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

[Daniel Henrique Barboza <dbarboza@ventanamicro.com>]

On 4/20/23 20:48, Alistair Francis wrote:
> 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.

That's all valid points. I believe it's ok to drop this patch to allow
for static CPUs to have more flexibility in the future. We're still have
misa_w.


Thanks,

Daniel

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