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From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 29 Apr 2019 12:24:23 -0700
Message-ID: <CALCETrXzVU0Q7u1q=QFPaDr=aojjF5cjbOi9CxxXnp5GqTqsWA@mail.gmail.com>
Subject: Re: [PATCH 3/4] x86/ftrace: make ftrace_int3_handler() not to skip
 fops invocation
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Andrew Lutomirski <luto@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Nicolai Stange <nstange@suse.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jiri Kosina <jikos@kernel.org>,
        Miroslav Benes <mbenes@suse.cz>,
        Petr Mladek <pmladek@suse.com>,
        Joe Lawrence <joe.lawrence@redhat.com>,
        Shuah Khan <shuah@kernel.org>,
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On Mon, Apr 29, 2019 at 12:13 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
>
>
> On Mon, Apr 29, 2019, 12:02 Linus Torvalds <torvalds@linux-foundation.org> wrote:
>>
>>
>>
>> If nmi were to break it, it would be a cpu bug.
>
>
> Side note: we *already* depend on sti shadow working in other parts of the kernel, namely sti->iret.
>

Where?  STI; IRET would be nuts.

Before:

commit 4214a16b02971c60960afd675d03544e109e0d75
Author: Andy Lutomirski <luto@kernel.org>
Date:   Thu Apr 2 17:12:12 2015 -0700

    x86/asm/entry/64/compat: Use SYSRETL to return from compat mode SYSENTER

we did sti; sysxit, but, when we discussed this, I don't recall anyone
speaking up in favor of the safely of the old code.

Not to mention that the crash we'll get if we get an NMI and a
rescheduling interrupt in this path will be very, very hard to debug.