On 03/10/2015 01:51 PM, Ingo Molnar wrote:
>
> * Denys Vlasenko <[email protected]> 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.
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