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Date: Wed, 4 Mar 2015 22:40:55 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org,
        Borislav Petkov <bp@alien8.de>, Oleg Nesterov <oleg@redhat.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>
Subject: Re: [PATCH 15/16] x86: entry.S: simplify optimistic SYSRET
Message-ID: <20150304214055.GB7247@gmail.com>
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 <1aef69b346a6db0d99cdfb0f5ba83e8c985e27d7.1424989793.git.luto@amacapital.net>
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* Andy Lutomirski <luto@amacapital.net> wrote:

> From: Denys Vlasenko <dvlasenk@redhat.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