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

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Message-ID: <7d06aaa0-cafc-4396-b5d9-8b64b6a1e643@rivosinc.com>
Date: Mon, 22 Apr 2024 13:41:25 +0200
From: Clément Léger <cleger@...osinc.com>
To: Conor Dooley <conor.dooley@...rochip.com>
Cc: Conor Dooley <conor@...nel.org>, Jonathan Corbet <corbet@....net>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt
 <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
 Anup Patel <anup@...infault.org>, Shuah Khan <shuah@...nel.org>,
 Atish Patra <atishp@...shpatra.org>, linux-doc@...r.kernel.org,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
 devicetree@...r.kernel.org, kvm@...r.kernel.org,
 kvm-riscv@...ts.infradead.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 04/12] riscv: add ISA parsing for Zca, Zcf, Zcd and Zcb



On 22/04/2024 13:36, Conor Dooley wrote:
> On Mon, Apr 22, 2024 at 01:14:26PM +0200, Clément Léger wrote:
>> 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@...osinc.com>
>>>>>> ---
>>>>>>  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
> 
> No it doesn't!
> There's a Kconfig option that controls whether or not we build with
> compressed instructions.

Acked, missed that. That is then advocating to keep separate Zc* extensions.

> 
>> 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
>