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

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Date:	Tue, 10 Mar 2015 14:21:48 +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:

> > 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. [...]

So I assumed that it was an equivalent transformation, given that none 
of the changelogs spelled out the increase in overhead ...

> [...] 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.

Hm, but that's the wrong direction, we should try to make it faster, 
and to clean it up - but making it slower without really good reasons 
isn't good.

Is 'usersp' really that much of a complication?

Thanks,

	Ingo
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