linux-kernel - Re: [PATCH 1/5] x86/asm/entry/32: Massage SYSENTER32 fast path to be nearly identical to SYSCALL32

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Date:	Mon, 27 Jul 2015 13:49:51 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Krzysztof A. Sobiecki" <sobkas@...il.com>,
	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 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/5] x86/asm/entry/32: Massage SYSENTER32 fast path to be
 nearly identical to SYSCALL32

On Mon, Jul 27, 2015 at 1:33 PM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
> This change swaps a few instructions in final register restoring/zeroing
> section of SYSENTER fast path, and adds/deletes a few empty lines.
>
> After this, the difference between SYSENTER and SYCALL fast paths
> (after the prologue which saved pt_regs) is very small:
> they differ merely in the choice of register to hold arg6 (EBP or R9)
> and in the value of EDX on exit: SYSENTER ABI doesn't need to preserve it,
> so it is zeroed. SYSCALL preserves it:

Acked-by: Andy Lutomirski <luto@...nel.org>

(I haven't tested it, but this looks Obviously Correct (tm) and
there's no funny stack manipulation in the middle of the changes that
could break things.)

--Andy
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