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Date:   Wed, 2 Aug 2023 23:07:38 +0900
From:   Masami Hiramatsu (Google) <mhiramat@kernel.org>
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        "Masami Hiramatsu (Google)" <mhiramat@kernel.org>,
        linux-trace-kernel@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        bpf <bpf@vger.kernel.org>, Sven Schnelle <svens@linux.ibm.com>,
        Alexei Starovoitov <ast@kernel.org>,
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        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v4 3/9] bpf/btf: Add a function to search a member of a
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On Tue, 1 Aug 2023 19:22:01 -0700
Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:

> On Tue, Aug 1, 2023 at 5:44 PM Steven Rostedt <rostedt@goodmis.org> wrote:
> >
> > On Tue, 1 Aug 2023 20:40:54 -0400
> > Steven Rostedt <rostedt@goodmis.org> wrote:
> >
> > > Maybe we can add a ftrace_partial_regs(fregs) that returns a
> > > partially filled pt_regs, and the caller that uses this obviously knows
> > > its partial (as it's in the name). But this doesn't quite help out arm64
> > > because unlike x86, struct ftrace_regs does not contain an address
> > > compatibility with pt_regs fields. It would need to do a copy.
> > >
> > >  ftrace_partial_regs(fregs, &regs) ?
> >
> > Well, both would be pointers so you wouldn't need the "&", but it was
> > to stress that it would be copying one to the other.
> >
> >   void ftrace_partial_regs(const struct ftrace_regs *fregs, struct pt_regs regs);
> 
> Copy works, but why did you pick a different layout?

I think it is for minimize the stack consumption. pt_regs on arm64 will
consume 42*u64 = 336 bytes, on the other hand ftrace_regs will use
14*unsigned long = 112 bytes. And most of the registers in pt_regs are not
accessed usually. (as you may know RISC processors usually have many
registers - and x86 will be if we use APX in kernel. So pt_regs is big.)

> Why not to use pt_regs ? if save of flags is slow, just skip that part
> and whatever else that is slow. You don't even need to zero out
> unsaved fields. Just ask the caller to zero out pt_regs before hand.
> Most users have per-cpu pt_regs that is being reused.
> So there will be one zero-out in the beginning and every partial
> save of regs will be fast.
> Then there won't be any need for copy-converter from ftrace_regs to pt_regs.
> Maybe too much churn at this point. copy is fine.

If there is no nested call, yeah, per-cpu pt_regs will work.

Thank you,

-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>