Re: [PATCH 3/4] x86: Rewrite switch_to() code

[Andy Lutomirski <luto@amacapital.net>]

cc: Josh Poimboeuf: do you care about the exact stack layout of the
bottom of the stack of an inactive task?

On May 21, 2016 9:05 AM, "Brian Gerst" <brgerst@gmail.com> wrote:
>
> Move the low-level context switch code to an out-of-line asm stub instead of
> using complex inline asm.  This allows constructing a new stack frame for the
> child process to make it seamlessly flow to ret_from_fork without an extra
> test and branch in __switch_to().  It also improves code generation for
> __schedule() by using the C calling convention instead of clobbering all
> registers.

I like the concept a lot.

>
> Signed-off-by: Brian Gerst <brgerst@gmail.com>
> ---
>  arch/x86/entry/entry_32.S          |  38 ++++++++++
>  arch/x86/entry/entry_64.S          |  42 +++++++++++-
>  arch/x86/include/asm/processor.h   |   3 -
>  arch/x86/include/asm/switch_to.h   | 137 ++++++-------------------------------
>  arch/x86/include/asm/thread_info.h |   2 -
>  arch/x86/kernel/asm-offsets.c      |   6 ++
>  arch/x86/kernel/asm-offsets_32.c   |   5 ++
>  arch/x86/kernel/asm-offsets_64.c   |   5 ++
>  arch/x86/kernel/process_32.c       |   8 ++-
>  arch/x86/kernel/process_64.c       |   7 +-
>  arch/x86/kernel/smpboot.c          |   1 -
>  11 files changed, 124 insertions(+), 130 deletions(-)
>
> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> index ee6fea0..05e5340 100644
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -204,6 +204,44 @@
>         POP_GS_EX
>  .endm
>
> +/*
> + * %eax: prev task
> + * %edx: next task
> + */
> +ENTRY(__switch_to_asm)
> +       /*
> +        * Save callee-saved registers
> +        * This must match the order in struct fork_frame
> +        * Frame pointer must be last for get_wchan
> +        */
> +       pushl   %ebx
> +       pushl   %edi
> +       pushl   %esi
> +       pushl   %ebp
> +
> +       /* switch stack */
> +       movl    %esp, TASK_threadsp(%eax)
> +       movl    TASK_threadsp(%edx), %esp
> +
> +#ifdef CONFIG_CC_STACKPROTECTOR
> +       movl    TASK_stack_canary(%edx), %ebx
> +       movl    %ebx, PER_CPU_VAR(stack_canary)+stack_canary_offset
> +#endif
> +
> +       /* restore callee-saved registers */
> +       popl    %ebp
> +       popl    %esi
> +       popl    %edi
> +       popl    %ebx

This is highly, highly magical.  eax and edx are prev and next, and:

> +
> +       jmp     __switch_to

leaves prev in eax.  This works, but it might be worth a comment.

> +END(__switch_to_asm)

>  /*
> + * %rdi: prev task
> + * %rsi: next task
> + */
> +ENTRY(__switch_to_asm)
> +       /*
> +        * Save callee-saved registers
> +        * This must match the order in struct fork_frame
> +        * Frame pointer must be last for get_wchan
> +        */
> +       pushq   %rbx
> +       pushq   %r12
> +       pushq   %r13
> +       pushq   %r14
> +       pushq   %r15
> +       pushq   %rbp
> +
> +       /* switch stack */
> +       movq    %rsp, TASK_threadsp(%rdi)
> +       movq    TASK_threadsp(%rsi), %rsp
> +
> +#ifdef CONFIG_CC_STACKPROTECTOR
> +       movq    TASK_stack_canary(%rsi), %rbx
> +       movq    %rbx, PER_CPU_VAR(irq_stack_union)+stack_canary_offset
> +#endif
> +
> +       /* restore callee-saved registers */
> +       popq    %rbp
> +       popq    %r15
> +       popq    %r14
> +       popq    %r13
> +       popq    %r12
> +       popq    %rbx
> +
> +       jmp     __switch_to

Ditto with the magic here.

> +struct fork_frame {
> +       unsigned long bp;
> +#ifdef CONFIG_X86_64
> +       unsigned long r15;
> +       unsigned long r14;
> +       unsigned long r13;
> +       unsigned long r12;
> +#else
> +       unsigned long si;
> +       unsigned long di;
> +#endif
> +       unsigned long bx;
> +       unsigned long ret_addr;
> +       struct pt_regs regs;
> +};

This, like the old implementation, is very much geared to the current
implementation of fork.  Can you split it up:

struct inactive_task_frame {
    unsigned long bp;
    ...
    unsigned long ret_addr;
};

/* fork works by setting up the child stack so that switch_to will
land at ret_from_fork with sp pointing at pt_regs */
struct fork_frame {
    struct inactive_task_frame switch_frame;
    struct pt_regs regs;
};

Then, if and when someone wants to fork into a different type of
context, they can reuse this.  Also, a future improved unwinder can
use inactive_task_frame directly to kick off its unwind.

--Andy