Re: [PATCH] target/riscv: Fix priority of csr related check in riscv_csrrw_check

[Anup Patel]

On Wed, Aug 3, 2022 at 6:16 PM Weiwei Li <liweiwei@iscas.ac.cn> wrote:
>
> Normally, riscv_csrrw_check is called when executing Zicsr instructions.
> And we can only do access control for existed CSRs. So the priority of
> CSR related check, from highest to lowest, should be as follows:
> 1) check whether Zicsr is supported: raise RISCV_EXCP_ILLEGAL_INST if not
> 2) check whether csr is existed: raise RISCV_EXCP_ILLEGAL_INST if not
> 3) do access control: raise RISCV_EXCP_ILLEGAL_INST or RISCV_EXCP_VIRT_
> INSTRUCTION_FAULT if not allowed
>
> The predicates contain parts of function of both 2) and 3), So they need
> to be placed in the middle of riscv_csrrw_check
>
> Signed-off-by: Weiwei Li <liweiwei@iscas.ac.cn>
> Signed-off-by: Junqiang Wang <wangjunqiang@iscas.ac.cn>
> ---
>  target/riscv/csr.c | 44 +++++++++++++++++++++++++-------------------
>  1 file changed, 25 insertions(+), 19 deletions(-)
>
> diff --git a/target/riscv/csr.c b/target/riscv/csr.c
> index 0fb042b2fd..d81f466c80 100644
> --- a/target/riscv/csr.c
> +++ b/target/riscv/csr.c
> @@ -3270,6 +3270,30 @@ static inline RISCVException 
> riscv_csrrw_check(CPURISCVState *env,
>      /* check privileges and return RISCV_EXCP_ILLEGAL_INST if check fails */
>      int read_only = get_field(csrno, 0xC00) == 3;
>      int csr_min_priv = csr_ops[csrno].min_priv_ver;
> +
> +    /* ensure the CSR extension is enabled. */
> +    if (!cpu->cfg.ext_icsr) {
> +        return RISCV_EXCP_ILLEGAL_INST;
> +    }
> +
> +    if (env->priv_ver < csr_min_priv) {
> +        return RISCV_EXCP_ILLEGAL_INST;

This line breaks nested virtualization because for nested virtualization
to work, the guest hypervisor accessing h<xyz> and vs<xyz> CSRs from
VS-mode should result in a virtual instruction trap not illegal
instruction trap.

Regards,
Anup

> +    }
> +
> +    /* check predicate */
> +    if (!csr_ops[csrno].predicate) {
> +        return RISCV_EXCP_ILLEGAL_INST;
> +    }
> +
> +    if (write_mask && read_only) {
> +        return RISCV_EXCP_ILLEGAL_INST;
> +    }
> +
> +    RISCVException ret = csr_ops[csrno].predicate(env, csrno);
> +    if (ret != RISCV_EXCP_NONE) {
> +        return ret;
> +    }
> +
>  #if !defined(CONFIG_USER_ONLY)
>      int csr_priv, effective_priv = env->priv;
>
> @@ -3290,25 +3314,7 @@ static inline RISCVException 
> riscv_csrrw_check(CPURISCVState *env,
>          return RISCV_EXCP_ILLEGAL_INST;
>      }
>  #endif
> -    if (write_mask && read_only) {
> -        return RISCV_EXCP_ILLEGAL_INST;
> -    }
> -
> -    /* ensure the CSR extension is enabled. */
> -    if (!cpu->cfg.ext_icsr) {
> -        return RISCV_EXCP_ILLEGAL_INST;
> -    }
> -
> -    /* check predicate */
> -    if (!csr_ops[csrno].predicate) {
> -        return RISCV_EXCP_ILLEGAL_INST;
> -    }
> -
> -    if (env->priv_ver < csr_min_priv) {
> -        return RISCV_EXCP_ILLEGAL_INST;
> -    }
> -
> -    return csr_ops[csrno].predicate(env, csrno);
> +    return RISCV_EXCP_NONE;
>  }
>
>  static RISCVException riscv_csrrw_do64(CPURISCVState *env, int csrno,
> --
> 2.17.1
>
>