Re: [PATCH v2 2/8] riscv: add support for misaligned trap handling in S-mode

[Björn Töpel]

Clément Léger <cleger@xxxxxxxxxxxx> writes:

> Misalignment trap handling is only supported for M-mode and uses direct
> accesses to user memory. In S-mode, when handling usermode fault, this
> requires to use the get_user()/put_user() accessors. Implement
> load_u8(), store_u8() and get_insn() using these accessors for
> userspace and direct text access for kernel.
>
> Signed-off-by: Clément Léger <cleger@xxxxxxxxxxxx>
> ---
>  arch/riscv/Kconfig                    |   8 ++
>  arch/riscv/include/asm/entry-common.h |  14 +++
>  arch/riscv/kernel/Makefile            |   2 +-
>  arch/riscv/kernel/traps.c             |   9 --
>  arch/riscv/kernel/traps_misaligned.c  | 119 +++++++++++++++++++++++---
>  5 files changed, 129 insertions(+), 23 deletions(-)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index d607ab0f7c6d..6e167358a897 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -636,6 +636,14 @@ config THREAD_SIZE_ORDER
>  	  Specify the Pages of thread stack size (from 4KB to 64KB), which also
>  	  affects irq stack size, which is equal to thread stack size.
>  
> +config RISCV_MISALIGNED
> +	bool "Support misaligned load/store traps for kernel and userspace"
> +	default y
> +	help
> +	  Say Y here if you want the kernel to embed support for misaligned
> +	  load/store for both kernel and userspace. When disable, misaligned
> +	  accesses will generate SIGBUS in userspace and panic in kernel.
> +
>  endmenu # "Platform type"
>  
>  menu "Kernel features"
> diff --git a/arch/riscv/include/asm/entry-common.h b/arch/riscv/include/asm/entry-common.h
> index 6e4dee49d84b..7ab5e34318c8 100644
> --- a/arch/riscv/include/asm/entry-common.h
> +++ b/arch/riscv/include/asm/entry-common.h
> @@ -8,4 +8,18 @@
>  void handle_page_fault(struct pt_regs *regs);
>  void handle_break(struct pt_regs *regs);
>  
> +#ifdef CONFIG_RISCV_MISALIGNED
> +int handle_misaligned_load(struct pt_regs *regs);
> +int handle_misaligned_store(struct pt_regs *regs);
> +#else
> +static inline int handle_misaligned_load(struct pt_regs *regs)
> +{
> +	return -1;
> +}
> +static inline int handle_misaligned_store(struct pt_regs *regs)
> +{
> +	return -1;
> +}
> +#endif
> +
>  #endif /* _ASM_RISCV_ENTRY_COMMON_H */
> diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
> index 95cf25d48405..0d874fb24b51 100644
> --- a/arch/riscv/kernel/Makefile
> +++ b/arch/riscv/kernel/Makefile
> @@ -59,7 +59,7 @@ obj-y	+= patch.o
>  obj-y	+= probes/
>  obj-$(CONFIG_MMU) += vdso.o vdso/
>  
> -obj-$(CONFIG_RISCV_M_MODE)	+= traps_misaligned.o
> +obj-$(CONFIG_RISCV_MISALIGNED)	+= traps_misaligned.o
>  obj-$(CONFIG_FPU)		+= fpu.o
>  obj-$(CONFIG_RISCV_ISA_V)	+= vector.o
>  obj-$(CONFIG_SMP)		+= smpboot.o
> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> index 19807c4d3805..d69779e4b967 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -179,14 +179,6 @@ asmlinkage __visible __trap_section void do_trap_insn_illegal(struct pt_regs *re
>  
>  DO_ERROR_INFO(do_trap_load_fault,
>  	SIGSEGV, SEGV_ACCERR, "load access fault");
> -#ifndef CONFIG_RISCV_M_MODE
> -DO_ERROR_INFO(do_trap_load_misaligned,
> -	SIGBUS, BUS_ADRALN, "Oops - load address misaligned");
> -DO_ERROR_INFO(do_trap_store_misaligned,
> -	SIGBUS, BUS_ADRALN, "Oops - store (or AMO) address misaligned");
> -#else
> -int handle_misaligned_load(struct pt_regs *regs);
> -int handle_misaligned_store(struct pt_regs *regs);
>  
>  asmlinkage __visible __trap_section void do_trap_load_misaligned(struct pt_regs *regs)
>  {
> @@ -229,7 +221,6 @@ asmlinkage __visible __trap_section void do_trap_store_misaligned(struct pt_regs
>  		irqentry_nmi_exit(regs, state);
>  	}
>  }
> -#endif
>  DO_ERROR_INFO(do_trap_store_fault,
>  	SIGSEGV, SEGV_ACCERR, "store (or AMO) access fault");
>  DO_ERROR_INFO(do_trap_ecall_s,
> diff --git a/arch/riscv/kernel/traps_misaligned.c b/arch/riscv/kernel/traps_misaligned.c
> index e7bfb33089c1..9daed7d756ae 100644
> --- a/arch/riscv/kernel/traps_misaligned.c
> +++ b/arch/riscv/kernel/traps_misaligned.c
> @@ -12,6 +12,7 @@
>  #include <asm/processor.h>
>  #include <asm/ptrace.h>
>  #include <asm/csr.h>
> +#include <asm/entry-common.h>
>  
>  #define INSN_MATCH_LB			0x3
>  #define INSN_MASK_LB			0x707f
> @@ -151,21 +152,25 @@
>  #define PRECISION_S 0
>  #define PRECISION_D 1
>  
> -static inline u8 load_u8(const u8 *addr)
> +#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 val;
> +	return 0;
>  }
>  
> -static inline void store_u8(u8 *addr, u8 val)
> +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 ulong get_insn(ulong mepc)
> +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;
> @@ -194,9 +199,87 @@ static inline ulong get_insn(ulong mepc)
>  	: [addr] "r" (__mepc), [rvc_mask] "r" (rvc_mask),
>  	  [xlen_minus_16] "i" (XLEN_MINUS_16));
>  
> -	return val;
> +	*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;
> +	}

One nit (...well two) ;-)

If you're respinning I'd get rid of the "inlines", and personally I
think early exit is easier to read. Applies to the whole patch.

  | {
  | 	if (user_mode(regs))
  | 		return __get_user(*r_val, addr);
  | 
  |         *r_val = *addr;
  | 	return 0;
  | }


Regardless if you change or not,

Reviewed-by: Björn Töpel <bjorn@xxxxxxxxxxxx>