Re: [PATCH v2 04/12] riscv: add ISA parsing for Zca, Zcf, Zcd and Zcb

[Clément Léger]

On 22/04/2024 11:35, Conor Dooley wrote:
> On Mon, Apr 22, 2024 at 10:53:10AM +0200, Clément Léger wrote:
>> On 19/04/2024 17:51, Conor Dooley wrote:
>>> On Thu, Apr 18, 2024 at 02:42:27PM +0200, Clément Léger wrote:
>>>> The Zc* standard extension for code reduction introduces new extensions.
>>>> This patch adds support for Zca, Zcf, Zcd and Zcb. Zce, Zcmt and Zcmp
>>>> are left out of this patch since they are targeting microcontrollers/
>>>> embedded CPUs instead of application processors.
>>>>
>>>> Signed-off-by: Clément Léger <cleger@xxxxxxxxxxxx>
>>>> ---
>>>>  arch/riscv/include/asm/hwcap.h | 4 ++++
>>>>  arch/riscv/kernel/cpufeature.c | 4 ++++
>>>>  2 files changed, 8 insertions(+)
>>>>
>>>> diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
>>>> index 543e3ea2da0e..b7551bad341b 100644
>>>> --- a/arch/riscv/include/asm/hwcap.h
>>>> +++ b/arch/riscv/include/asm/hwcap.h
>>>> @@ -82,6 +82,10 @@
>>>>  #define RISCV_ISA_EXT_ZACAS		73
>>>>  #define RISCV_ISA_EXT_XANDESPMU		74
>>>>  #define RISCV_ISA_EXT_ZIMOP		75
>>>> +#define RISCV_ISA_EXT_ZCA		76
>>>> +#define RISCV_ISA_EXT_ZCB		77
>>>> +#define RISCV_ISA_EXT_ZCD		78
>>>> +#define RISCV_ISA_EXT_ZCF		79
>>>>  
>>>>  #define RISCV_ISA_EXT_XLINUXENVCFG	127
>>>>  
>>>> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
>>>> index 115ba001f1bc..09dee071274d 100644
>>>> --- a/arch/riscv/kernel/cpufeature.c
>>>> +++ b/arch/riscv/kernel/cpufeature.c
>>>> @@ -261,6 +261,10 @@ const struct riscv_isa_ext_data riscv_isa_ext[] = {
>>>>  	__RISCV_ISA_EXT_DATA(zfa, RISCV_ISA_EXT_ZFA),
>>>>  	__RISCV_ISA_EXT_DATA(zfh, RISCV_ISA_EXT_ZFH),
>>>>  	__RISCV_ISA_EXT_DATA(zfhmin, RISCV_ISA_EXT_ZFHMIN),
>>>> +	__RISCV_ISA_EXT_DATA(zca, RISCV_ISA_EXT_ZCA),
>>>> +	__RISCV_ISA_EXT_DATA(zcb, RISCV_ISA_EXT_ZCB),
>>>> +	__RISCV_ISA_EXT_DATA(zcd, RISCV_ISA_EXT_ZCD),
>>>> +	__RISCV_ISA_EXT_DATA(zcf, RISCV_ISA_EXT_ZCF),
>>>>  	__RISCV_ISA_EXT_DATA(zba, RISCV_ISA_EXT_ZBA),
>>>>  	__RISCV_ISA_EXT_DATA(zbb, RISCV_ISA_EXT_ZBB),
>>>>  	__RISCV_ISA_EXT_DATA(zbc, RISCV_ISA_EXT_ZBC),
>>>
>>> Ye, this looks exactly like what I "feared".
>>
>> Ok but for instance, Qemu actually set Zc* based on C/F/D. So the ISA
>> string containing theses dependencies should actually also be allowed.
>> So should we simply ignore them in the ISA string and always do our own
>> "post-processing" based on C/F/D?
> 
> I'm not familiar with the contents of all of these extensions, but I
> assume the reasoning for splitting them out is that you can implement
> them but not maybe not implement C (or something similar)? If that's the
> case, you cannot always imply.

Yeah, they can be implemented independently so we need to be able to
parse them independently. However, the kernel currently requires C so we
should always have Zca/Zcf/Zcd. But if that changes in the future, then,
that won't be true anymore. Better keep it generic probably

Clément