linux-kernel - RE: [PATCH v2] riscv, bpf: Optimize zextw insn with Zba extension

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Message-ID: <DM8PR11MB575179A3EB8D056B3EEECA74B8E32@DM8PR11MB5751.namprd11.prod.outlook.com>
Date: Tue, 14 May 2024 07:36:04 +0000
From: "Wang, Xiao W" <xiao.w.wang@...el.com>
To: Andrew Jones <ajones@...tanamicro.com>
CC: "paul.walmsley@...ive.com" <paul.walmsley@...ive.com>,
	"palmer@...belt.com" <palmer@...belt.com>, "aou@...s.berkeley.edu"
	<aou@...s.berkeley.edu>, "luke.r.nels@...il.com" <luke.r.nels@...il.com>,
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	"conor@...nel.org" <conor@...nel.org>, Ben Dooks <ben.dooks@...ethink.co.uk>
Subject: RE: [PATCH v2] riscv, bpf: Optimize zextw insn with Zba extension



> -----Original Message-----
> From: Andrew Jones <ajones@...tanamicro.com>
> Sent: Tuesday, May 14, 2024 12:53 AM
> To: Wang, Xiao W <xiao.w.wang@...el.com>
> Cc: paul.walmsley@...ive.com; palmer@...belt.com;
> aou@...s.berkeley.edu; luke.r.nels@...il.com; xi.wang@...il.com;
> bjorn@...nel.org; ast@...nel.org; daniel@...earbox.net; andrii@...nel.org;
> martin.lau@...ux.dev; eddyz87@...il.com; song@...nel.org;
> yonghong.song@...ux.dev; john.fastabend@...il.com; kpsingh@...nel.org;
> sdf@...gle.com; haoluo@...gle.com; jolsa@...nel.org; linux-
> riscv@...ts.infradead.org; linux-kernel@...r.kernel.org; bpf@...r.kernel.org;
> pulehui@...wei.com; Li, Haicheng <haicheng.li@...el.com>;
> conor@...nel.org
> Subject: Re: [PATCH v2] riscv, bpf: Optimize zextw insn with Zba extension
> 
> On Sat, May 11, 2024 at 10:34:36AM GMT, Xiao Wang wrote:
> > The Zba extension provides add.uw insn which can be used to implement
> > zext.w with rs2 set as ZERO.
> >
> > Signed-off-by: Xiao Wang <xiao.w.wang@...el.com>
> > ---
> > v2:
> > * Add Zba description in the Kconfig. (Lehui)
> > * Reword the Kconfig help message to make it clearer. (Conor)
> > ---
> >  arch/riscv/Kconfig       | 22 ++++++++++++++++++++++
> >  arch/riscv/net/bpf_jit.h | 18 ++++++++++++++++++
> >  2 files changed, 40 insertions(+)
> >
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index 6bec1bce6586..e262a8668b41 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -586,6 +586,14 @@ config RISCV_ISA_V_PREEMPTIVE
> >  	  preemption. Enabling this config will result in higher memory
> >  	  consumption due to the allocation of per-task's kernel Vector
> context.
> >
> > +config TOOLCHAIN_HAS_ZBA
> > +	bool
> > +	default y
> > +	depends on !64BIT || $(cc-option,-mabi=lp64 -march=rv64ima_zba)
> > +	depends on !32BIT || $(cc-option,-mabi=ilp32 -march=rv32ima_zba)
> > +	depends on LLD_VERSION >= 150000 || LD_VERSION >= 23900
> > +	depends on AS_HAS_OPTION_ARCH
> > +
> >  config TOOLCHAIN_HAS_ZBB
> >  	bool
> >  	default y
> > @@ -601,6 +609,20 @@ config TOOLCHAIN_HAS_VECTOR_CRYPTO
> >  	def_bool $(as-instr, .option arch$(comma) +v$(comma) +zvkb)
> >  	depends on AS_HAS_OPTION_ARCH
> >
> > +config RISCV_ISA_ZBA
> > +	bool "Zba extension support for bit manipulation instructions"
> > +	depends on TOOLCHAIN_HAS_ZBA
> 
> We handcraft the instruction, so why do we need toolchain support?

Good point, we don't need toolchain support for this bpf jit case.

> 
> > +	depends on RISCV_ALTERNATIVE
> 
> Also, while riscv_has_extension_likely() will be accelerated with
> RISCV_ALTERNATIVE, it's not required.

Agree, it's not required. For this bpf jit case, we should drop these two dependencies.

BTW, Zbb is used in bpf jit, the usage there also doesn't depend on toolchain and
RISCV_ALTERNATIVE, but the Kconfig for RISCV_ISA_ZBB has forced the dependencies
due to Zbb assembly programming elsewhere.
Maybe we could just dynamically check the existence of RISCV_ISA_ZB* before jit code
emission? or introduce new config options for bpf jit? I prefer the first method and
welcome any comments.

Thanks,
Xiao

[...]
> >  {
> > +	if (rvzba_enabled()) {
> > +		emit(rvzba_zextw(rd, rs), ctx);
> > +		return;
> > +	}
> > +
> >  	emit_slli(rd, rs, 32, ctx);
> >  	emit_srli(rd, rd, 32, ctx);
> >  }
> > --
> > 2.25.1
> >
> 
> Thanks,
> drew