Re: [PATCH RESEND v7 12/12] target/riscv: forbid write_misa() for static CPUs

[Alistair Francis]

On Thu, Apr 20, 2023 at 7:22 PM Daniel Henrique Barboza
<dbarboza@ventanamicro.com> wrote:
>
> Static CPUs don't want their extensions changed by user interaction. We
> can prevent it during init by not exposing user facing properties, but
> write_misa() is also capable of disabling/enabling extension during
> runtime.
>
> We have a way of telling whether a CPU is static or not by checking for
> TYPE_RISCV_DYNAMIC_CPU. Use it to make write_misa() a no-op for these
> CPUs.
>
> Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
> ---
>  target/riscv/cpu.c | 5 +++++
>  target/riscv/cpu.h | 2 ++
>  target/riscv/csr.c | 5 +++++
>  3 files changed, 12 insertions(+)
>
> diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
> index 4fa720a39d..3cbcf6d320 100644
> --- a/target/riscv/cpu.c
> +++ b/target/riscv/cpu.c
> @@ -1452,6 +1452,11 @@ static void riscv_cpu_init(Object *obj)
>  #endif /* CONFIG_USER_ONLY */
>  }
>
> +bool riscv_cpu_is_static(RISCVCPU *cpu)
> +{
> +    return object_dynamic_cast(OBJECT(cpu), TYPE_RISCV_DYNAMIC_CPU) == NULL;
> +}
> +
>  typedef struct RISCVCPUMisaExtConfig {
>      const char *name;
>      const char *description;
> diff --git a/target/riscv/cpu.h b/target/riscv/cpu.h
> index 1f39edc687..1913ab9d8d 100644
> --- a/target/riscv/cpu.h
> +++ b/target/riscv/cpu.h
> @@ -587,6 +587,8 @@ G_NORETURN void riscv_raise_exception(CPURISCVState *env,
>  target_ulong riscv_cpu_get_fflags(CPURISCVState *env);
>  void riscv_cpu_set_fflags(CPURISCVState *env, target_ulong);
>
> +bool riscv_cpu_is_static(RISCVCPU *cpu);
> +
>  #include "exec/cpu-all.h"
>
>  FIELD(TB_FLAGS, MEM_IDX, 0, 3)
> diff --git a/target/riscv/csr.c b/target/riscv/csr.c
> index d449da2657..929c5477dd 100644
> --- a/target/riscv/csr.c
> +++ b/target/riscv/csr.c
> @@ -1391,6 +1391,11 @@ static RISCVException write_misa(CPURISCVState *env, 
> int csrno,
>      uint32_t orig_misa_ext = env->misa_ext;
>      Error *local_err = NULL;
>
> +    if (riscv_cpu_is_static(cpu)) {
> +        /* never write MISA for static CPUs */
> +        return RISCV_EXCP_NONE;
> +    }

Do we actually need this? We already check misa_w which would be
disabled. What's the harm in allowing someone to manually enable
misa_w and then change the MISA?

Also, it's possible that static CPUs actually support changing the
MISA value at run time.

Alistair

> +
>      if (!riscv_cpu_cfg(env)->misa_w) {
>          /* drop write to misa */
>          return RISCV_EXCP_NONE;
> --
> 2.40.0
>
>