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

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Date:	Thu, 26 Jun 2014 13:00:22 -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 12:59 PM, Andy Lutomirski <luto@...capital.net> wrote:
> 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.

Also, if anyone ever wants to add some code to switch back from iret
to sysret when sysret will work, this is a prerequisite.  Otherwise
sysret will never match iret.  (I'm not immediately planning on doing
this, but I can imagine workloads (e.g. UML) for which it would be a
big improvement.)

--Andy
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