[PATCH v2 3/7] riscv: convert bottom half of exception handling to C

Jisheng Zhang
2024-07-20T17:26:55Z

For readability, maintainability and future scalability, convert the
bottom half of the exception handling to C.

Mostly the assembly code is converted to C in a relatively
straightforward manner.

However, there are two modifications I need to mention:

1. the cause I.E the CSR_CAUSE value is passed to do_traps() via. 2nd
param, do_traps() doesn't get it from pt_regs because this way an extra
memory load is needed, the exception handling sits in hot code path,
every instruction matters.

2.To cope with SIFIVE_CIP_453 errata, it looks like we don't need
alternative mechanism any more after the asm->c conversion. Just
replace the excp_vect_table two entries.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
---
arch/riscv/errata/sifive/errata.c | 25 ++++++++---
arch/riscv/include/asm/asm-prototypes.h | 1 +
arch/riscv/include/asm/errata_list.h | 5 +--
arch/riscv/kernel/entry.S | 57 +------------------------
arch/riscv/kernel/traps.c | 37 ++++++++++++++++
5 files changed, 61 insertions(+), 64 deletions(-)

diff --git a/arch/riscv/errata/sifive/errata.c b/arch/riscv/errata/sifive/errata.c
index 716cfedad3a2..bbba99f207ca 100644
--- a/arch/riscv/errata/sifive/errata.c
+++ b/arch/riscv/errata/sifive/errata.c
@@ -10,9 +10,14 @@
#include <linux/bug.h>
#include <asm/patch.h>
#include <asm/alternative.h>
+#include <asm/csr.h>
#include <asm/vendorid_list.h>
#include <asm/errata_list.h>

+extern void (*excp_vect_table[])(struct pt_regs *regs);
+extern void sifive_cip_453_insn_fault_trp(struct pt_regs *regs);
+extern void sifive_cip_453_page_fault_trp(struct pt_regs *regs);
+
struct errata_info_t {
char name[32];
bool (*check_func)(unsigned long arch_id, unsigned long impid);
@@ -20,6 +25,9 @@ struct errata_info_t {

static bool errata_cip_453_check_func(unsigned long arch_id, unsigned long impid)
{
+ if (!IS_ENABLED(CONFIG_ERRATA_SIFIVE_CIP_453))
+ return false;
+
/*
* Affected cores:
* Architecture ID: 0x8000000000000007
@@ -51,10 +59,6 @@ static bool errata_cip_1200_check_func(unsigned long arch_id, unsigned long imp
}

static struct errata_info_t errata_list[ERRATA_SIFIVE_NUMBER] = {
- {
- .name = "cip-453",
- .check_func = errata_cip_453_check_func
- },
{
.name = "cip-1200",
.check_func = errata_cip_1200_check_func
@@ -62,11 +66,20 @@ static struct errata_info_t errata_list[ERRATA_SIFIVE_NUMBER] = {
};

static u32 __init_or_module sifive_errata_probe(unsigned long archid,
- unsigned long impid)
+ unsigned long impid,
+ unsigned int stage)
{
int idx;
u32 cpu_req_errata = 0;

+ if (stage == RISCV_ALTERNATIVES_BOOT) {
+ if (IS_ENABLED(CONFIG_MMU) &&
+ errata_cip_453_check_func(archid, impid)) {
+ excp_vect_table[EXC_INST_ACCESS] = sifive_cip_453_insn_fault_trp;
+ excp_vect_table[EXC_INST_PAGE_FAULT] = sifive_cip_453_page_fault_trp;
+ }
+ }
+
for (idx = 0; idx < ERRATA_SIFIVE_NUMBER; idx++)
if (errata_list[idx].check_func(archid, impid))
cpu_req_errata |= (1U << idx);
@@ -99,7 +112,7 @@ void sifive_errata_patch_func(struct alt_entry *begin, struct alt_entry *end,
if (stage == RISCV_ALTERNATIVES_EARLY_BOOT)
return;

- cpu_req_errata = sifive_errata_probe(archid, impid);
+ cpu_req_errata = sifive_errata_probe(archid, impid, stage);

for (alt = begin; alt < end; alt++) {
if (alt->vendor_id != SIFIVE_VENDOR_ID)
diff --git a/arch/riscv/include/asm/asm-prototypes.h b/arch/riscv/include/asm/asm-prototypes.h
index cd627ec289f1..c6691e9032dd 100644
--- a/arch/riscv/include/asm/asm-prototypes.h
+++ b/arch/riscv/include/asm/asm-prototypes.h
@@ -55,5 +55,6 @@ DECLARE_DO_ERROR_INFO(do_trap_break);
asmlinkage void handle_bad_stack(struct pt_regs *regs);
asmlinkage void do_page_fault(struct pt_regs *regs);
asmlinkage void do_irq(struct pt_regs *regs);
+asmlinkage void do_traps(struct pt_regs *regs, unsigned long cause);

#endif /* _ASM_RISCV_PROTOTYPES_H */
diff --git a/arch/riscv/include/asm/errata_list.h b/arch/riscv/include/asm/errata_list.h
index 7c8a71a526a3..95b79afc4061 100644
--- a/arch/riscv/include/asm/errata_list.h
+++ b/arch/riscv/include/asm/errata_list.h
@@ -17,9 +17,8 @@
#endif

#ifdef CONFIG_ERRATA_SIFIVE
-#define ERRATA_SIFIVE_CIP_453 0
-#define ERRATA_SIFIVE_CIP_1200 1
-#define ERRATA_SIFIVE_NUMBER 2
+#define ERRATA_SIFIVE_CIP_1200 0
+#define ERRATA_SIFIVE_NUMBER 1
#endif

#ifdef CONFIG_ERRATA_THEAD
diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index 81dec627a8d4..37c3c2068fef 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -83,36 +83,10 @@ SYM_CODE_START(handle_exception)
/* Load the kernel shadow call stack pointer if coming from userspace */
scs_load_current_if_task_changed s5

-#ifdef CONFIG_RISCV_ISA_V_PREEMPTIVE
- move a0, sp
- call riscv_v_context_nesting_start
-#endif
move a0, sp /* pt_regs */
-
- /*
- * MSB of cause differentiates between
- * interrupts and exceptions
- */
- bge s4, zero, 1f
-
- /* Handle interrupts */
- call do_irq
+ move a1, s4 /* cause */
+ call do_traps
j ret_from_exception
-1:
- /* Handle other exceptions */
- slli t0, s4, RISCV_LGPTR
- la t1, excp_vect_table
- la t2, excp_vect_table_end
- add t0, t1, t0
- /* Check if exception code lies within bounds */
- bgeu t0, t2, 3f
- REG_L t1, 0(t0)
-2: jalr t1
- j ret_from_exception
-3:
-
- la t1, do_trap_unknown
- j 2b
SYM_CODE_END(handle_exception)
ASM_NOKPROBE(handle_exception)

@@ -329,33 +303,6 @@ SYM_FUNC_START(__switch_to)
ret
SYM_FUNC_END(__switch_to)

-#ifndef CONFIG_MMU
-#define do_page_fault do_trap_unknown
-#endif
-
- .section ".rodata"
- .align LGREG
- /* Exception vector table */
-SYM_DATA_START_LOCAL(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 */
- 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 */
-SYM_DATA_END_LABEL(excp_vect_table, SYM_L_LOCAL, excp_vect_table_end)
-
#ifndef CONFIG_MMU
SYM_DATA_START(__user_rt_sigreturn)
li a7, __NR_rt_sigreturn
diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index 3d1f84cb6eac..3eaa7c72f2be 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -390,6 +390,43 @@ asmlinkage void noinstr do_irq(struct pt_regs *regs)
irqentry_exit(regs, state);
}

+void (*excp_vect_table[])(struct pt_regs *regs) __ro_after_init = {
+ do_trap_insn_misaligned, /* 0 Instruction address misaligned */
+ do_trap_insn_fault, /* 1 Instruction access fault */
+ do_trap_insn_illegal, /* 2 Illegal instruction */
+ do_trap_break, /* 3 Breakpoint */
+ do_trap_load_misaligned, /* 4 Load address misaligned */
+ do_trap_load_fault, /* 5 Load access fault */
+ do_trap_store_misaligned, /* 6 Store/AMO address misaligned */
+ do_trap_store_fault, /* 7 Store/AMO access fault */
+ do_trap_ecall_u, /* 8 Environment call from U-mode */
+ do_trap_ecall_s, /* 9 Environment call from S-mode */
+ do_trap_unknown, /* 10 Reserved */
+ do_trap_ecall_m, /* 11 Environment call from M-mode */
+#ifdef CONFIG_MMU
+ do_page_fault, /* 12 Instruciton page fault */
+ do_page_fault, /* 13 Load page fault */
+ do_trap_unknown, /* 14 Reserved */
+ do_page_fault, /* 15 Store/AMO page fault */
+#endif
+};
+
+asmlinkage void noinstr do_traps(struct pt_regs *regs, unsigned long cause)
+{
+#ifdef CONFIG_RISCV_ISA_V_PREEMPTIVE
+ riscv_v_context_nesting_start(regs);
+#endif
+ if (cause & CAUSE_IRQ_FLAG) {
+ do_irq(regs);
+ } else {
+ if (cause >= ARRAY_SIZE(excp_vect_table)) {
+ do_trap_unknown(regs);
+ return;
+ }
+ excp_vect_table[cause](regs);
+ }
+}
+
#ifdef CONFIG_GENERIC_BUG
int is_valid_bugaddr(unsigned long pc)
{
--
2.43.0