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

[Daniel Borkmann]

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?

Thanks,
Daniel

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