Re: [PATCH 3/4] x86: save user rsp in pt_regs->sp on SYSCALL64 fastpath

[Andy Lutomirski <luto@amacapital.net>]

On Tue, Mar 10, 2015 at 6:18 AM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
> On 03/10/2015 01:51 PM, Ingo Molnar wrote:
>>
>> * Denys Vlasenko <dvlasenk@redhat.com> wrote:
>>
>>> PER_CPU(old_rsp) usage is simplified - now it is used only
>>> as temp storage, and userspace stack pointer is immediately stored
>>> in pt_regs->sp on syscall entry, instead of being used later,
>>> on syscall exit.
>>>
>>> Instead of PER_CPU(old_rsp) and task->thread.usersp, C code
>>> uses pt_regs->sp now.
>>>
>>> FIXUP/RESTORE_TOP_OF_STACK are simplified.
>>
>> Just trying to judge the performance impact:
>>
>>> --- a/arch/x86/kernel/entry_64.S
>>> +++ b/arch/x86/kernel/entry_64.S
>>> @@ -128,8 +128,6 @@ ENDPROC(native_usergs_sysret64)
>>>   * manipulation.
>>>   */
>>>      .macro FIXUP_TOP_OF_STACK tmp offset=0
>>> -    movq PER_CPU_VAR(old_rsp),\tmp
>>> -    movq \tmp,RSP+\offset(%rsp)
>>>      movq $__USER_DS,SS+\offset(%rsp)
>>>      movq $__USER_CS,CS+\offset(%rsp)
>>>      movq RIP+\offset(%rsp),\tmp  /* get rip */
>>> @@ -139,8 +137,7 @@ ENDPROC(native_usergs_sysret64)
>>>      .endm
>>>
>>>      .macro RESTORE_TOP_OF_STACK tmp offset=0
>>> -    movq RSP+\offset(%rsp),\tmp
>>> -    movq \tmp,PER_CPU_VAR(old_rsp)
>>> +    /* nothing to do */
>>>      .endm
>>>
>>>  /*
>>> @@ -253,11 +247,13 @@ GLOBAL(system_call_after_swapgs)
>>>       */
>>>      ENABLE_INTERRUPTS(CLBR_NONE)
>>>      ALLOC_PT_GPREGS_ON_STACK 8              /* +8: space for orig_ax */
>>> +    movq    %rcx,RIP(%rsp)
>>> +    movq    PER_CPU_VAR(old_rsp),%rcx
>>> +    movq    %r11,EFLAGS(%rsp)
>>> +    movq    %rcx,RSP(%rsp)
>>> +    movq_cfi rax,ORIG_RAX
>>>      SAVE_C_REGS_EXCEPT_RAX_RCX_R11
>>>      movq    $-ENOSYS,RAX(%rsp)
>>> -    movq_cfi rax,ORIG_RAX
>>> -    movq    %r11,EFLAGS(%rsp)
>>> -    movq    %rcx,RIP(%rsp)
>>>      CFI_REL_OFFSET rip,RIP
>>>      testl $_TIF_WORK_SYSCALL_ENTRY,TI_flags+THREAD_INFO(%rsp,RIP)
>>>      jnz tracesys
>>
>> So there are now +2 instructions (5 instead of 3) in the system_call
>> path, but there are -2 instructions in the SYSRETQ path,
>
> Unfortunately, no. There is only this change in SYSRETQ path,
> which simply changes where we get RSP from:
>
> @@ -293,7 +289,7 @@ ret_from_sys_call:
>         CFI_REGISTER    rip,rcx
>         movq    EFLAGS(%rsp),%r11
>         /*CFI_REGISTER  rflags,r11*/
> -       movq    PER_CPU_VAR(old_rsp), %rsp
> +       movq    RSP(%rsp),%rsp
>         /*
>          * 64bit SYSRET restores rip from rcx,
>          * rflags from r11 (but RF and VM bits are forced to 0),
>
> Most likely, no change in execution speed here.
> At best, it is one cycle faster somewhere in address generation unit
> because for PER_CPU_VAR() address evaluation, GS base is nonzero.
>
>
> Since this patch does add two extra MOVs,
> I did benchmark these patches. They add exactly one cycle
> to system call code path on my Sandy Bridge CPU.
>

Personally, I'm willing to pay that cycle.  It could be a bigger
savings on context switch, and the simplification it enables is pretty
good.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC