linux-kernel - Re: [RFC PATCH 2/4] x86/entry/64: move ENTER_IRQ_STACK from interrupt macro to helper function

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Message-ID: <CAMzpN2j4a166dt5M7QHmJE-m4XVBER42nUF-h8Wq-Tssq8Fgog@mail.gmail.com>
Date:   Wed, 14 Feb 2018 13:36:06 -0500
From:   Brian Gerst <brgerst@...il.com>
To:     Dominik Brodowski <linux@...inikbrodowski.net>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [RFC PATCH 2/4] x86/entry/64: move ENTER_IRQ_STACK from interrupt
 macro to helper function

On Wed, Feb 14, 2018 at 1:21 PM, Dominik Brodowski
<linux@...inikbrodowski.net> wrote:
> Moving the switch to IRQ stack from the interrupt macro to the helper
> function requires some trickery: All ENTER_IRQ_STACK really cares about
> is where the "original" stack -- meaning the GP registers etc. -- is
> stored. Therefore, we need to offset the stored RSP value by 8 whenever
> ENTER_IRQ_STACK is called from within a function. In such cases, and
> after switching to the IRQ stack, we need to push the "original" return
> address (i.e. the return address from the call to the interrupt entry
> function) to the IRQ stack.
>
> This trickery allows us to carve another 1k from the text size:
>
>    text    data     bss     dec     hex filename
>   17905       0       0   17905    45f1 entry_64.o-orig
>   16897       0       0   16897    4201 entry_64.o
>
> Signed-off-by: Dominik Brodowski <linux@...inikbrodowski.net>
> ---
>  arch/x86/entry/entry_64.S | 53 +++++++++++++++++++++++++++++++----------------
>  1 file changed, 35 insertions(+), 18 deletions(-)
>
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index de8a0da0d347..3046b12a1acb 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -449,10 +449,18 @@ END(irq_entries_start)
>   *
>   * The invariant is that, if irq_count != -1, then the IRQ stack is in use.
>   */
> -.macro ENTER_IRQ_STACK regs=1 old_rsp
> +.macro ENTER_IRQ_STACK regs=1 old_rsp save_ret=0
>         DEBUG_ENTRY_ASSERT_IRQS_OFF
>         movq    %rsp, \old_rsp
>
> +       .if \save_ret
> +       /*
> +        * If save_ret is set, the original stack contains one additional
> +        * entry -- the return address.
> +        */
> +       addq    $8, \old_rsp
> +       .endif

Combine the mov and add into leaq 8(%rsp), \old_rsp.

--
Brian Gerst