linux-kernel - Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call functions

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Message-ID: <20190506215353.14a8ef78@oasis.local.home>
Date:   Mon, 6 May 2019 21:53:53 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Nicolai Stange <nstange@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Juergen Gross <jgross@...e.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nayna Jain <nayna@...ux.ibm.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Joerg Roedel <jroedel@...e.de>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>, stable <stable@...r.kernel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call
 functions

On Mon, 6 May 2019 18:34:59 -0700
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Mon, May 6, 2019 at 6:04 PM Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> > That iterator does something special for each individual record. All
> > 40,000 of them.  
> 
> .. yes, but the 'int3' only happens for *one* of them at a time.
> 
> Why would it bother with the other 39,999 calls?
> 
> You could easily just look up the record at the int3 time, and just
> use the record. Exactly the same way you use the one-at-a-time ones.
> 
> Instead, you emulate a fake call to a function that *wouldn't* get
> called, which now does the lookup there. That's the part I don't get.
> Why are you emulating something else than what you'd be rewriting?
>

Ah, now I see what you are saying. Yes, I could pass in what it is
suppose to call. But I was trying to use the same code for all the
alternative solutions we were passing around, and this became the
"default" case that would work with any int3_emulate_call
implementation we came up with.

That is, if we call ftrace_regs_caller() for any scenario it should
work. Even if the call was suppose to be a nop, because in that case,
all the ftrace_ops registered in the iterator would refuse to have
their handler be called for that function.

I sent you a single patch, but that was really just a diff of several
applied patches against your unmodified tree. The last patch implements
the ftrace code. And I had it this way because it should work for any
of the implementations.

I could modify it so that it picks what function to call when the int3
is triggered. I think all the solutions we are down to allow that now.
Some of the early ideas had me call one function for all int3s due to
trampolines and such.

Also, I figured just calling ftrace_regs_caller() was simpler then
having that int3 handler do the hash look ups to determine what handler
it needs to call.

-- Steve