Messages in this thread | | | Date | Tue, 10 Mar 2015 13:51:51 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH 3/4] x86: save user rsp in pt_regs->sp on SYSCALL64 fastpath |
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* 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, so combined it's a wash performance-wise, right?
Thanks,
Ingo
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