netdev - Re: [PATCH bpf-next v2 4/4] riscv, bpf: Mixing bpf2bpf and tailcalls

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Message-ID: <878r44mr4g.fsf@all.your.base.are.belong.to.us>
Date: Thu, 01 Feb 2024 11:10:55 +0100
From: Björn Töpel <bjorn@...nel.org>
To: Pu Lehui <pulehui@...weicloud.com>, bpf@...r.kernel.org,
 linux-riscv@...ts.infradead.org, netdev@...r.kernel.org
Cc: Song Liu <song@...nel.org>, 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>, 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 v2 4/4] riscv, bpf: Mixing bpf2bpf and tailcalls

Pu Lehui <pulehui@...weicloud.com> writes:

>>> @@ -252,10 +220,7 @@ static void __build_epilogue(bool is_tail_call, struct rv_jit_context *ctx)
>>>   		emit_ld(RV_REG_S5, store_offset, RV_REG_SP, ctx);
>>>   		store_offset -= 8;
>>>   	}
>>> -	if (seen_reg(RV_REG_S6, ctx)) {
>>> -		emit_ld(RV_REG_S6, store_offset, RV_REG_SP, ctx);
>>> -		store_offset -= 8;
>>> -	}
>>> +	emit_ld(RV_REG_TCC, store_offset, RV_REG_SP, ctx);
>> 
>> Why do you need to restore RV_REG_TCC? We're passing RV_REG_TCC (a6) as
>> an argument at all call-sites, and for tailcalls we're loading from the
>> stack.
>> 
>> Is this to fake the a6 argument for the tail-call? If so, it's better to
>> move it to emit_bpf_tail_call(), instead of letting all programs pay for
>> it.
>
> Yes, we can remove this duplicate load. will do that at next version.

Hmm, no remove, but *move* right? Otherwise a6 can contain gargabe on
entering the tailcall?

Move it before __emit_epilogue() in the tailcall, no?


Björn