On 03/18/2015 09:54 PM, Andy Lutomirski wrote: > On Wed, Mar 18, 2015 at 12:47 PM, Denys Vlasenko <dvlas...@redhat.com> 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. 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. Second (why do this *after* this patch set, not in it?): the removal of KERNEL_STACK_OFFSET has no strong reason behind it unless we do MOV -> PUSH conversion - in which case it is mandatory: we must have RSP start at top of stack in 64-bit mode. I want to post a minimal patch set which results in PUSH conversion. Want to avoid polluting it with tangential goal of removing duplicated percpu variable. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
