linux-kernel - Re: [PATCH 3/4] x86: save user rsp in pt

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Date:	Tue, 10 Mar 2015 13:51:51 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] x86: save user rsp in pt_regs->sp on SYSCALL64
 fastpath


* Denys Vlasenko <dvlasenk@...hat.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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