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

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Date: Tue, 30 Jan 2024 07:18:03 +0100
From: Björn Töpel <bjorn@...nel.org>
To: Daniel Borkmann <daniel@...earbox.net>, Pu Lehui <pulehui@...wei.com>,
 Pu Lehui <pulehui@...weicloud.com>, bpf@...r.kernel.org,
 linux-riscv@...ts.infradead.org, netdev@...r.kernel.org
Cc: Alexei Starovoitov <ast@...nel.org>, Andrii Nakryiko
 <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu
 <song@...nel.org>, Yonghong Song <yhs@...com>, John Fastabend
 <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>, Stanislav
 Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, Jiri Olsa
 <jolsa@...nel.org>, Palmer Dabbelt <palmer@...belt.com>, Conor Dooley
 <conor@...nel.org>, Luke Nelson <luke.r.nels@...il.com>, Andrew Jones
 <ajones@...tanamicro.com>
Subject: Re: [PATCH RESEND bpf-next v3 4/6] riscv, bpf: Add necessary Zbb
 instructions

Daniel Borkmann <daniel@...earbox.net> writes:

> On 1/29/24 10:13 AM, Pu Lehui wrote:
>> On 2024/1/28 1:16, Björn Töpel wrote:
>>> Pu Lehui <pulehui@...weicloud.com> writes:
>>>
>>>> From: Pu Lehui <pulehui@...wei.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@...wei.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