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From: Andy Lutomirski <luto@amacapital.net>
Date: Mon, 27 Jul 2015 13:49:51 -0700
Message-ID: <CALCETrW0xCXzNm34tBORChrDnrNC6oB4BDPJLvyYONZgbsP3sw@mail.gmail.com>
Subject: Re: [PATCH 1/5] x86/asm/entry/32: Massage SYSENTER32 fast path to be
 nearly identical to SYSCALL32
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "Krzysztof A. Sobiecki" <sobkas@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>, X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Mon, Jul 27, 2015 at 1:33 PM, Denys Vlasenko <dvlasenk@redhat.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@kernel.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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