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

From: Borislav Petkov
Date: Wed Mar 18 2015 - 16:49:20 EST

On Wed, Mar 18, 2015 at 08:47:34PM +0100, Denys Vlasenko 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".

Can we please add those last two lines to the definition of THREAD_INFO
in the header? It is good to have.



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