netdev - Re: [PATCH bpf-next v2 02/18] x86,bpf: add bpf_global

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Message-ID: <CADxym3bT8P796Qqcd0mtJwR09bgs0Bc45GuPkb39ELLsg1MQ_g@mail.gmail.com>
Date: Wed, 16 Jul 2025 22:40:54 +0800
From: Menglong Dong <menglong8.dong@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Menglong Dong <menglong.dong@...ux.dev>, Steven Rostedt <rostedt@...dmis.org>, 
	Jiri Olsa <jolsa@...nel.org>, bpf <bpf@...r.kernel.org>, "H. Peter Anvin" <hpa@...or.com>, 
	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@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	LKML <linux-kernel@...r.kernel.org>, Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 02/18] x86,bpf: add bpf_global_caller for
 global trampoline

On Wed, Jul 16, 2025 at 12:35 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Tue, Jul 15, 2025 at 1:37 AM Menglong Dong <menglong.dong@...ux.dev> wrote:
> >
> >
> > On 7/15/25 10:25, Alexei Starovoitov wrote:
[......]
> >
> > According to my benchmark, it has ~5% overhead to save/restore
> > *5* variants when compared with *0* variant. The save/restore of regs
> > is fast, but it still need 12 insn, which can produce ~6% overhead.
>
> I think it's an ok trade off, because with one global trampoline
> we do not need to call rhashtable lookup before entering bpf prog.
> bpf prog will do it on demand if/when it needs to access arguments.
> This will compensate for a bit of lost performance due to extra save/restore.

I just think of another benefit of defining multiple global trampolines
here, which you may be interested in. In the feature, we can make
the global trampoline supports functions that have 7+ arguments.
If we use _one_ global trampoline, it's not possible, as we can't handle
the arguments in the stack. However, it's possible if we define
different global trampoline for the functions that have different arguments
count, and what we need to do in the feature is do some adjustment
to CALLER_DEFINE().

Wish you are interested in this idea :)

Thanks!
Menglong Dong


>
> PS
> pls don't add your chinatelecom.cn email in cc.
> gmail just cannot deliver there and it's annoying to keep deleting
> it manually in every reply.