Re: [Qemu-devel] [Qemu-arm] [PATCH v3 4/7] target/arm: add "cortex-m0" CPU model

["Philippe Mathieu-Daudé" <f4bug@amsat.org>]

On 07/30/2018 02:51 PM, Peter Maydell wrote:
> On 27 July 2018 at 06:26, Philippe Mathieu-Daudé <f4bug@amsat.org> wrote:
>> Hi Stefan,
>>
>> On 07/25/2018 05:59 AM, Stefan Hajnoczi wrote:
>>> Define a "cortex-m0" ARMv6-M CPU model.
>>>
>>> Most of the register reset values set by other CPU models are not
>>> relevant for the cut-down ARMv6-M architecture.
>>>
>>> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
>>> ---
>>>  target/arm/cpu.c | 11 +++++++++++
>>>  1 file changed, 11 insertions(+)
>>>
>>> diff --git a/target/arm/cpu.c b/target/arm/cpu.c
>>> index 3848ef46aa..7e477c0d23 100644
>>> --- a/target/arm/cpu.c
>>> +++ b/target/arm/cpu.c
>>> @@ -1255,6 +1255,15 @@ static void arm11mpcore_initfn(Object *obj)
>>>      cpu->reset_auxcr = 1;
>>>  }
>>>
>>> +static void cortex_m0_initfn(Object *obj)
>>> +{
>>> +    ARMCPU *cpu = ARM_CPU(obj);
>>> +    set_feature(&cpu->env, ARM_FEATURE_V6);
>>> +    set_feature(&cpu->env, ARM_FEATURE_M);
>>
>> What about ARM_FEATURE_THUMB2 (T32)?
> 
> No, the M0 doesn't have Thumb2. It has Thumb1 plus a tiny set
> of 32-bit instructions (which we deal with specially in translate.c).

Oh I thought it as Thumb2, ok.

> 
> (As it happens we generally can't get to the checks on the THUMB2
> feature for a v6M core, so I think but have not checked thoroughly
> that it would make no difference to QEMU's behaviour whether the
> feature bit was set or not.)
> 
>> Peter: Since the M0 (optionally?) supports 32x32 multiply, should this
>> cpu use the ARM_FEATURE_THUMB_DSP feature? Else this might trigger an
>> 'Undefined Instruction' in disas_thumb2_insn().
> 
> ARM_FEATURE_THUMB_DSP only enables checks in disas_thumb2_insn()
> (and 32x32->32 multiply is not one of the insns it gates).
> The only insns in that function that a v6M core can execute are
> msr, mrs, dsb, dmb, isb and bl, none of which are affected by that
> feature switch.

Yes.

> 
>> And what about optional ARM_FEATURE_PMSA?
>> Oh this would be cortex_m0plus_initfn() for "cortex-m0-plus", ok.
> 
> Yep, plain M0 has no MPU (and so we're postponing the work
> of implementing the v6M PMSA, which IIRC isn't quite the
> same as the v7M one).

Thanks for your detailed explanations :)

Regards,

Phil.