Re: [PATCH] riscv: misaligned: remove CONFIG_RISCV_M_MODE specific code

[Charlie Jenkins]

On Tue, Nov 28, 2023 at 05:52:06PM +0100, Clément Léger wrote:
> While reworking code to fix sparse errors, it appears that the
> RISCV_M_MODE specific could actually be removed and use the one for
> normal mode. Even though RISCV_M_MODE can do direct user memory access,
> using the user uaccess helpers is also going to work. Since there is no
> need anymore for specific accessors (load_u8()/store_u8()), we can
> directly use memcpy()/copy_{to/from}_user() and get rid of the copy
> loop entirely. __read_insn() is also fixed to use an unsigned long
> instead of a pointer which was cast in __user address space. The
> insn_addr parameter is now cast from unsigned long to the correct
> address space directly.
> 
> Signed-off-by: Clément Léger <cleger@xxxxxxxxxxxx>
> 
> ---
> 
> Notes: This patch is a complete rework of a previous one [1] and thus is
> not a V3.
> 
> [1] https://lore.kernel.org/linux-riscv/d156242a-f104-4925-9736-624a4ba8210d@xxxxxxxxxxxx/
> ---
>  arch/riscv/kernel/traps_misaligned.c | 108 +++++----------------------
>  1 file changed, 18 insertions(+), 90 deletions(-)
> 
> diff --git a/arch/riscv/kernel/traps_misaligned.c b/arch/riscv/kernel/traps_misaligned.c
> index 5eba37147caa..38af97f4819c 100644
> --- a/arch/riscv/kernel/traps_misaligned.c
> +++ b/arch/riscv/kernel/traps_misaligned.c
> @@ -264,86 +264,14 @@ static unsigned long get_f32_rs(unsigned long insn, u8 fp_reg_offset,
>  #define GET_F32_RS2C(insn, regs) (get_f32_rs(insn, 2, regs))
>  #define GET_F32_RS2S(insn, regs) (get_f32_rs(RVC_RS2S(insn), 0, regs))
>  
> -#ifdef CONFIG_RISCV_M_MODE
> -static inline int load_u8(struct pt_regs *regs, const u8 *addr, u8 *r_val)
> -{
> -	u8 val;
> -
> -	asm volatile("lbu %0, %1" : "=&r" (val) : "m" (*addr));
> -	*r_val = val;
> -
> -	return 0;
> -}
> -
> -static inline int store_u8(struct pt_regs *regs, u8 *addr, u8 val)
> -{
> -	asm volatile ("sb %0, %1\n" : : "r" (val), "m" (*addr));
> -
> -	return 0;
> -}
> -
> -static inline int get_insn(struct pt_regs *regs, ulong mepc, ulong *r_insn)
> -{
> -	register ulong __mepc asm ("a2") = mepc;
> -	ulong val, rvc_mask = 3, tmp;
> -
> -	asm ("and %[tmp], %[addr], 2\n"
> -		"bnez %[tmp], 1f\n"
> -#if defined(CONFIG_64BIT)
> -		__stringify(LWU) " %[insn], (%[addr])\n"
> -#else
> -		__stringify(LW) " %[insn], (%[addr])\n"
> -#endif
> -		"and %[tmp], %[insn], %[rvc_mask]\n"
> -		"beq %[tmp], %[rvc_mask], 2f\n"
> -		"sll %[insn], %[insn], %[xlen_minus_16]\n"
> -		"srl %[insn], %[insn], %[xlen_minus_16]\n"
> -		"j 2f\n"
> -		"1:\n"
> -		"lhu %[insn], (%[addr])\n"
> -		"and %[tmp], %[insn], %[rvc_mask]\n"
> -		"bne %[tmp], %[rvc_mask], 2f\n"
> -		"lhu %[tmp], 2(%[addr])\n"
> -		"sll %[tmp], %[tmp], 16\n"
> -		"add %[insn], %[insn], %[tmp]\n"
> -		"2:"
> -	: [insn] "=&r" (val), [tmp] "=&r" (tmp)
> -	: [addr] "r" (__mepc), [rvc_mask] "r" (rvc_mask),
> -	  [xlen_minus_16] "i" (XLEN_MINUS_16));
> -
> -	*r_insn = val;
> -
> -	return 0;
> -}
> -#else
> -static inline int load_u8(struct pt_regs *regs, const u8 *addr, u8 *r_val)
> -{
> -	if (user_mode(regs)) {
> -		return __get_user(*r_val, addr);
> -	} else {
> -		*r_val = *addr;
> -		return 0;
> -	}
> -}
> -
> -static inline int store_u8(struct pt_regs *regs, u8 *addr, u8 val)
> -{
> -	if (user_mode(regs)) {
> -		return __put_user(val, addr);
> -	} else {
> -		*addr = val;
> -		return 0;
> -	}
> -}
> -
> -#define __read_insn(regs, insn, insn_addr)		\
> +#define __read_insn(regs, insn, insn_addr, type)	\
>  ({							\
>  	int __ret;					\
>  							\
> -	if (user_mode(regs)) {				\
> -		__ret = __get_user(insn, insn_addr);	\
> +	if (user_mode(regs)) { \
Nit: formatting was changed here (previous indentation seems better)

> +		__ret = __get_user(insn, (type __user *) insn_addr); \
>  	} else {					\
> -		insn = *insn_addr;			\
> +		insn = *(type *)insn_addr;		\
>  		__ret = 0;				\
>  	}						\
>  							\
> @@ -356,9 +284,8 @@ static inline int get_insn(struct pt_regs *regs, ulong epc, ulong *r_insn)
>  
>  	if (epc & 0x2) {
>  		ulong tmp = 0;
> -		u16 __user *insn_addr = (u16 __user *)epc;
>  
> -		if (__read_insn(regs, insn, insn_addr))
> +		if (__read_insn(regs, insn, epc, u16))
>  			return -EFAULT;
>  		/* __get_user() uses regular "lw" which sign extend the loaded
>  		 * value make sure to clear higher order bits in case we "or" it
> @@ -369,16 +296,14 @@ static inline int get_insn(struct pt_regs *regs, ulong epc, ulong *r_insn)
>  			*r_insn = insn;
>  			return 0;
>  		}
> -		insn_addr++;
> -		if (__read_insn(regs, tmp, insn_addr))
> +		epc += sizeof(u16);
> +		if (__read_insn(regs, tmp, epc, u16))
>  			return -EFAULT;
>  		*r_insn = (tmp << 16) | insn;
>  
>  		return 0;
>  	} else {
> -		u32 __user *insn_addr = (u32 __user *)epc;
> -
> -		if (__read_insn(regs, insn, insn_addr))
> +		if (__read_insn(regs, insn, epc, u32))
>  			return -EFAULT;
>  		if ((insn & __INSN_LENGTH_MASK) == __INSN_LENGTH_32) {
>  			*r_insn = insn;
> @@ -390,7 +315,6 @@ static inline int get_insn(struct pt_regs *regs, ulong epc, ulong *r_insn)
>  		return 0;
>  	}
>  }
> -#endif
>  
>  union reg_data {
>  	u8 data_bytes[8];
> @@ -409,7 +333,7 @@ int handle_misaligned_load(struct pt_regs *regs)
>  	unsigned long epc = regs->epc;
>  	unsigned long insn;
>  	unsigned long addr = regs->badaddr;
> -	int i, fp = 0, shift = 0, len = 0;
> +	int fp = 0, shift = 0, len = 0;
>  
>  	perf_sw_event(PERF_COUNT_SW_ALIGNMENT_FAULTS, 1, regs, addr);
>  
> @@ -490,9 +414,11 @@ int handle_misaligned_load(struct pt_regs *regs)
>  		return -EOPNOTSUPP;
>  
>  	val.data_u64 = 0;
> -	for (i = 0; i < len; i++) {
> -		if (load_u8(regs, (void *)(addr + i), &val.data_bytes[i]))
> +	if (user_mode(regs)) {
> +		if (raw_copy_from_user(&val, (u8 __user *)addr, len))
>  			return -1;
> +	} else {
> +		memcpy(&val, (u8 *)addr, len);
>  	}
>  
>  	if (!fp)
> @@ -513,7 +439,7 @@ int handle_misaligned_store(struct pt_regs *regs)
>  	unsigned long epc = regs->epc;
>  	unsigned long insn;
>  	unsigned long addr = regs->badaddr;
> -	int i, len = 0, fp = 0;
> +	int len = 0, fp = 0;
>  
>  	perf_sw_event(PERF_COUNT_SW_ALIGNMENT_FAULTS, 1, regs, addr);
>  
> @@ -588,9 +514,11 @@ int handle_misaligned_store(struct pt_regs *regs)
>  	if (!IS_ENABLED(CONFIG_FPU) && fp)
>  		return -EOPNOTSUPP;
>  
> -	for (i = 0; i < len; i++) {
> -		if (store_u8(regs, (void *)(addr + i), val.data_bytes[i]))
> +	if (user_mode(regs)) {
> +		if (raw_copy_to_user((u8 __user *)addr, &val, len))
>  			return -1;
> +	} else {
> +		memcpy((u8 *)addr, &val, len);
>  	}
>  
>  	regs->epc = epc + INSN_LEN(insn);
> -- 
> 2.42.0
> 
> 
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The changes here logically make sense but I don't know how to test this
misaligned access code on M-mode or in S-mode. Can you provide how this
was tested?

There are also some merge conflicts that were introduced in 6.8-rc1.

- Charlie