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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
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        "Peter Zijlstra \(Intel\)" <peterz@infradead.org>
Subject: Re: [patch V6 19/37] x86/irq: Convey vector as argument and not in ptregs
In-Reply-To: <CALCETrWjT+A_AAuv=zZ52vJhR2ZADktB3XZnO8n_qu09S0P0vQ@mail.gmail.com>
Date:   Thu, 21 May 2020 15:22:43 +0200
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Andy Lutomirski <luto@kernel.org> writes:
> On Fri, May 15, 2020 at 5:10 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
>  +       .align 8
>> +SYM_CODE_START(irq_entries_start)
>> +    vector=FIRST_EXTERNAL_VECTOR
>> +    .rept (FIRST_SYSTEM_VECTOR - FIRST_EXTERNAL_VECTOR)
>> +       UNWIND_HINT_IRET_REGS
>> +       .byte   0x6a, vector
>> +       jmp     common_interrupt
>> +       .align  8
>> +    vector=vector+1
>> +    .endr
>> +SYM_CODE_END(irq_entries_start)
>
> Having battled code like this in the past (for early exceptions), I
> prefer the variant like:
>
> pos = .;
> .rept blah blah blah
>   .byte whatever
>   jmp whatever
>   . = pos + 8;
>  vector = vector + 1
> .endr
>
> or maybe:
>
> .rept blah blah blah
>   .byte whatever
>   jmp whatever;
>   . = irq_entries_start + 8 * vector;
>   vector = vector + 1
> .endr
>
> The reason is that these variants will fail to assemble if something
> goes wrong and the code expands to more than 8 bytes, whereas using
> .align will cause gas to happily emit 16 bytes and result in
> hard-to-debug mayhem.

Yes. They just make objtool very unhappy:

arch/x86/entry/entry_64.o: warning: objtool: .entry.text+0xfd0: special:
can't find orig instruction

Peter suggested to use:

      .pos = .
      .byte..
      jmp
      .nops (pos + 8) - .

That works ...