linux-kernel - Re: [PATCH 1/4] x86/thread_info: introduce ->ftrace_int3

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Date:   Sun, 28 Apr 2019 13:51:43 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     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>, 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>,
        Andy Lutomirski <luto@...nel.org>,
        Joerg Roedel <jroedel@...e.de>, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 1/4] x86/thread_info: introduce ->ftrace_int3_stack
 member

On Sun, 28 Apr 2019 10:41:10 -0700
Andy Lutomirski <luto@...capital.net> wrote:


> > Note that at any given point
> > in time, there can be at most four such call insn emulations pending:
> > namely at most one per "process", "irq", "softirq" and "nmi" context.
> >   
> 
> That’s quite an assumption. I think your list should also contain
> exception, exceptions nested inside that exception, and machine
> check, at the very least.  I’m also wondering why irq and softirq are
> treated separately.

4 has usually been the context count we choose. But I guess in theory,
if we get exceptions then I could potentially be more.

As for irq vs softirq, an interrupt can preempt a softirq. Interrupts
are enabled while softirqs are running. When sofirqs run, softirqs are
disabled to prevent nested softirqs. But interrupts are enabled again,
and another interrupt may come in while a softirq is executing.

> 
> All this makes me think that one of the other solutions we came up
> with last time we discussed this might be better.

+100

Perhaps adding another slot into pt_regs that gets used by int3 to
store a slot to emulate a call on return?

-- Steve