linux-kernel - Re: [PATCH V2 1/3] riscv: Add Zicbop instruction definitions & cpufeature

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Message-ID: <78afc7ce-c6ce-4edd-b91a-1f8a94ce298a@rivosinc.com>
Date: Thu, 11 Jan 2024 11:49:48 +0100
From: Clément Léger <cleger@...osinc.com>
To: Andrew Jones <ajones@...tanamicro.com>
Cc: guoren@...nel.org, linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org, Guo Ren <guoren@...ux.alibaba.com>,
 paul.walmsley@...ive.com, palmer@...belt.com, panqinglin2020@...as.ac.cn,
 bjorn@...osinc.com, conor.dooley@...rochip.com, leobras@...hat.com,
 peterz@...radead.org, keescook@...omium.org, wuwei2016@...as.ac.cn,
 xiaoguang.xing@...hgo.com, chao.wei@...hgo.com, unicorn_wang@...look.com,
 uwu@...nowy.me, jszhang@...nel.org, wefu@...hat.com, atishp@...shpatra.org
Subject: Re: [PATCH V2 1/3] riscv: Add Zicbop instruction definitions &
 cpufeature



On 11/01/2024 11:45, Andrew Jones wrote:
> On Thu, Jan 11, 2024 at 11:31:32AM +0100, Clément Léger wrote:
>>
>>
>> On 03/01/2024 13:00, Andrew Jones wrote:
>>> On Wed, Jan 03, 2024 at 10:31:37AM +0100, Clément Léger wrote:
>>>>
>>>>
>>>> On 31/12/2023 09:29, guoren@...nel.org wrote:
>>>>> From: Guo Ren <guoren@...ux.alibaba.com>
>>>>>
>>>>> Cache-block prefetch instructions are HINTs to the hardware to
>>>>> indicate that software intends to perform a particular type of
>>>>> memory access in the near future. This patch adds prefetch.i,
>>>>> prefetch.r and prefetch.w instruction definitions by
>>>>> RISCV_ISA_EXT_ZICBOP cpufeature.
>>>>>
>>>>> Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
>>>>> Signed-off-by: Guo Ren <guoren@...nel.org>
>>>>> ---
>>>>>  arch/riscv/Kconfig                | 15 ++++++++
>>>>>  arch/riscv/include/asm/hwcap.h    |  1 +
>>>>>  arch/riscv/include/asm/insn-def.h | 60 +++++++++++++++++++++++++++++++
>>>>>  arch/riscv/kernel/cpufeature.c    |  1 +
>>>>>  4 files changed, 77 insertions(+)
>>>>>
>>>>> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
>>>>> index 24c1799e2ec4..fcbd417d65ea 100644
>>>>> --- a/arch/riscv/Kconfig
>>>>> +++ b/arch/riscv/Kconfig
>>>>> @@ -579,6 +579,21 @@ config RISCV_ISA_ZICBOZ
>>>>>  
>>>>>  	   If you don't know what to do here, say Y.
>>>>>  
>>>>> +config RISCV_ISA_ZICBOP
>>>>> +	bool "Zicbop extension support for cache block prefetch"
>>>>> +	depends on MMU
>>>>> +	depends on RISCV_ALTERNATIVE
>>>>> +	default y
>>>>> +	help
>>>>> +	  Adds support to dynamically detect the presence of the ZICBOP
>>>>> +	  extension (Cache Block Prefetch Operations) and enable its
>>>>> +	  usage.
>>>>> +
>>>>> +	  The Zicbop extension can be used to prefetch cache block for
>>>>> +	  read/write fetch.
>>>>> +
>>>>> +	  If you don't know what to do here, say Y.
>>>>> +
>>>>>  config TOOLCHAIN_HAS_ZIHINTPAUSE
>>>>>  	bool
>>>>>  	default y
>>>>> diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
>>>>> index 06d30526ef3b..77d3b6ee25ab 100644
>>>>> --- a/arch/riscv/include/asm/hwcap.h
>>>>> +++ b/arch/riscv/include/asm/hwcap.h
>>>>> @@ -57,6 +57,7 @@
>>>>>  #define RISCV_ISA_EXT_ZIHPM		42
>>>>>  #define RISCV_ISA_EXT_SMSTATEEN		43
>>>>>  #define RISCV_ISA_EXT_ZICOND		44
>>>>> +#define RISCV_ISA_EXT_ZICBOP		45
>>>>
>>>> Hi Guo,
>>>>
>>>> Since you are adding support for the Zicbop extension, you could
>>>> probably also allow to probe it from userspace using hwprobe. Add a few
>>>> definitions to sys_riscv.c/hwprobe.h and it will be fine.
>>>
>>> To expose to userspace, we should also start parsing the block size,
>>> so it can also be exposed to userspace. Starting to parse the block
>>> size first requires that we decide we need to parse the block size
>>> (see [1]).
>>
>> Hi Andrew, thanks for the thread.
>>
>> I read it (and the other ones that are related to it) and basically, it
>> seems there was a first decision (expose Zicbop block size indivudally)
>> due to the fact the specification did not mentioned anything specific
>> about clock sizes  but then after that, there was a clarification in the
>> spec stating that Zicbop and Zicbom have the same block size so the
>> first decision was questioned again.
>>
>> From a user coherency point of view, I think it would make more sense to
>> expose it individually in hwprobe  so that zicboz, zicbop and zicbom
>> have their "own" block size (even though zicbop and zicbom would use the
>> same one). Moreover, it would allow us for future evolution easily
>> without breaking any userspace later if zicbop and zicbom block size are
>> decoupled.
> 
> I agree and QEMU has already headed down the road of generating
> riscv,cbop-block-size (I guess Conor's ack on [1] was interpreted as
> being sufficient to merge the QEMU bits), so we can add the Linux
> support and test with QEMU now. The work could probably be a separate
> series to this one, though.

Yes, it QEMU had it merged. and agreed, since this requires a bit more
plumbing, it can probably be left out of this series. I could probably
take care of that later.

Thanks,

Clément