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" <[email protected]> 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 <[email protected]> > --- > 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
