From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1752460AbbCJMwA (ORCPT <rfc822;w@1wt.eu>);
	Tue, 10 Mar 2015 08:52:00 -0400
Received: from mail-wg0-f43.google.com ([74.125.82.43]:38392 "EHLO
	mail-wg0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1750848AbbCJMv4 (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Tue, 10 Mar 2015 08:51:56 -0400
Date: Tue, 10 Mar 2015 13:51:51 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/4] x86: save user rsp in pt_regs->sp on SYSCALL64
 fastpath
Message-ID: <20150310125151.GB21686@gmail.com>
References: <1425926364-9526-1-git-send-email-dvlasenk@redhat.com>
 <1425926364-9526-4-git-send-email-dvlasenk@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1425926364-9526-4-git-send-email-dvlasenk@redhat.com>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org


* 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