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Date: Wed, 14 Feb 2018 20:06:29 +0100
From: Dominik Brodowski <linux@...inikbrodowski.net>
To: Brian Gerst <brgerst@...il.com>
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 3/4] x86/entry/64: move switch_to_thread_stack to
interrupt helper function
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@...inikbrodowski.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@...inikbrodowski.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
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