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Subject: Re: [PATCH 1/4] x86/thread_info: introduce ->ftrace_int3_stack member
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20190428135143.09d35bb6@oasis.local.home>
Date:   Sun, 28 Apr 2019 11:08:34 -0700
Cc:     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>, 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>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Mimi Zohar <zohar@linux.ibm.com>,
        Juergen Gross <jgross@suse.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nayna Jain <nayna@linux.ibm.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Andy Lutomirski <luto@kernel.org>,
        Joerg Roedel <jroedel@suse.de>, linux-kernel@vger.kernel.org,
        live-patching@vger.kernel.org, linux-kselftest@vger.kernel.org
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References: <20190427100639.15074-1-nstange@suse.de> <20190427100639.15074-2-nstange@suse.de> <F6268634-96D3-4267-B739-6D375FAB26A8@amacapital.net> <20190428135143.09d35bb6@oasis.local.home>
To:     Steven Rostedt <rostedt@goodmis.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Apr 28, 2019, at 10:51 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
>=20
> On Sun, 28 Apr 2019 10:41:10 -0700
> Andy Lutomirski <luto@amacapital.net> wrote:
>=20
>=20
>>> 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.
>>>=20
>>=20
>> That=E2=80=99s quite an assumption. I think your list should also contain=

>> exception, exceptions nested inside that exception, and machine
>> check, at the very least.  I=E2=80=99m also wondering why irq and softirq=
 are
>> treated separately.
>=20
> 4 has usually been the context count we choose. But I guess in theory,
> if we get exceptions then I could potentially be more.
>=20
> 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.
>=20
>>=20
>> All this makes me think that one of the other solutions we came up
>> with last time we discussed this might be better.
>=20
> +100
>=20
> Perhaps adding another slot into pt_regs that gets used by int3 to
> store a slot to emulate a call on return?
>=20
>=20

That=E2=80=99s not totally nuts, although finding pt_regs isn=E2=80=99t enti=
rely trivial.

I still think I prefer an approach where we just emulate the call directly.=