Re: [PATCH v8 06/19] target/riscv: add rv64i CPU

[Andrew Jones]

On Wed, Nov 01, 2023 at 05:41:51PM -0300, Daniel Henrique Barboza wrote:
> We don't have any form of a 'bare bones' CPU. rv64, our default CPUs,
> comes with a lot of defaults. This is fine for most regular uses but
> it's not suitable when more control of what is actually loaded in the
> CPU is required.
> 
> A bare-bones CPU would be annoying to deal with if not by profile
> support, a way to load a multitude of extensions with a single flag.
> Profile support is going to be implemented shortly, so let's add a CPU
> for it.
> 
> The new 'rv64i' CPU will have only RVI loaded. It is inspired in the
> profile specification that dictates, for RVA22U64 [1]:
> 
> "RVA22U64 Mandatory Base
>  RV64I is the mandatory base ISA for RVA22U64"
> 
> And so it seems that RV64I is the mandatory base ISA for all profiles
> listed in [1], making it an ideal CPU to use with profile support.
> 
> rv64i is a CPU of type TYPE_RISCV_BARE_CPU. It has a mix of features
> from pre-existent CPUs:
> 
> - it allows extensions to be enabled, like generic CPUs;
> - it will not inherit extension defaults, like vendor CPUs.
> 
> This is the minimum extension set to boot OpenSBI and buildroot using
> rv64i:
> 
> ./build/qemu-system-riscv64 -nographic -M virt \
>     -cpu rv64i,sv39=true,g=true,c=true,s=true,u=true
> 
> Our minimal riscv,isa in this case will be:
> 
>  # cat /proc/device-tree/cpus/cpu@0/riscv,isa
> rv64imafdc_zicntr_zicsr_zifencei_zihpm_zca_zcd#
> 
> [1] https://github.com/riscv/riscv-profiles/blob/main/profiles.adoc
> 
> Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
> ---
>  target/riscv/cpu-qom.h |  2 ++
>  target/riscv/cpu.c     | 23 +++++++++++++++++++++++
>  2 files changed, 25 insertions(+)
> 
> diff --git a/target/riscv/cpu-qom.h b/target/riscv/cpu-qom.h
> index 7831e86d37..ea9a752280 100644
> --- a/target/riscv/cpu-qom.h
> +++ b/target/riscv/cpu-qom.h
> @@ -25,6 +25,7 @@
>  #define TYPE_RISCV_CPU "riscv-cpu"
>  #define TYPE_RISCV_DYNAMIC_CPU "riscv-dynamic-cpu"
>  #define TYPE_RISCV_VENDOR_CPU "riscv-vendor-cpu"
> +#define TYPE_RISCV_BARE_CPU "riscv-bare-cpu"
>  
>  #define RISCV_CPU_TYPE_SUFFIX "-" TYPE_RISCV_CPU
>  #define RISCV_CPU_TYPE_NAME(name) (name RISCV_CPU_TYPE_SUFFIX)
> @@ -35,6 +36,7 @@
>  #define TYPE_RISCV_CPU_BASE32           RISCV_CPU_TYPE_NAME("rv32")
>  #define TYPE_RISCV_CPU_BASE64           RISCV_CPU_TYPE_NAME("rv64")
>  #define TYPE_RISCV_CPU_BASE128          RISCV_CPU_TYPE_NAME("x-rv128")
> +#define TYPE_RISCV_CPU_RV64I            RISCV_CPU_TYPE_NAME("rv64i")
>  #define TYPE_RISCV_CPU_IBEX             RISCV_CPU_TYPE_NAME("lowrisc-ibex")
>  #define TYPE_RISCV_CPU_SHAKTI_C         RISCV_CPU_TYPE_NAME("shakti-c")
>  #define TYPE_RISCV_CPU_SIFIVE_E31       RISCV_CPU_TYPE_NAME("sifive-e31")
> diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
> index f7c1989d14..4a6e544eaf 100644
> --- a/target/riscv/cpu.c
> +++ b/target/riscv/cpu.c
> @@ -544,6 +544,16 @@ static void rv128_base_cpu_init(Object *obj)
>      set_satp_mode_max_supported(RISCV_CPU(obj), VM_1_10_SV57);
>  #endif
>  }
> +
> +static void rv64i_bare_cpu_init(Object *obj)
> +{
> +    CPURISCVState *env = &RISCV_CPU(obj)->env;
> +    riscv_cpu_set_misa(env, MXL_RV64, RVI);
> +
> +    /* Remove the defaults from the parent class */
> +    RISCV_CPU(obj)->cfg.ext_zicntr = false;
> +    RISCV_CPU(obj)->cfg.ext_zihpm = false;

Good catch since v1, but having to do this is a bit gross. I'd prefer the
parent class not enable any extensions. Each CPU type that needs these
can just set them themselves.

> +}
>  #else
>  static void rv32_base_cpu_init(Object *obj)
>  {
> @@ -1753,6 +1763,13 @@ void riscv_cpu_list(void)
>          .instance_init = initfn              \
>      }
>  
> +#define DEFINE_BARE_CPU(type_name, initfn) \
> +    {                                      \
> +        .name = type_name,                 \
> +        .parent = TYPE_RISCV_BARE_CPU,     \
> +        .instance_init = initfn            \
> +    }
> +
>  static const TypeInfo riscv_cpu_type_infos[] = {
>      {
>          .name = TYPE_RISCV_CPU,
> @@ -1775,6 +1792,11 @@ static const TypeInfo riscv_cpu_type_infos[] = {
>          .parent = TYPE_RISCV_CPU,
>          .abstract = true,
>      },
> +    {
> +        .name = TYPE_RISCV_BARE_CPU,
> +        .parent = TYPE_RISCV_CPU,
> +        .abstract = true,
> +    },
>      DEFINE_DYNAMIC_CPU(TYPE_RISCV_CPU_ANY,      riscv_any_cpu_init),
>      DEFINE_DYNAMIC_CPU(TYPE_RISCV_CPU_MAX,      riscv_max_cpu_init),
>  #if defined(TARGET_RISCV32)
> @@ -1791,6 +1813,7 @@ static const TypeInfo riscv_cpu_type_infos[] = {
>      DEFINE_VENDOR_CPU(TYPE_RISCV_CPU_THEAD_C906,  rv64_thead_c906_cpu_init),
>      DEFINE_VENDOR_CPU(TYPE_RISCV_CPU_VEYRON_V1,   rv64_veyron_v1_cpu_init),
>      DEFINE_DYNAMIC_CPU(TYPE_RISCV_CPU_BASE128,  rv128_base_cpu_init),
> +    DEFINE_BARE_CPU(TYPE_RISCV_CPU_RV64I, rv64i_bare_cpu_init),
>  #endif
>  };
>  
> -- 
> 2.41.0
>

We'll probably need to bring back the satp supported mode setting, as
suggested on a previous patch, but otherwise

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>

Thanks,
drew