 >> SR_UXL_SHIFT)
 	sub t0, s0, t0
 	bnez t0, 1f

 	/* Call compat_syscall */
 	la s0, compat_sys_call_table
 	j 2f
 1:
 #endif
 	/* Call syscall */
 	la s0, sys_call_table
 2:
 	slli t0, a7, RISCV_LGPTR
 	add s0, s0, t0
 	REG_L s0, 0(s0)
 3:
 	jalr s0

 ret_from_syscall:
 	/* Set user a0 to kernel a0 */
 	REG_S a0, PT_A0(sp)
 	/*
 	 * We didn't execute the actual syscall.
 	 * Seccomp already set return value for the current task pt_regs.
 	 * (If it was configured with SECCOMP_RET_ERRNO/TRACE)
 	 */
 ret_from_syscall_rejected:
 #ifdef CONFIG_DEBUG_RSEQ
 	move a0, sp
 	call rseq_syscall
 #endif
 	/* Trace syscalls, but only if requested by the user. */
 	REG_L t0, TASK_TI_FLAGS(tp)
 	andi t0, t0, _TIF_SYSCALL_WORK
 	bnez t0, handle_syscall_trace_exit

 ret_from_exception:
 	REG_L s0, PT_STATUS(sp)
 	csrc CSR_STATUS, SR_IE
 #ifdef CONFIG_TRACE_IRQFLAGS
 	call __trace_hardirqs_off
 #endif
 #ifdef CONFIG_RISCV_M_MODE
 	/* the MPP value is too large to be used as an immediate arg for addi */
 	li t0, SR_MPP
 	and s0, s0, t0
 #else
 	andi s0, s0, SR_SPP
 #endif
 	bnez s0, resume_kernel

 resume_userspace:
 	/* Interrupts must be disabled here so flags are checked atomically */
 	REG_L s0, TASK_TI_FLAGS(tp) /* current_thread_info->flags */
 	andi s1, s0, _TIF_WORK_MASK
 	bnez s1, work_pending

 #ifdef CONFIG_CONTEXT_TRACKING_USER
 	call user_enter_callable
 #endif

 	/* Save unwound kernel stack pointer in thread_info */
 	addi s0, sp, PT_SIZE_ON_STACK
 	REG_S s0, TASK_TI_KERNEL_SP(tp)

 	/*
 	 * Save TP into the scratch register , so we can find the kernel data
 	 * structures again.
 	 */
 	csrw CSR_SCRATCH, tp

 restore_all:
 #ifdef CONFIG_TRACE_IRQFLAGS
 	REG_L s1, PT_STATUS(sp)
 	andi t0, s1, SR_PIE
 	beqz t0, 1f
 	call __trace_hardirqs_on
 	j 2f
 1:
 	call __trace_hardirqs_off
 2:
 #endif
 	REG_L a0, PT_STATUS(sp)
 	/*
 	 * The current load reservation is effectively part of the processor's
 	 * state, in the sense that load reservations cannot be shared between
 	 * different hart contexts.  We can't actually save and restore a load
 	 * reservation, so instead here we clear any existing reservation --
 	 * it's always legal for implementations to clear load reservations at
 	 * any point (as long as the forward progress guarantee is kept, but
 	 * we'll ignore that here).
 	 *
 	 * Dangling load reservations can be the result of taking a trap in the
 	 * middle of an LR/SC sequence, but can also be the result of a taken
 	 * forward branch around an SC -- which is how we implement CAS.  As a
 	 * result we need to clear reservations between the last CAS and the
 	 * jump back to the new context.  While it is unlikely the store
 	 * completes, implementations are allowed to expand reservations to be
 	 * arbitrarily large.
 	 */
 	REG_L  a2, PT_EPC(sp)
 	REG_SC x0, a2, PT_EPC(sp)

 	csrw CSR_STATUS, a0
 	csrw CSR_EPC, a2

 	REG_L x1,  PT_RA(sp)
 	REG_L x3,  PT_GP(sp)
 	REG_L x4,  PT_TP(sp)
 	REG_L x5,  PT_T0(sp)
 	REG_L x6,  PT_T1(sp)
 	REG_L x7,  PT_T2(sp)
 	REG_L x8,  PT_S0(sp)
 	REG_L x9,  PT_S1(sp)
 	REG_L x10, PT_A0(sp)
 	REG_L x11, PT_A1(sp)
 	REG_L x12, PT_A2(sp)
 	REG_L x13, PT_A3(sp)
 	REG_L x14, PT_A4(sp)
 	REG_L x15, PT_A5(sp)
 	REG_L x16, PT_A6(sp)
 	REG_L x17, PT_A7(sp)
 	REG_L x18, PT_S2(sp)
 	REG_L x19, PT_S3(sp)
 	REG_L x20, PT_S4(sp)
 	REG_L x21, PT_S5(sp)
 	REG_L x22, PT_S6(sp)
 	REG_L x23, PT_S7(sp)
 	REG_L x24, PT_S8(sp)
 	REG_L x25, PT_S9(sp)
 	REG_L x26, PT_S10(sp)
 	REG_L x27, PT_S11(sp)
 	REG_L x28, PT_T3(sp)
 	REG_L x29, PT_T4(sp)
 	REG_L x30, PT_T5(sp)
 	REG_L x31, PT_T6(sp)

 	REG_L x2,  PT_SP(sp)

 #ifdef CONFIG_RISCV_M_MODE
 	mret
 #else
 	sret
 #endif

 #if IS_ENABLED(CONFIG_PREEMPTION)
 resume_kernel:
 	REG_L s0, TASK_TI_PREEMPT_COUNT(tp)
 	bnez s0, restore_all
 	REG_L s0, TASK_TI_FLAGS(tp)
 	andi s0, s0, _TIF_NEED_RESCHED
 	beqz s0, restore_all
 	call preempt_schedule_irq
 	j restore_all
 #endif

 work_pending:
 	/* Enter slow path for supplementary processing */
 	la ra, ret_from_exception
 	andi s1, s0, _TIF_NEED_RESCHED
 	bnez s1, work_resched
 work_notifysig:
 	/* Handle pending signals and notify-resume requests */
 	csrs CSR_STATUS, SR_IE /* Enable interrupts for do_notify_resume() */
 	move a0, sp /* pt_regs */
 	move a1, s0 /* current_thread_info->flags */
 	tail do_notify_resume
 work_resched:
 	tail schedule

 /* Slow paths for ptrace. */
 handle_syscall_trace_enter:
 	move a0, sp
 	call do_syscall_trace_enter
 	move t0, a0
 	REG_L a0, PT_A0(sp)
 	REG_L a1, PT_A1(sp)
 	REG_L a2, PT_A2(sp)
 	REG_L a3, PT_A3(sp)
 	REG_L a4, PT_A4(sp)
 	REG_L a5, PT_A5(sp)
 	REG_L a6, PT_A6(sp)
 	REG_L a7, PT_A7(sp)
 	bnez t0, ret_from_syscall_rejected
 	j check_syscall_nr
 handle_syscall_trace_exit:
 	move a0, sp
 	call do_syscall_trace_exit
 	j ret_from_exception

 #ifdef CONFIG_VMAP_STACK
 handle_kernel_stack_overflow:
 	/*
 	 * Takes the psuedo-spinlock for the shadow stack, in case multiple
 	 * harts are concurrently overflowing their kernel stacks.  We could
 	 * store any value here, but since we're overflowing the kernel stack
 	 * already we only have SP to use as a scratch register.  So we just
 	 * swap in the address of the spinlock, as that's definately non-zero.
 	 *
 	 * Pairs with a store_release in handle_bad_stack().
 	 */
 1:	la sp, spin_shadow_stack
 	REG_AMOSWAP_AQ sp, sp, (sp)
 	bnez sp, 1b

 	la sp, shadow_stack
 	addi sp, sp, SHADOW_OVERFLOW_STACK_SIZE

 	//save caller register to shadow stack
 	addi sp, sp, -(PT_SIZE_ON_STACK)
 	REG_S x1,  PT_RA(sp)
 	REG_S x5,  PT_T0(sp)
 	REG_S x6,  PT_T1(sp)
 	REG_S x7,  PT_T2(sp)
 	REG_S x10, PT_A0(sp)
 	REG_S x11, PT_A1(sp)
 	REG_S x12, PT_A2(sp)
 	REG_S x13, PT_A3(sp)
 	REG_S x14, PT_A4(sp)
 	REG_S x15, PT_A5(sp)
 	REG_S x16, PT_A6(sp)
 	REG_S x17, PT_A7(sp)
 	REG_S x28, PT_T3(sp)
 	REG_S x29, PT_T4(sp)
 	REG_S x30, PT_T5(sp)
 	REG_S x31, PT_T6(sp)

 	la ra, restore_caller_reg
 	tail get_overflow_stack

 restore_caller_reg:
 	//save per-cpu overflow stack
 	REG_S a0, -8(sp)
 	//restore caller register from shadow_stack
 	REG_L x1,  PT_RA(sp)
 	REG_L x5,  PT_T0(sp)
 	REG_L x6,  PT_T1(sp)
 	REG_L x7,  PT_T2(sp)
 	REG_L x10, PT_A0(sp)
 	REG_L x11, PT_A1(sp)
 	REG_L x12, PT_A2(sp)
 	REG_L x13, PT_A3(sp)
 	REG_L x14, PT_A4(sp)
 	REG_L x15, PT_A5(sp)
 	REG_L x16, PT_A6(sp)
 	REG_L x17, PT_A7(sp)
 	REG_L x28, PT_T3(sp)
 	REG_L x29, PT_T4(sp)
 	REG_L x30, PT_T5(sp)
 	REG_L x31, PT_T6(sp)

 	//load per-cpu overflow stack
 	REG_L sp, -8(sp)
 	addi sp, sp, -(PT_SIZE_ON_STACK)

 	//save context to overflow stack
 	REG_S x1,  PT_RA(sp)
 	REG_S x3,  PT_GP(sp)
 	REG_S x5,  PT_T0(sp)
 	REG_S x6,  PT_T1(sp)
 	REG_S x7,  PT_T2(sp)
 	REG_S x8,  PT_S0(sp)
 	REG_S x9,  PT_S1(sp)
 	REG_S x10, PT_A0(sp)
 	REG_S x11, PT_A1(sp)
 	REG_S x12, PT_A2(sp)
 	REG_S x13, PT_A3(sp)
 	REG_S x14, PT_A4(sp)
 	REG_S x15, PT_A5(sp)
 	REG_S x16, PT_A6(sp)
 	REG_S x17, PT_A7(sp)
 	REG_S x18, PT_S2(sp)
 	REG_S x19, PT_S3(sp)
 	REG_S x20, PT_S4(sp)
 	REG_S x21, PT_S5(sp)
 	REG_S x22, PT_S6(sp)
 	REG_S x23, PT_S7(sp)
 	REG_S x24, PT_S8(sp)
 	REG_S x25, PT_S9(sp)
 	REG_S x26, PT_S10(sp)
 	REG_S x27, PT_S11(sp)
 	REG_S x28, PT_T3(sp)
 	REG_S x29, PT_T4(sp)
 	REG_S x30, PT_T5(sp)
 	REG_S x31, PT_T6(sp)

 	REG_L s0, TASK_TI_KERNEL_SP(tp)
 	csrr s1, CSR_STATUS
 	csrr s2, CSR_EPC
 	csrr s3, CSR_TVAL
 	csrr s4, CSR_CAUSE
 	csrr s5, CSR_SCRATCH
 	REG_S s0, PT_SP(sp)
 	REG_S s1, PT_STATUS(sp)
 	REG_S s2, PT_EPC(sp)
 	REG_S s3, PT_BADADDR(sp)
 	REG_S s4, PT_CAUSE(sp)
 	REG_S s5, PT_TP(sp)
 	move a0, sp
 	tail handle_bad_stack
 #endif

 END(handle_exception)

 ENTRY(ret_from_fork)
 	la ra, ret_from_exception
 	tail schedule_tail
 ENDPROC(ret_from_fork)

 ENTRY(ret_from_kernel_thread)
 	call schedule_tail
 	/* Call fn(arg) */
 	la ra, ret_from_exception
 	move a0, s1
 	jr s0
 ENDPROC(ret_from_kernel_thread)


 /*
  * Integer register context switch
  * The callee-saved registers must be saved and restored.
  *
  *   a0: previous task_struct (must be preserved across the switch)
  *   a1: next task_struct
  *
  * The value of a0 and a1 must be preserved by this function, as that's how
  * arguments are passed to schedule_tail.
  */
 ENTRY(__switch_to)
 	/* Save context into prev->thread */
 	li    a4,  TASK_THREAD_RA
 	add   a3, a0, a4
 	add   a4, a1, a4
 	REG_S ra,  TASK_THREAD_RA_RA(a3)
 	REG_S sp,  TASK_THREAD_SP_RA(a3)
 	REG_S s0,  TASK_THREAD_S0_RA(a3)
 	REG_S s1,  TASK_THREAD_S1_RA(a3)
 	REG_S s2,  TASK_THREAD_S2_RA(a3)
 	REG_S s3,  TASK_THREAD_S3_RA(a3)
 	REG_S s4,  TASK_THREAD_S4_RA(a3)
 	REG_S s5,  TASK_THREAD_S5_RA(a3)
 	REG_S s6,  TASK_THREAD_S6_RA(a3)
 	REG_S s7,  TASK_THREAD_S7_RA(a3)
 	REG_S s8,  TASK_THREAD_S8_RA(a3)
 	REG_S s9,  TASK_THREAD_S9_RA(a3)
 	REG_S s10, TASK_THREAD_S10_RA(a3)
 	REG_S s11, TASK_THREAD_S11_RA(a3)
 	/* Restore context from next->thread */
 	REG_L ra,  TASK_THREAD_RA_RA(a4)
 	REG_L sp,  TASK_THREAD_SP_RA(a4)
 	REG_L s0,  TASK_THREAD_S0_RA(a4)
 	REG_L s1,  TASK_THREAD_S1_RA(a4)
 	REG_L s2,  TASK_THREAD_S2_RA(a4)
 	REG_L s3,  TASK_THREAD_S3_RA(a4)
 	REG_L s4,  TASK_THREAD_S4_RA(a4)
 	REG_L s5,  TASK_THREAD_S5_RA(a4)
 	REG_L s6,  TASK_THREAD_S6_RA(a4)
 	REG_L s7,  TASK_THREAD_S7_RA(a4)
 	REG_L s8,  TASK_THREAD_S8_RA(a4)
 	REG_L s9,  TASK_THREAD_S9_RA(a4)
 	REG_L s10, TASK_THREAD_S10_RA(a4)
 	REG_L s11, TASK_THREAD_S11_RA(a4)
 	/* The offset of thread_info in task_struct is zero. */
 	move tp, a1
 	ret
 ENDPROC(__switch_to)

 #ifndef CONFIG_MMU
 #define do_page_fault do_trap_unknown
 #endif

 	.section ".rodata"
 	.align LGREG
 	/* Exception vector table */
 ENTRY(excp_vect_table)
 	RISCV_PTR do_trap_insn_misaligned
 	ALT_INSN_FAULT(RISCV_PTR do_trap_insn_fault)
 	RISCV_PTR do_trap_insn_illegal
 	RISCV_PTR do_trap_break
 	RISCV_PTR do_trap_load_misaligned
 	RISCV_PTR do_trap_load_fault
 	RISCV_PTR do_trap_store_misaligned
 	RISCV_PTR do_trap_store_fault
 	RISCV_PTR do_trap_ecall_u /* system call, gets intercepted */
 	RISCV_PTR do_trap_ecall_s
 	RISCV_PTR do_trap_unknown
 	RISCV_PTR do_trap_ecall_m
 	/* instruciton page fault */
 	ALT_PAGE_FAULT(RISCV_PTR do_page_fault)
 	RISCV_PTR do_page_fault   /* load page fault */
 	RISCV_PTR do_trap_unknown
 	RISCV_PTR do_page_fault   /* store page fault */
 excp_vect_table_end:
 END(excp_vect_table)

 #ifndef CONFIG_MMU
 ENTRY(__user_rt_sigreturn)
 	li a7, __NR_rt_sigreturn
 	scall
 END(__user_rt_sigreturn)
 #endif
	/* SPDX-License-Identifier: GPL-2.0-only */
	/*
	* Copyright (C) 2012 Regents of the University of California
	* Copyright (C) 2017 SiFive
	*/

	#include <linux/init.h>
	#include <linux/linkage.h>

	#include <asm/asm.h>
	#include <asm/csr.h>
	#include <asm/unistd.h>
	#include <asm/thread_info.h>
	#include <asm/asm-offsets.h>
	#include <asm/errata_list.h>

	#if !IS_ENABLED(CONFIG_PREEMPTION)
	.set resume_kernel, restore_all
	#endif

	ENTRY(handle_exception)
	/*
	* If coming from userspace, preserve the user thread pointer and load
	* the kernel thread pointer. If we came from the kernel, the scratch
	* register will contain 0, and we should continue on the current TP.
	*/
	csrrw tp, CSR_SCRATCH, tp
	bnez tp, _save_context

	_restore_kernel_tpsp:
	csrr tp, CSR_SCRATCH
	REG_S sp, TASK_TI_KERNEL_SP(tp)

	#ifdef CONFIG_VMAP_STACK
	addi sp, sp, -(PT_SIZE_ON_STACK)
	srli sp, sp, THREAD_SHIFT
	andi sp, sp, 0x1
	bnez sp, handle_kernel_stack_overflow
	REG_L sp, TASK_TI_KERNEL_SP(tp)
	#endif

	_save_context:
	REG_S sp, TASK_TI_USER_SP(tp)
	REG_L sp, TASK_TI_KERNEL_SP(tp)
	addi sp, sp, -(PT_SIZE_ON_STACK)
	REG_S x1, PT_RA(sp)
	REG_S x3, PT_GP(sp)
	REG_S x5, PT_T0(sp)
	REG_S x6, PT_T1(sp)
	REG_S x7, PT_T2(sp)
	REG_S x8, PT_S0(sp)
	REG_S x9, PT_S1(sp)
	REG_S x10, PT_A0(sp)
	REG_S x11, PT_A1(sp)
	REG_S x12, PT_A2(sp)
	REG_S x13, PT_A3(sp)
	REG_S x14, PT_A4(sp)
	REG_S x15, PT_A5(sp)
	REG_S x16, PT_A6(sp)
	REG_S x17, PT_A7(sp)
	REG_S x18, PT_S2(sp)
	REG_S x19, PT_S3(sp)
	REG_S x20, PT_S4(sp)
	REG_S x21, PT_S5(sp)
	REG_S x22, PT_S6(sp)
	REG_S x23, PT_S7(sp)
	REG_S x24, PT_S8(sp)
	REG_S x25, PT_S9(sp)
	REG_S x26, PT_S10(sp)
	REG_S x27, PT_S11(sp)
	REG_S x28, PT_T3(sp)
	REG_S x29, PT_T4(sp)
	REG_S x30, PT_T5(sp)
	REG_S x31, PT_T6(sp)

	/*
	* Disable user-mode memory access as it should only be set in the
	* actual user copy routines.
	*
	* Disable the FPU to detect illegal usage of floating point in kernel
	* space.
	*/
	li t0, SR_SUM \| SR_FS

	REG_L s0, TASK_TI_USER_SP(tp)
	csrrc s1, CSR_STATUS, t0
	csrr s2, CSR_EPC
	csrr s3, CSR_TVAL
	csrr s4, CSR_CAUSE
	csrr s5, CSR_SCRATCH
	REG_S s0, PT_SP(sp)
	REG_S s1, PT_STATUS(sp)
	REG_S s2, PT_EPC(sp)
	REG_S s3, PT_BADADDR(sp)
	REG_S s4, PT_CAUSE(sp)
	REG_S s5, PT_TP(sp)

	/*
	* Set the scratch register to 0, so that if a recursive exception
	* occurs, the exception vector knows it came from the kernel
	*/
	csrw CSR_SCRATCH, x0

	/* Load the global pointer */
	.option push
	.option norelax
	la gp, __global_pointer$
	.option pop

	#ifdef CONFIG_TRACE_IRQFLAGS
	call __trace_hardirqs_off
	#endif

	#ifdef CONFIG_CONTEXT_TRACKING_USER
	/* If previous state is in user mode, call user_exit_callable(). */
	li a0, SR_PP
	and a0, s1, a0
	bnez a0, skip_context_tracking
	call user_exit_callable
	skip_context_tracking:
	#endif

	/*
	* MSB of cause differentiates between
	* interrupts and exceptions
	*/
	bge s4, zero, 1f

	la ra, ret_from_exception

	/* Handle interrupts */
	move a0, sp /* pt_regs */
	la a1, generic_handle_arch_irq
	jr a1
	1:
	/*
	* Exceptions run with interrupts enabled or disabled depending on the
	* state of SR_PIE in m/sstatus.
	*/
	andi t0, s1, SR_PIE
	beqz t0, 1f
	/* kprobes, entered via ebreak, must have interrupts disabled. */
	li t0, EXC_BREAKPOINT
	beq s4, t0, 1f
	#ifdef CONFIG_TRACE_IRQFLAGS
	call __trace_hardirqs_on
	#endif
	csrs CSR_STATUS, SR_IE

	1:
	la ra, ret_from_exception
	/* Handle syscalls */
	li t0, EXC_SYSCALL
	beq s4, t0, handle_syscall

	/* Handle other exceptions */
	slli t0, s4, RISCV_LGPTR
	la t1, excp_vect_table
	la t2, excp_vect_table_end
	move a0, sp /* pt_regs */
	add t0, t1, t0
	/* Check if exception code lies within bounds */
	bgeu t0, t2, 1f
	REG_L t0, 0(t0)
	jr t0
	1:
	tail do_trap_unknown

	handle_syscall:
	#ifdef CONFIG_RISCV_M_MODE
	/*
	* When running is M-Mode (no MMU config), MPIE does not get set.
	* As a result, we need to force enable interrupts here because
	* handle_exception did not do set SR_IE as it always sees SR_PIE
	* being cleared.
	*/
	csrs CSR_STATUS, SR_IE
	#endif
	#if defined(CONFIG_TRACE_IRQFLAGS) \|\| defined(CONFIG_CONTEXT_TRACKING_USER)
	/* Recover a0 - a7 for system calls */
	REG_L a0, PT_A0(sp)
	REG_L a1, PT_A1(sp)
	REG_L a2, PT_A2(sp)
	REG_L a3, PT_A3(sp)
	REG_L a4, PT_A4(sp)
	REG_L a5, PT_A5(sp)
	REG_L a6, PT_A6(sp)
	REG_L a7, PT_A7(sp)
	#endif
	/* save the initial A0 value (needed in signal handlers) */
	REG_S a0, PT_ORIG_A0(sp)
	/*
	* Advance SEPC to avoid executing the original
	* scall instruction on sret
	*/
	addi s2, s2, 0x4
	REG_S s2, PT_EPC(sp)
	/* Trace syscalls, but only if requested by the user. */
	REG_L t0, TASK_TI_FLAGS(tp)
	andi t0, t0, _TIF_SYSCALL_WORK
	bnez t0, handle_syscall_trace_enter
	check_syscall_nr:
	/* Check to make sure we don't jump to a bogus syscall number. */
	li t0, __NR_syscalls
	la s0, sys_ni_syscall
	/*
	* Syscall number held in a7.
	* If syscall number is above allowed value, redirect to ni_syscall.
	*/
	bgeu a7, t0, 3f
	#ifdef CONFIG_COMPAT
	REG_L s0, PT_STATUS(sp)
	srli s0, s0, SR_UXL_SHIFT
	andi s0, s0, (SR_UXL >> SR_UXL_SHIFT)
	li t0, (SR_UXL_32 >> SR_UXL_SHIFT)
	sub t0, s0, t0
	bnez t0, 1f

	/* Call compat_syscall */
	la s0, compat_sys_call_table
	j 2f
	1:
	#endif
	/* Call syscall */
	la s0, sys_call_table
	2:
	slli t0, a7, RISCV_LGPTR
	add s0, s0, t0
	REG_L s0, 0(s0)
	3:
	jalr s0

	ret_from_syscall:
	/* Set user a0 to kernel a0 */
	REG_S a0, PT_A0(sp)
	/*
	* We didn't execute the actual syscall.
	* Seccomp already set return value for the current task pt_regs.
	* (If it was configured with SECCOMP_RET_ERRNO/TRACE)
	*/
	ret_from_syscall_rejected:
	#ifdef CONFIG_DEBUG_RSEQ
	move a0, sp
	call rseq_syscall
	#endif
	/* Trace syscalls, but only if requested by the user. */
	REG_L t0, TASK_TI_FLAGS(tp)
	andi t0, t0, _TIF_SYSCALL_WORK
	bnez t0, handle_syscall_trace_exit

	ret_from_exception:
	REG_L s0, PT_STATUS(sp)
	csrc CSR_STATUS, SR_IE
	#ifdef CONFIG_TRACE_IRQFLAGS
	call __trace_hardirqs_off
	#endif
	#ifdef CONFIG_RISCV_M_MODE
	/* the MPP value is too large to be used as an immediate arg for addi */
	li t0, SR_MPP
	and s0, s0, t0
	#else
	andi s0, s0, SR_SPP
	#endif
	bnez s0, resume_kernel

	resume_userspace:
	/* Interrupts must be disabled here so flags are checked atomically */
	REG_L s0, TASK_TI_FLAGS(tp) /* current_thread_info->flags */
	andi s1, s0, _TIF_WORK_MASK
	bnez s1, work_pending

	#ifdef CONFIG_CONTEXT_TRACKING_USER
	call user_enter_callable
	#endif

	/* Save unwound kernel stack pointer in thread_info */
	addi s0, sp, PT_SIZE_ON_STACK
	REG_S s0, TASK_TI_KERNEL_SP(tp)

	/*
	* Save TP into the scratch register , so we can find the kernel data
	* structures again.
	*/
	csrw CSR_SCRATCH, tp

	restore_all:
	#ifdef CONFIG_TRACE_IRQFLAGS
	REG_L s1, PT_STATUS(sp)
	andi t0, s1, SR_PIE
	beqz t0, 1f
	call __trace_hardirqs_on
	j 2f
	1:
	call __trace_hardirqs_off
	2:
	#endif
	REG_L a0, PT_STATUS(sp)
	/*
	* The current load reservation is effectively part of the processor's
	* state, in the sense that load reservations cannot be shared between
	* different hart contexts. We can't actually save and restore a load
	* reservation, so instead here we clear any existing reservation --
	* it's always legal for implementations to clear load reservations at
	* any point (as long as the forward progress guarantee is kept, but
	* we'll ignore that here).
	*
	* Dangling load reservations can be the result of taking a trap in the
	* middle of an LR/SC sequence, but can also be the result of a taken
	* forward branch around an SC -- which is how we implement CAS. As a
	* result we need to clear reservations between the last CAS and the
	* jump back to the new context. While it is unlikely the store
	* completes, implementations are allowed to expand reservations to be
	* arbitrarily large.
	*/
	REG_L a2, PT_EPC(sp)
	REG_SC x0, a2, PT_EPC(sp)

	csrw CSR_STATUS, a0
	csrw CSR_EPC, a2

	REG_L x1, PT_RA(sp)
	REG_L x3, PT_GP(sp)
	REG_L x4, PT_TP(sp)
	REG_L x5, PT_T0(sp)
	REG_L x6, PT_T1(sp)
	REG_L x7, PT_T2(sp)
	REG_L x8, PT_S0(sp)
	REG_L x9, PT_S1(sp)
	REG_L x10, PT_A0(sp)
	REG_L x11, PT_A1(sp)
	REG_L x12, PT_A2(sp)
	REG_L x13, PT_A3(sp)
	REG_L x14, PT_A4(sp)
	REG_L x15, PT_A5(sp)
	REG_L x16, PT_A6(sp)
	REG_L x17, PT_A7(sp)
	REG_L x18, PT_S2(sp)
	REG_L x19, PT_S3(sp)
	REG_L x20, PT_S4(sp)
	REG_L x21, PT_S5(sp)
	REG_L x22, PT_S6(sp)
	REG_L x23, PT_S7(sp)
	REG_L x24, PT_S8(sp)
	REG_L x25, PT_S9(sp)
	REG_L x26, PT_S10(sp)
	REG_L x27, PT_S11(sp)
	REG_L x28, PT_T3(sp)
	REG_L x29, PT_T4(sp)
	REG_L x30, PT_T5(sp)
	REG_L x31, PT_T6(sp)

	REG_L x2, PT_SP(sp)

	#ifdef CONFIG_RISCV_M_MODE
	mret
	#else
	sret
	#endif

	#if IS_ENABLED(CONFIG_PREEMPTION)
	resume_kernel:
	REG_L s0, TASK_TI_PREEMPT_COUNT(tp)
	bnez s0, restore_all
	REG_L s0, TASK_TI_FLAGS(tp)
	andi s0, s0, _TIF_NEED_RESCHED
	beqz s0, restore_all
	call preempt_schedule_irq
	j restore_all
	#endif

	work_pending:
	/* Enter slow path for supplementary processing */
	la ra, ret_from_exception
	andi s1, s0, _TIF_NEED_RESCHED
	bnez s1, work_resched
	work_notifysig:
	/* Handle pending signals and notify-resume requests */
	csrs CSR_STATUS, SR_IE /* Enable interrupts for do_notify_resume() */
	move a0, sp /* pt_regs */
	move a1, s0 /* current_thread_info->flags */
	tail do_notify_resume
	work_resched:
	tail schedule

	/* Slow paths for ptrace. */
	handle_syscall_trace_enter:
	move a0, sp
	call do_syscall_trace_enter
	move t0, a0
	REG_L a0, PT_A0(sp)
	REG_L a1, PT_A1(sp)
	REG_L a2, PT_A2(sp)
	REG_L a3, PT_A3(sp)
	REG_L a4, PT_A4(sp)
	REG_L a5, PT_A5(sp)
	REG_L a6, PT_A6(sp)
	REG_L a7, PT_A7(sp)
	bnez t0, ret_from_syscall_rejected
	j check_syscall_nr
	handle_syscall_trace_exit:
	move a0, sp
	call do_syscall_trace_exit
	j ret_from_exception

	#ifdef CONFIG_VMAP_STACK
	handle_kernel_stack_overflow:
	/*
	* Takes the psuedo-spinlock for the shadow stack, in case multiple
	* harts are concurrently overflowing their kernel stacks. We could
	* store any value here, but since we're overflowing the kernel stack
	* already we only have SP to use as a scratch register. So we just
	* swap in the address of the spinlock, as that's definately non-zero.
	*
	* Pairs with a store_release in handle_bad_stack().
	*/
	1: la sp, spin_shadow_stack
	REG_AMOSWAP_AQ sp, sp, (sp)
	bnez sp, 1b

	la sp, shadow_stack
	addi sp, sp, SHADOW_OVERFLOW_STACK_SIZE

	//save caller register to shadow stack
	addi sp, sp, -(PT_SIZE_ON_STACK)
	REG_S x1, PT_RA(sp)
	REG_S x5, PT_T0(sp)
	REG_S x6, PT_T1(sp)
	REG_S x7, PT_T2(sp)
	REG_S x10, PT_A0(sp)
	REG_S x11, PT_A1(sp)
	REG_S x12, PT_A2(sp)
	REG_S x13, PT_A3(sp)
	REG_S x14, PT_A4(sp)
	REG_S x15, PT_A5(sp)
	REG_S x16, PT_A6(sp)
	REG_S x17, PT_A7(sp)
	REG_S x28, PT_T3(sp)
	REG_S x29, PT_T4(sp)
	REG_S x30, PT_T5(sp)
	REG_S x31, PT_T6(sp)

	la ra, restore_caller_reg
	tail get_overflow_stack

	restore_caller_reg:
	//save per-cpu overflow stack
	REG_S a0, -8(sp)
	//restore caller register from shadow_stack
	REG_L x1, PT_RA(sp)
	REG_L x5, PT_T0(sp)
	REG_L x6, PT_T1(sp)
	REG_L x7, PT_T2(sp)
	REG_L x10, PT_A0(sp)
	REG_L x11, PT_A1(sp)
	REG_L x12, PT_A2(sp)
	REG_L x13, PT_A3(sp)
	REG_L x14, PT_A4(sp)
	REG_L x15, PT_A5(sp)
	REG_L x16, PT_A6(sp)
	REG_L x17, PT_A7(sp)
	REG_L x28, PT_T3(sp)
	REG_L x29, PT_T4(sp)
	REG_L x30, PT_T5(sp)
	REG_L x31, PT_T6(sp)

	//load per-cpu overflow stack
	REG_L sp, -8(sp)
	addi sp, sp, -(PT_SIZE_ON_STACK)

	//save context to overflow stack
	REG_S x1, PT_RA(sp)
	REG_S x3, PT_GP(sp)
	REG_S x5, PT_T0(sp)
	REG_S x6, PT_T1(sp)
	REG_S x7, PT_T2(sp)
	REG_S x8, PT_S0(sp)
	REG_S x9, PT_S1(sp)
	REG_S x10, PT_A0(sp)
	REG_S x11, PT_A1(sp)
	REG_S x12, PT_A2(sp)
	REG_S x13, PT_A3(sp)
	REG_S x14, PT_A4(sp)
	REG_S x15, PT_A5(sp)
	REG_S x16, PT_A6(sp)
	REG_S x17, PT_A7(sp)
	REG_S x18, PT_S2(sp)
	REG_S x19, PT_S3(sp)
	REG_S x20, PT_S4(sp)
	REG_S x21, PT_S5(sp)
	REG_S x22, PT_S6(sp)
	REG_S x23, PT_S7(sp)
	REG_S x24, PT_S8(sp)
	REG_S x25, PT_S9(sp)
	REG_S x26, PT_S10(sp)
	REG_S x27, PT_S11(sp)
	REG_S x28, PT_T3(sp)
	REG_S x29, PT_T4(sp)
	REG_S x30, PT_T5(sp)
	REG_S x31, PT_T6(sp)

	REG_L s0, TASK_TI_KERNEL_SP(tp)
	csrr s1, CSR_STATUS
	csrr s2, CSR_EPC
	csrr s3, CSR_TVAL
	csrr s4, CSR_CAUSE
	csrr s5, CSR_SCRATCH
	REG_S s0, PT_SP(sp)
	REG_S s1, PT_STATUS(sp)
	REG_S s2, PT_EPC(sp)
	REG_S s3, PT_BADADDR(sp)
	REG_S s4, PT_CAUSE(sp)
	REG_S s5, PT_TP(sp)
	move a0, sp
	tail handle_bad_stack
	#endif

	END(handle_exception)

	ENTRY(ret_from_fork)
	la ra, ret_from_exception
	tail schedule_tail
	ENDPROC(ret_from_fork)

	ENTRY(ret_from_kernel_thread)
	call schedule_tail
	/* Call fn(arg) */
	la ra, ret_from_exception
	move a0, s1
	jr s0
	ENDPROC(ret_from_kernel_thread)


	/*
	* Integer register context switch
	* The callee-saved registers must be saved and restored.
	*
	* a0: previous task_struct (must be preserved across the switch)
	* a1: next task_struct
	*
	* The value of a0 and a1 must be preserved by this function, as that's how
	* arguments are passed to schedule_tail.
	*/
	ENTRY(__switch_to)
	/* Save context into prev->thread */
	li a4, TASK_THREAD_RA
	add a3, a0, a4
	add a4, a1, a4
	REG_S ra, TASK_THREAD_RA_RA(a3)
	REG_S sp, TASK_THREAD_SP_RA(a3)
	REG_S s0, TASK_THREAD_S0_RA(a3)
	REG_S s1, TASK_THREAD_S1_RA(a3)
	REG_S s2, TASK_THREAD_S2_RA(a3)
	REG_S s3, TASK_THREAD_S3_RA(a3)
	REG_S s4, TASK_THREAD_S4_RA(a3)
	REG_S s5, TASK_THREAD_S5_RA(a3)
	REG_S s6, TASK_THREAD_S6_RA(a3)
	REG_S s7, TASK_THREAD_S7_RA(a3)
	REG_S s8, TASK_THREAD_S8_RA(a3)
	REG_S s9, TASK_THREAD_S9_RA(a3)
	REG_S s10, TASK_THREAD_S10_RA(a3)
	REG_S s11, TASK_THREAD_S11_RA(a3)
	/* Restore context from next->thread */
	REG_L ra, TASK_THREAD_RA_RA(a4)
	REG_L sp, TASK_THREAD_SP_RA(a4)
	REG_L s0, TASK_THREAD_S0_RA(a4)
	REG_L s1, TASK_THREAD_S1_RA(a4)
	REG_L s2, TASK_THREAD_S2_RA(a4)
	REG_L s3, TASK_THREAD_S3_RA(a4)
	REG_L s4, TASK_THREAD_S4_RA(a4)
	REG_L s5, TASK_THREAD_S5_RA(a4)
	REG_L s6, TASK_THREAD_S6_RA(a4)
	REG_L s7, TASK_THREAD_S7_RA(a4)
	REG_L s8, TASK_THREAD_S8_RA(a4)
	REG_L s9, TASK_THREAD_S9_RA(a4)
	REG_L s10, TASK_THREAD_S10_RA(a4)
	REG_L s11, TASK_THREAD_S11_RA(a4)
	/* The offset of thread_info in task_struct is zero. */
	move tp, a1
	ret
	ENDPROC(__switch_to)

	#ifndef CONFIG_MMU
	#define do_page_fault do_trap_unknown
	#endif

	.section ".rodata"
	.align LGREG
	/* Exception vector table */
	ENTRY(excp_vect_table)
	RISCV_PTR do_trap_insn_misaligned
	ALT_INSN_FAULT(RISCV_PTR do_trap_insn_fault)
	RISCV_PTR do_trap_insn_illegal
	RISCV_PTR do_trap_break
	RISCV_PTR do_trap_load_misaligned
	RISCV_PTR do_trap_load_fault
	RISCV_PTR do_trap_store_misaligned
	RISCV_PTR do_trap_store_fault
	RISCV_PTR do_trap_ecall_u /* system call, gets intercepted */
	RISCV_PTR do_trap_ecall_s
	RISCV_PTR do_trap_unknown
	RISCV_PTR do_trap_ecall_m
	/* instruciton page fault */
	ALT_PAGE_FAULT(RISCV_PTR do_page_fault)
	RISCV_PTR do_page_fault /* load page fault */
	RISCV_PTR do_trap_unknown
	RISCV_PTR do_page_fault /* store page fault */
	excp_vect_table_end:
	END(excp_vect_table)

	#ifndef CONFIG_MMU
	ENTRY(__user_rt_sigreturn)
	li a7, __NR_rt_sigreturn
	scall
	END(__user_rt_sigreturn)
	#endif