[PATCH RESEND bpf-next v3 4/6] riscv, bpf: Add necessary Zbb instructions

[Pu Lehui]

On 2024/1/29 23:32, Daniel Borkmann wrote:
> On 1/29/24 10:13 AM, Pu Lehui wrote:
>> On 2024/1/28 1:16, Björn Töpel wrote:
>>> Pu Lehui <pulehui at huaweicloud.com> writes:
>>>
>>>> From: Pu Lehui <pulehui at huawei.com>
>>>>
>>>> Add necessary Zbb instructions introduced by [0] to reduce code size 
>>>> and
>>>> improve performance of RV64 JIT. Meanwhile, a runtime deteted helper is
>>>> added to check whether the CPU supports Zbb instructions.
>>>>
>>>> Link: 
>>>> https://github.com/riscv/riscv-bitmanip/releases/download/1.0.0/bitmanip-1.0.0-38-g865e7a7.pdf [0]
>>>> Signed-off-by: Pu Lehui <pulehui at huawei.com>
>>>> ---
>>>>   arch/riscv/net/bpf_jit.h | 32 ++++++++++++++++++++++++++++++++
>>>>   1 file changed, 32 insertions(+)
>>>>
>>>> diff --git a/arch/riscv/net/bpf_jit.h b/arch/riscv/net/bpf_jit.h
>>>> index e30501b46f8f..51f6d214086f 100644
>>>> --- a/arch/riscv/net/bpf_jit.h
>>>> +++ b/arch/riscv/net/bpf_jit.h
>>>> @@ -18,6 +18,11 @@ static inline bool rvc_enabled(void)
>>>>       return IS_ENABLED(CONFIG_RISCV_ISA_C);
>>>>   }
>>>> +static inline bool rvzbb_enabled(void)
>>>> +{
>>>> +    return IS_ENABLED(CONFIG_RISCV_ISA_ZBB) && 
>>>> riscv_has_extension_likely(RISCV_ISA_EXT_ZBB);
>>>
>>> Hmm, I'm thinking about the IS_ENABLED(CONFIG_RISCV_ISA_ZBB) semantics
>>> for a kernel JIT compiler.
>>>
>>> IS_ENABLED(CONFIG_RISCV_ISA_ZBB) affects the kernel compiler flags.
>>> Should it be enough to just have the run-time check? Should a kernel
>>> built w/o Zbb be able to emit Zbb from the JIT?
>>
>> Not enough, because riscv_has_extension_likely(RISCV_ISA_EXT_ZBB) is a 
>> platform capability check, and the other one is a kernel image 
>> capability check. We can pass the check 
>> riscv_has_extension_likely(RISCV_ISA_EXT_ZBB) when 
>> CONFIG_RISCV_ISA_ZBB=n. And my local test prove it.
> 
> So if I understand you correctly, only relying on the 
> riscv_has_extension_likely(RISCV_ISA_EXT_ZBB)
> part would not work - iow, the IS_ENABLED(CONFIG_RISCV_ISA_ZBB) is 
> mandatory here?
> 

Yes, it should be IS_ENABLED(CONFIG_RISCV_ISA_ZBB) && 
riscv_has_extension_likely(RISCV_ISA_EXT_ZBB).

> Thanks,
> Daniel
> 
> P.s.: Given Bjorn's review and tests I took the series into bpf-next 
> now. Thanks everyone!

Thanks Daniel and Björn