Re: [RFC PATCH 3/4] x86/entry/64: move switch_to_thread_stack to interrupt helper function

[Brian Gerst <brgerst@gmail.com>]

On Wed, Feb 14, 2018 at 1:21 PM, Dominik Brodowski
<linux@dominikbrodowski.net> wrote:
> We can also move the SWAPGS and the switch_to_thread_stack to the
> interrupt helper function. As we do not want call depths of two,
> convert switch_to_thread_stack to a macro. However, as entry_64_compat.S
> expects switch_to_thread_stack to be a function, provide a wrapper for
> that, which leads to some code duplication if CONFIG_IA32_EMULATION is
> enabled. Therefore, the size reduction differs slightly:
>
> With CONFIG_IA32_EMULATION enabled (-0.13k):
>    text    data     bss     dec     hex filename
>   16897       0       0   16897    4201 entry_64.o-orig
>   16767       0       0   16767    417f entry_64.o
>
> With CONFIG_IA32_EMULATION disabled (-0.27k):
>    text    data     bss     dec     hex filename
>   16897       0       0   16897    4201 entry_64.o-orig
>   16622       0       0   16622    40ee entry_64.o
>
> Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
> ---
>  arch/x86/entry/entry_64.S | 65 ++++++++++++++++++++++++++---------------------
>  1 file changed, 36 insertions(+), 29 deletions(-)
>
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 3046b12a1acb..b60a3b692ca9 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -536,6 +536,31 @@ END(irq_entries_start)
>         decl    PER_CPU_VAR(irq_count)
>  .endm
>
> +/*
> + * Switch to the thread stack.  This is called with the IRET frame and
> + * orig_ax on the stack.  (That is, RDI..R12 are not on the stack and
> + * space has not been allocated for them.)
> + */
> +.macro DO_SWITCH_TO_THREAD_STACK
> +       pushq   %rdi
> +       /* Need to switch before accessing the thread stack. */
> +       SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi
> +       movq    %rsp, %rdi
> +       movq    PER_CPU_VAR(cpu_current_top_of_stack), %rsp
> +       UNWIND_HINT sp_offset=16 sp_reg=ORC_REG_DI
> +
> +       pushq   7*8(%rdi)               /* regs->ss */
> +       pushq   6*8(%rdi)               /* regs->rsp */
> +       pushq   5*8(%rdi)               /* regs->eflags */
> +       pushq   4*8(%rdi)               /* regs->cs */
> +       pushq   3*8(%rdi)               /* regs->ip */
> +       pushq   2*8(%rdi)               /* regs->orig_ax */
> +       pushq   8(%rdi)                 /* return address */
> +       UNWIND_HINT_FUNC
> +
> +       movq    (%rdi), %rdi
> +.endm
> +
>  /*
>   * Interrupt entry/exit.
>   *
> @@ -543,10 +568,17 @@ END(irq_entries_start)
>   *
>   * Entry runs with interrupts off.
>   */
> +/* 8(%rsp): ~(interrupt number) */
>  ENTRY(interrupt_helper)
>         UNWIND_HINT_FUNC
>         cld
>
> +       testb   $3, CS-ORIG_RAX+8(%rsp)
> +       jz      1f
> +       SWAPGS
> +       DO_SWITCH_TO_THREAD_STACK
> +1:
> +
>         PUSH_AND_CLEAR_REGS save_ret=1
>         ENCODE_FRAME_POINTER 8
>
> @@ -579,12 +611,6 @@ END(interrupt_helper)
>         .macro interrupt func
>         cld
>
> -       testb   $3, CS-ORIG_RAX(%rsp)
> -       jz      1f
> -       SWAPGS
> -       call    switch_to_thread_stack
> -1:
> -
>         call    interrupt_helper
>
>         call    \func   /* rdi points to pt_regs */
> @@ -853,33 +879,14 @@ apicinterrupt IRQ_WORK_VECTOR                     irq_work_interrupt              smp_irq_work_interrupt
>   */
>  #define CPU_TSS_IST(x) PER_CPU_VAR(cpu_tss_rw) + (TSS_ist + ((x) - 1) * 8)
>
> -/*
> - * Switch to the thread stack.  This is called with the IRET frame and
> - * orig_ax on the stack.  (That is, RDI..R12 are not on the stack and
> - * space has not been allocated for them.)
> - */
> +#if defined(CONFIG_IA32_EMULATION)
> +/* entry_64_compat.S::entry_INT80_compat expects this to be an ASM function */
>  ENTRY(switch_to_thread_stack)
>         UNWIND_HINT_FUNC
> -
> -       pushq   %rdi
> -       /* Need to switch before accessing the thread stack. */
> -       SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi
> -       movq    %rsp, %rdi
> -       movq    PER_CPU_VAR(cpu_current_top_of_stack), %rsp
> -       UNWIND_HINT sp_offset=16 sp_reg=ORC_REG_DI
> -
> -       pushq   7*8(%rdi)               /* regs->ss */
> -       pushq   6*8(%rdi)               /* regs->rsp */
> -       pushq   5*8(%rdi)               /* regs->eflags */
> -       pushq   4*8(%rdi)               /* regs->cs */
> -       pushq   3*8(%rdi)               /* regs->ip */
> -       pushq   2*8(%rdi)               /* regs->orig_ax */
> -       pushq   8(%rdi)                 /* return address */
> -       UNWIND_HINT_FUNC
> -
> -       movq    (%rdi), %rdi
> +       DO_SWITCH_TO_THREAD_STACK
>         ret
>  END(switch_to_thread_stack)
> +#endif
>
>  .macro idtentry sym do_sym has_error_code:req paranoid=0 shift_ist=-1
>  ENTRY(\sym)
> --
> 2.16.1
>

Move the macro to calling.h, and inline it into the compat entry.

--
Brian Gerst