Re: [RFC 1/2] riscv: process: Introduce idle thread using Zawrs extension

[Samuel Holland <samuel.holland@sifive.com>]

Hi Drew,

On 2024-04-18 2:10 PM, Andrew Jones wrote:
> On Thu, Apr 18, 2024 at 04:05:55PM +0100, Conor Dooley wrote:
>> + Drew,
>>
>> On Thu, Apr 18, 2024 at 07:49:41PM +0800, Xu Lu wrote:
>>> The Zawrs extension introduces a new instruction WRS.NTO, which will
>>> register a reservation set and causes the hart to temporarily stall
>>> execution in a low-power state until a store occurs to the reservation
>>> set or an interrupt is observed.
>>>
>>> This commit implements new version of idle thread for RISC-V via Zawrs
>>> extension.
>>>
>>> Signed-off-by: Xu Lu <luxu.kernel@bytedance.com>
>>> Reviewed-by: Hangjing Li <lihangjing@bytedance.com>
>>> Reviewed-by: Liang Deng <dengliang.1214@bytedance.com>
>>> Reviewed-by: Wen Chai <chaiwen.cc@bytedance.com>
>>> ---
>>>  arch/riscv/Kconfig                 | 24 +++++++++++++++++
>>>  arch/riscv/include/asm/cpuidle.h   | 11 +-------
>>>  arch/riscv/include/asm/hwcap.h     |  1 +
>>>  arch/riscv/include/asm/processor.h | 17 +++++++++++++
>>>  arch/riscv/kernel/cpu.c            |  5 ++++
>>>  arch/riscv/kernel/cpufeature.c     |  1 +
>>>  arch/riscv/kernel/process.c        | 41 +++++++++++++++++++++++++++++-
>>>  7 files changed, 89 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
>>> index be09c8836d56..a0d344e9803f 100644
>>> --- a/arch/riscv/Kconfig
>>> +++ b/arch/riscv/Kconfig
>>> @@ -19,6 +19,7 @@ config RISCV
>>>  	select ARCH_ENABLE_SPLIT_PMD_PTLOCK if PGTABLE_LEVELS > 2
>>>  	select ARCH_ENABLE_THP_MIGRATION if TRANSPARENT_HUGEPAGE
>>>  	select ARCH_HAS_BINFMT_FLAT
>>> +	select ARCH_HAS_CPU_FINALIZE_INIT
>>>  	select ARCH_HAS_CURRENT_STACK_POINTER
>>>  	select ARCH_HAS_DEBUG_VIRTUAL if MMU
>>>  	select ARCH_HAS_DEBUG_VM_PGTABLE
>>> @@ -525,6 +526,20 @@ config RISCV_ISA_SVPBMT
>>>  
>>>  	   If you don't know what to do here, say Y.
>>>  
>>> +config RISCV_ISA_ZAWRS
>>> +	bool "Zawrs extension support for wait-on-reservation-set instructions"
>>> +	depends on RISCV_ALTERNATIVE
>>> +	default y
>>> +	help
>>> +	   Adds support to dynamically detect the presence of the Zawrs
>>> +	   extension and enable its usage.
>>
>> Drew, could you, in your update, use the wording:
>> 	   Add support for enabling optimisations in the kernel when the
>> 	   Zawrs extension is detected at boot.
> 
> How about
> 
>   The Zawrs extension defines a pair of instructions to be used in
>   polling loops which allow a hart to enter a low-power state or to
>   trap to the hypervisor while waiting on a store to a memory location.
>   Enable the use of these instructions when the Zawrs extension is

                                        ^ in the kernel

I believe "in the kernel" was an important part of the clarification that these
Kconfig options do not affect whether userspace can use these instructions.

Regards,
Samuel

>   detected at boot.
> 
> Thanks,
> drew
> 
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