linux-kernel - Re: [patch V6 19/37] x86/irq: Convey vector as argument and not in ptregs

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Date:   Thu, 21 May 2020 15:22:43 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Wei Liu <wei.liu@...nel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        Jason Chen CJ <jason.cj.chen@...el.com>,
        Zhao Yakui <yakui.zhao@...el.com>,
        "Peter Zijlstra \(Intel\)" <peterz@...radead.org>
Subject: Re: [patch V6 19/37] x86/irq: Convey vector as argument and not in ptregs

Andy Lutomirski <luto@...nel.org> writes:
> On Fri, May 15, 2020 at 5:10 PM Thomas Gleixner <tglx@...utronix.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 ...