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

* 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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