linux-kernel - Re: [PATCH v2 6/8] x86/entry: add unwind hint annotations

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Date:   Fri, 30 Jun 2017 08:56:09 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        live-patching@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Slaby <jslaby@...e.cz>, Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mike Galbraith <efault@....de>
Subject: Re: [PATCH v2 6/8] x86/entry: add unwind hint annotations

On Fri, Jun 30, 2017 at 8:55 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> On Fri, Jun 30, 2017 at 08:44:40AM -0700, Andy Lutomirski wrote:
>> > A few comments about the first patch:
>> >
>> >   https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/entry_irq_stack&id=3e2aa2102cc1c5e60d4a8637bff78d0478a55059
>> >
>> > - It uses a '693:' label instead of '.Lirqs_off_\@:'
>>
>> Touché!
>>
>> >
>> > - There's a comment I don't follow:
>> >
>> >     "Anything that can interrupt us here without using IST must be
>> >     *extremely* careful to limit its stack usage."
>> >
>> >   What specifically could interrupt there without using IST?
>>
>> #DB, later on in the series.  I'll update the comment.
>>
>> >
>> > - Since do_softirq_own_stack() is a callable function, I think it still
>> >   needs to save rbp.
>>
>> Whoops.
>>
>> >
>> > - Why change the "jmp error_exit" to "ret" in
>> >   xen_do_hypervisor_callback()?
>>
>> To match the other change I made there.  I removed both.
>
> One more thing I forgot to mention: if you could use r10 instead of r11,
> that would be helpful because it means one less register undwarf needs
> to know about.  (It already deals with r10 because of GCC stack
> realignment).

I'll let you figure that one out :)

(Although I think I agree with hpa: why not let it support all regs?
Or am I missing something  still?)

>
> --
> Josh