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

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Date:   Fri, 22 May 2020 14:48:53 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "Paul E. McKenney" <paulmck@...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>,
        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

On 5/21/20 9:22 AM, Thomas Gleixner wrote:
> 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) - .


Unfortunately this (.nops directive) only works for newer assemblers
(2.31, per
https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=blob_plain;f=gas/NEWS;h=9a3f352108e439754688e19e63a6235b38801182;hb=5eb617a71463fa6810cd14f57adfe7a1efc93a96)


I have 2.27 and things don't go well.


-boris


>
> That works ...