linux-kernel - Re: [patch V4 part 5 13/31] x86/irq: Convey vector as argument and not in ptregs

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Date:   Sun, 10 May 2020 10:44:52 +0800
From:   Lai Jiangshan <jiangshanlai+lkml@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, 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>
Subject: Re: [patch V4 part 5 13/31] x86/irq: Convey vector as argument and
 not in ptregs

On Tue, May 5, 2020 at 10:23 PM Thomas Gleixner <tglx@...utronix.de> wrote:

> +/*
> + * ASM code to emit the common vector entry stubs where each stub is
> + * packed into 8 bytes.
> + *
> + * Note, that the 'pushq imm8' is emitted via '.byte 0x6a, vector' because
> + * GCC treats the local vector variable as unsigned int and would expand
> + * all vectors above 0x7F to a 5 byte push. The original code did an
> + * adjustment of the vector number to be in the signed byte range to avoid
> + * this. While clever it's mindboggling counterintuitive and requires the
> + * odd conversion back to a real vector number in the C entry points. Using
> + * .byte achieves the same thing and the only fixup needed in the C entry
> + * point is to mask off the bits above bit 7 because the push is sign
> + * extending.
> + */
> +       .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)

Hello, tglx

Using ".byte   0x6a, vector" is somewhat ugly.

I hope it should be " pushq $(s8_to_s64(vector))", which can also
help to reduce bunches of comments about ".byte   0x6a, vector".

However, I don't know how to implement s8_to_s64() here. But at
least the following code works (generates the same two-byte machine
code as ".byte   0x6a, vector" does):

        .if vector < 128
        pushq $(vector)
        .else
        pushq $(0xffffffffffffff00+vector)
        .endif

Thanks,
Lai