On 03/19/2015 04:43 PM, Andy Lutomirski wrote: > On Thu, Mar 19, 2015 at 8:28 AM, Denys Vlasenko <[email protected]> wrote: >> On 03/18/2015 09:54 PM, Andy Lutomirski wrote: >>> 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) >> >> PER_CPU(cpu_current_top_of_stack) exists only in 32 bits. >> Can't use it in 64-bit code. >> >> PER_CPU(cpu_current_top_of_stack) becomes equal >> to PER_CPU(kernel_stack) only after this patch. > > You could have a #define that gives the top of the stack from asm, though. > >> >> I plan to clean up PER_CPU(cpu_current_top_of_stack)/ >> PER_CPU(kernel_stack)/PER_CPU(tss->sp0) mess >> on top of my patches. >> >> First: this seems to be the easier way. > > It's probably easier, but it's harder to review since you're making > two change at once (changing the rsp loaded during syscall and > changing the way that GET_THREAD_INFO works).
I will split off the THREAD_INFO() change. -- 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/
