linux-kernel - Re: [PATCH bpf-next 2/2] bpf,riscv: Implement bpf_addr_space

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Date: Sun, 24 Mar 2024 10:39:43 +0800
From: Pu Lehui <pulehui@...wei.com>
To: Puranjay Mohan <puranjay12@...il.com>
CC: Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann
	<daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau
	<martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>, Song Liu
	<song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>, 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>,
	Björn Töpel <bjorn@...nel.org>, Luke Nelson
	<luke.r.nels@...il.com>, Xi Wang <xi.wang@...il.com>, Paul Walmsley
	<paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, Albert Ou
	<aou@...s.berkeley.edu>, <bpf@...r.kernel.org>,
	<linux-riscv@...ts.infradead.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next 2/2] bpf,riscv: Implement bpf_addr_space_cast
 instruction



On 2024/3/24 0:49, Puranjay Mohan wrote:
> Pu Lehui <pulehui@...wei.com> writes:
> 
>> On 2024/3/23 23:46, Puranjay Mohan wrote:
>>> LLVM generates bpf_addr_space_cast instruction while translating
>> [snip]
>>>    
>>>    /* Convert from ninsns to bytes. */
>>> diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
>>> index f51b832eafb6..3c389e75cb96 100644
>>> --- a/arch/riscv/net/bpf_jit_comp64.c
>>> +++ b/arch/riscv/net/bpf_jit_comp64.c
>>> @@ -1083,6 +1083,16 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
>>>    	/* dst = src */
>>>    	case BPF_ALU | BPF_MOV | BPF_X:
>>>    	case BPF_ALU64 | BPF_MOV | BPF_X:
>>> +		if (BPF_CLASS(insn->code) == BPF_ALU64 && insn->off == BPF_ADDR_SPACE_CAST &&
>>> +		    insn->imm == 1U << 16) {
>>> +			emit_mv(RV_REG_T1, rs, ctx); > +			emit_zextw(RV_REG_T1, RV_REG_T1, ctx);
>> combine mv and zextw will be better
> 
> Do you suggest doing:
> 
> emit_zextw(RV_REG_T1, rs, ctx);
> 
> Will do it in next version.
> 
>>> +			emit_imm(rd, (ctx->user_vm_start >> 32) << 32, ctx);
>>> +			emit(rv_beq(RV_REG_T1, RV_REG_ZERO, 4), ctx);
>>> +			emit_or(RV_REG_T1, rd, RV_REG_T1, ctx);
>>> +			emit_mv(rd, RV_REG_T1, ctx);
>> ditto, but for or and mv
> 
> How would we combine or and mv?
> also, we have a beq above and in one case both or and mv should happen,
> but in other case only mv should happen.
> 
Okay, another branch is that t1 is zero, but not rd.

> Thanks,
> Puranjay