linux-kernel - Re: [PATCH 15/16] x86: entry.S: simplify optimistic SYSRET

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Message-ID: <20150304214055.GB7247@gmail.com>
Date:	Wed, 4 Mar 2015 22:40:55 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	Borislav Petkov <bp@...en8.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 15/16] x86: entry.S: simplify optimistic SYSRET


* Andy Lutomirski <luto@...capital.net> wrote:

> From: Denys Vlasenko <dvlasenk@...hat.com>
> 
> Avoid redundant load of %r11 (it is already loaded a few instructions before).

Note, this comment is incorrect:

> Do not needlessly increment %rsp - we are going to return to 
> userspace via SYSRET, this insn doesn't use stack for return.

The reason we don't need to increment %rsp because in the very next 
instruction we are restoring it, and then do we execute a SYSRET:

So instead of:

	add $0x80, %rsp
	mov 0x18(%rsp), %rsp

we can do a simplified:

	mov 0x98(%rsp), %rsp

to get the same result.

(I've clarified the changelog with this when applying the patch.)

Thanks,

	Ingo
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