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

[Pu Lehui <pulehui@huawei.com>]

On 2024/1/28 1:16, Björn Töpel wrote:
> Pu Lehui <pulehui@huaweicloud.com> writes:
> 
>> From: Pu Lehui <pulehui@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@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.

> 
> Björn