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From:   Andy Lutomirski <luto@kernel.org>
Date:   Tue, 19 May 2020 13:19:51 -0700
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Subject: Re: [patch V6 19/37] x86/irq: Convey vector as argument and not in ptregs
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
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        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
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        Brian Gerst <brgerst@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
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        Will Deacon <will@kernel.org>,
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        Michael Kelley <mikelley@microsoft.com>,
        Jason Chen CJ <jason.cj.chen@intel.com>,
        Zhao Yakui <yakui.zhao@intel.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>
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On Fri, May 15, 2020 at 5:10 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
>
> Device interrupts which go through do_IRQ() or the spurious interrupt
> handler have their separate entry code on 64 bit for no good reason.
>
> Both 32 and 64 bit transport the vector number through ORIG_[RE]AX in
> pt_regs. Further the vector number is forced to fit into an u8 and is
> complemented and offset by 0x80 so it's in the signed character
> range. Otherwise GAS would expand the pushq to a 5 byte instruction for any
> vector > 0x7F.
>
> Treat the vector number like an error code and hand it to the C function as
> argument. This allows to get rid of the extra entry code in a later step.
>
> Simplify the error code push magic by implementing the pushq imm8 via a
> '.byte 0x6a, vector' sequence so GAS is not able to screw it up. As the
> pushq imm8 is sign extending the resulting error code needs to be truncated
> to 8 bits in C code.


Acked-by: Andy Lutomirski <luto@kernel.org>

although you may be giving me more credit than deserved :)

 +       .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.