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

Samuel Holland
2024-04-18T22:00:59Z

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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