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

[Dominik Brodowski <linux@dominikbrodowski.net>]

On Wed, Feb 14, 2018 at 01:57:15PM -0500, Brian Gerst wrote:
> 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.

That certainly sounds possible, but makes the macro more complex: Inlining
means that the offsets need to be reduced by -8. But we need the current
offset for the call from interrupt_helper. So such a change might make the
code less readable.

Thanks,
	Dominik