linux-kernel - Re: [PATCH] x86_64,entry: Fix RCX for traced syscalls

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Message-ID: <CALCETrVBKno2skh3zR=1GYuojwPhYPLD=1HFcLMXjStS+0SPJQ@mail.gmail.com>
Date:	Thu, 26 Jun 2014 12:59:01 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] x86_64,entry: Fix RCX for traced syscalls

On Thu, Jun 26, 2014 at 12:56 PM, Andi Kleen <andi@...stfloor.org> wrote:
>> show RCX == RIP even under strace.
>
> If you think it's really worth the extra instruction?

Hard to say.  That extra instruction only happens on slow paths, so I
suspect the slowdown is negligible.  On the other hand, having syscall
show a blatant difference in behavior between traced and untraced
processes seems unfortunate.

>
> It's not wrong, but it's not clear if it's useful.

--Andy
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