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

[Andy Lutomirski]

On Sun, May 22, 2016 at 12:31 PM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
> On Sun, May 22, 2016 at 1:59 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> 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@xxxxxxxxx> 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@xxxxxxxxx>
>>> ---
>>>  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:
>
> What is so magical about the standard C calling convention (regarm(3)
> in the 32-bit case)?  This just passes them right though to
> __switch_to().
>

I guess it's not highly magical, just a bit different from what I
expected.  I guess it's okay.

--Andy

>>> +
>>> +       jmp     __switch_to
>>
>> leaves prev in eax.  This works, but it might be worth a comment.
>
> Not quite, __switch_to() returns 'last', not 'prev'.  The previous
> task when this is called is not the same task when the thread  wakes
> up.

Right.

I wish switch_to were a normal function instead of a silly macro.

--Andy