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

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Message-ID: <20230914-ought-hypnotize-64cee0e27ed2@wendy>
Date: Thu, 14 Sep 2023 14:02:04 +0100
From: Conor Dooley <conor.dooley@...rochip.com>
To: Conor Dooley <conor@...nel.org>
CC: Pu Lehui <pulehui@...weicloud.com>, <bpf@...r.kernel.org>,
	<linux-riscv@...ts.infradead.org>, <netdev@...r.kernel.org>,
	Björn Töpel <bjorn@...nel.org>, Alexei Starovoitov
	<ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 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>, Luke Nelson <luke.r.nels@...il.com>, Pu
 Lehui <pulehui@...wei.com>
Subject: Re: [PATCH bpf-next 4/6] riscv, bpf: Add necessary Zbb instructions

On Wed, Sep 13, 2023 at 05:23:48PM +0100, Conor Dooley wrote:
> On Wed, Sep 13, 2023 at 11:34:11PM +0800, Pu Lehui wrote:
> > From: Pu Lehui <pulehui@...wei.com>
> > 
> > Add necessary Zbb instructions introduced by [0] to reduce code size and
> > improve performance of RV64 JIT. At the same time, a helper is added to
> > check whether the CPU supports Zbb instructions.
> > 
> > [0] https://github.com/riscv/riscv-bitmanip/releases/download/1.0.0/bitmanip-1.0.0-38-g865e7a7.pdf
> > 
> > Signed-off-by: Pu Lehui <pulehui@...wei.com>
> > ---
> >  arch/riscv/net/bpf_jit.h | 26 ++++++++++++++++++++++++++
> >  1 file changed, 26 insertions(+)
> > 
> > diff --git a/arch/riscv/net/bpf_jit.h b/arch/riscv/net/bpf_jit.h
> > index 8e0ef4d08..7ee59d1f6 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);
> > +}
> 
> I dunno much about bpf, so passing question that may be a bit obvious:
> Is this meant to be a test as to whether the kernel binary is built with
> support for the extension, or whether the underlying platform is capable
> of executing zbb instructions.
> 
> Sorry if that would be obvious to a bpf aficionado, context I have here
> is the later user and the above rvc_enabled() test, which functions
> differently to Zbb and so doesn't really help me.

FTR, I got an off-list reply about this & it is meant to be a check as
to whether the underlying platform supports the extension. The current
test here is insufficient for that.

Thanks,
Conor.

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