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Date: Thu, 20 Jun 2024 09:49:48 +1000
From: Cyril Bur <cyrilbur@...storrent.com>
To: Jisheng Zhang <jszhang@...nel.org>
Cc: Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>,
Albert Ou <aou@...s.berkeley.edu>, Samuel Holland <samuel.holland@...ive.com>,
linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] riscv: convert bottom half of exception handling to C
On Mon, Jun 17, 2024 at 3:21 AM Jisheng Zhang <jszhang@...nel.org> wrote:
>
> 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 CSR_CAUSE reg reading and saving is moved to the C code
> because we need the cause to dispatch the exception handling,
> if we keep the cause reading and saving, we either pass it to
> do_traps() via. 2nd param or get it from pt_regs which an extra
> memory load is needed, I don't like any of the two solutions becase
> 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 convertion. Just
> replace the excp_vect_table two entries.
>
HI Jisheng,
Patch looks great. I've been going through to understand, this is more
a learning exercise for me than anything else.
I have a question though.
> Signed-off-by: Jisheng Zhang <jszhang@...nel.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 | 58 +------------------------
> arch/riscv/kernel/traps.c | 41 +++++++++++++++++
> 5 files changed, 64 insertions(+), 66 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..81a1f27fa54f 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);
>
> #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..401bfe85a098 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -62,13 +62,11 @@ SYM_CODE_START(handle_exception)
> 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)
>
> /*
> @@ -83,36 +81,9 @@ 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
Along with removing this, can the asmlinkage in asm-prototypes.h be removed?
Or are you keeping the _start() in because the _end() needs to stay?
That leads me to think about leaving the call to
riscv_context_nesting_start() in asm
here for the symmetry of _start() and _end() being called from entry.S.
> move a0, sp /* pt_regs */
> -
> - /*
> - * MSB of cause differentiates between
> - * interrupts and exceptions
> - */
> - bge s4, zero, 1f
> -
> - /* Handle interrupts */
> - call do_irq
> - 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
> + call do_traps
> j ret_from_exception
> -3:
> -
> - la t1, do_trap_unknown
> - j 2b
> SYM_CODE_END(handle_exception)
> ASM_NOKPROBE(handle_exception)
>
> @@ -329,33 +300,6 @@ 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 05a16b1f0aee..b44d4a8d4083 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -390,6 +390,47 @@ 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 = csr_read(CSR_CAUSE);
> +
> + regs->cause = 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
>
>
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