On Wed, Mar 18, 2015 at 12:47 PM, Denys Vlasenko <[email protected]> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
