Re: [PATCH] target/riscv: raise an exception when CSRRS/CSRRC writes a read-only CSR

[Richard Henderson]

On 3/7/24 22:40, Yu-Ming Chang via wrote:
> Both CSRRS and CSRRC always read the addressed CSR and cause any read side
> effects regardless of rs1 and rd fields. Note that if rs1 specifies a register
> holding a zero value other than x0, the instruction will still attempt to write
> the unmodified value back to the CSR and will cause any attendant side effects.
>
> So if CSRRS or CSRRC tries to write a read-only CSR with rs1 which specifies
> a register holding a zero value, an illegal instruction exception should be
> raised.
>
> Signed-off-by: Yu-Ming Chang <yumin686@andestech.com>
> ---
>   target/riscv/cpu.h       |  3 +++
>   target/riscv/csr.c       | 18 +++++++++++++++---
>   target/riscv/op_helper.c |  2 +-
>   3 files changed, 19 insertions(+), 4 deletions(-)
>
> diff --git a/target/riscv/cpu.h b/target/riscv/cpu.h
> index 5d291a7092..087ef64889 100644
> --- a/target/riscv/cpu.h
> +++ b/target/riscv/cpu.h
> @@ -710,6 +710,9 @@ void cpu_get_tb_cpu_state(CPURISCVState *env, vaddr *pc,
>   void riscv_cpu_update_mask(CPURISCVState *env);
>   bool riscv_cpu_is_32bit(RISCVCPU *cpu);
>   
> +RISCVException riscv_csrr(CPURISCVState *env, int csrno,
> +                          target_ulong *ret_value,
> +                          target_ulong new_value, target_ulong write_mask);
>   RISCVException riscv_csrrw(CPURISCVState *env, int csrno,
>                              target_ulong *ret_value,
>                              target_ulong new_value, target_ulong write_mask);
> diff --git a/target/riscv/csr.c b/target/riscv/csr.c
> index d4e8ac13b9..3e49cf0df1 100644
> --- a/target/riscv/csr.c
> +++ b/target/riscv/csr.c
> @@ -4306,7 +4306,7 @@ static RISCVException rmw_seed(CPURISCVState *env, int 
> csrno,
>   
>   static inline RISCVException riscv_csrrw_check(CPURISCVState *env,
>                                                  int csrno,
> -                                               bool write_mask)
> +                                               bool write)
>   {
>       /* check privileges and return RISCV_EXCP_ILLEGAL_INST if check fails */
>       bool read_only = get_field(csrno, 0xC00) == 3;
> @@ -4328,7 +4328,7 @@ static inline RISCVException 
> riscv_csrrw_check(CPURISCVState *env,
>       }
>   
>       /* read / write check */
> -    if (write_mask && read_only) {
> +    if (write && read_only) {
>           return RISCV_EXCP_ILLEGAL_INST;
>       }
>   
> @@ -4415,11 +4415,23 @@ static RISCVException riscv_csrrw_do64(CPURISCVState *env, int csrno,
>       return RISCV_EXCP_NONE;
>   }
>   
> +RISCVException riscv_csrr(CPURISCVState *env, int csrno,
> +                           target_ulong *ret_value,
> +                           target_ulong new_value, target_ulong write_mask)

Remove new_value and write_mask arguments, as they don't make sense for read.

> +{
> +    RISCVException ret = riscv_csrrw_check(env, csrno, false);
> +    if (ret != RISCV_EXCP_NONE) {
> +        return ret;
> +    }
> +
> +    return riscv_csrrw_do64(env, csrno, ret_value, new_value, write_mask);

... and pass zeros ...

> -    RISCVException ret = riscv_csrrw(env, csr, &val, 0, 0);
> +    RISCVException ret = riscv_csrr(env, csr, &val, 0, 0);

... like you do from the only caller.


r~