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Date:   Sun, 28 Apr 2019 23:22:54 +0200
From:   Nicolai Stange <>
To:     Steven Rostedt <>
Cc:     Andy Lutomirski <>,
        Nicolai Stange <>,
        Thomas Gleixner <>,
        Ingo Molnar <>, Borislav Petkov <>,
        "H. Peter Anvin" <>,,
        Josh Poimboeuf <>,
        Jiri Kosina <>,
        Miroslav Benes <>,
        Petr Mladek <>,
        Joe Lawrence <>,
        Shuah Khan <>,
        Konrad Rzeszutek Wilk <>,
        Tim Chen <>,
        Sebastian Andrzej Siewior <>,
        Mimi Zohar <>,
        Juergen Gross <>,
        Nick Desaulniers <>,
        Nayna Jain <>,
        Masahiro Yamada <>,
        Andy Lutomirski <>,
        Joerg Roedel <>,,,
Subject: Re: [PATCH 1/4] x86/thread_info: introduce ->ftrace_int3_stack member

Steven Rostedt <> writes:

> On Sun, 28 Apr 2019 10:41:10 -0700
> Andy Lutomirski <> 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.

You're right, I missed the machine check case.

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

After having seen the static_call discussion, I'm in no way defending
this limited approach here, but out of curiosity: can the code between
the push onto the stack from ftrace_int3_handler() and the subsequent
pop from the stub actually trigger an (non-#MC) exception? There's an
iret inbetween, but that can fault only when returning to user space,

> 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.



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