Re: [PATCH 1/3] x86: get rid of KERNEL_STACK_OFFSET

[Andy Lutomirski]

On Wed, Mar 18, 2015 at 12:47 PM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
> PER_CPU_VAR(kernel_stack) was set up in a way where it points
> five stack slots below the top of stack.
>
> Presumably, it was done to avoid one "sub $5*8,%rsp"
> in syscall/sysenter code paths, where iret frame needs to be
> created by hand.
>
> Ironically, none of them benefits from this optimization,
> since all of them need to allocate additional data on stack
> (struct pt_regs), so they still have to perform subtraction.
>
> This patch eliminates KERNEL_STACK_OFFSET.
>
> PER_CPU_VAR(kernel_stack) now points directly to top of stack.
> pt_regs allocations are adjusted to allocate iret frame as well.
> Hopefully we can merge it later with 32-bit specific
> PER_CPU_VAR(cpu_current_top_of_stack) variable...
>
> Semi-mysterious expressions THREAD_INFO(%rsp,RIP) - "why RIP??"
> are now replaced by more logical THREAD_INFO(%rsp,SIZEOF_PTREGS) -
> "calculate thread_info's address using information that
> rsp is SIZEOF_PTREGS bytes below the stack top".
>
> Net result in generated code is that constants in several insns
> are changed.
>
> This change is necessary for changing struct pt_regs creation
> in SYSCALL64 code path from MOV to PUSH instructions.
>

Would it be reasonable to break this up into two pieces: first, remove
KERNEL_STACK_OFFSET from THREAD_INFO and related macros (i.e. make
them relative to current_top_of_stack instead) and second, remove
KERNEL_STACK_OFFSET?

Arguably the second half would be better accomplished by switching
everything to use sp0 or its x86_32 equivalent and then eliminating
kernel_stack entirely.

--Andy
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