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

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Message-ID: <20150310132845.GA26490@gmail.com>
Date:	Tue, 10 Mar 2015 14:28:45 +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 ...

Also, for future reference, you need to spell out performance costs of 
fast path patches very clearly - and that wasn't done here.

That's a big no-no, please don't do it again ...

Thanks,

	Ingo
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