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

[Björn Töpel]

Daniel Borkmann <daniel at iogearbox.net> writes:

> 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.

What I'm trying to say (and drew as well in the other reply) is that
"riscv_has_extension_likely(RISCV_ISA_EXT_ZBB) when
CONFIG_RISCV_ISA_ZBB=n" should also make the JIT emit Zbb insns. The
platform check should be sufficient.

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

Thanks! Yes, this is mainly a semantic discussion, and it can be further
relaxed later with a follow up -- if applicable.


Björn