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

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Date:   Fri, 30 Jun 2017 08:44:40 -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 6:11 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> On Thu, Jun 29, 2017 at 10:41:44PM -0700, Andy Lutomirski wrote:
>> On Thu, Jun 29, 2017 at 10:05 PM, Andy Lutomirski <luto@...nel.org> wrote:
>> > Hmm.  There's another option that might be considerably nicer, though:
>> > put the IRQ stack at a known (at link time) position *in percpu
>> > space*.  (Presumably it already is -- I haven't checked.)  Then we do:
>> >
>> > .macro ENTER_IRQ_STACK old_rsp
>> >     DEBUG_ENTRY_ASSERT_IRQS_OFF
>> >     movq    %rsp, \old_rsp
>> >     incl    PER_CPU_VAR(irq_count)
>> >
>> >     /*
>> >      * Right now, if we just incremented irq_count to zero, we've
>> >      * claimed the IRQ stack but we haven't switched to it yet.
>> >      * Anything that can interrupt us here without using IST
>> >      * must be *extremely* careful to limit its stack usage.
>> >      */
>> >     jnz .Lpush_old_rsp_\@
>> >     movq    \old_rsp, PER_CPU_VAR(top_word_in_irq_stack)
>> >     movq    PER_CPU_VAR(irq_stack_ptr), %rsp
>> >     .Lpush_old_rsp_\@:
>> >     pushq    \old_rsp
>> > .endm
>> >
>>
>> How about the two commits here (well, soon to be there once gitweb catches up):
>>
>> https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/entry_irq_stack&id=0f56a55bb133cd53ccb78ca51378086296618322
>>
>> If you like them, want to add them to your series?
>
> The second patch looks good to me, thanks.  I can pick up the patches.
>
> 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.