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

[H. Peter Anvin]

The real question is if we care that sysret and iter don't match.  On 32 bits the situation is even more complex.

On June 26, 2014 1:00:22 PM PDT, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>On Thu, Jun 26, 2014 at 12:59 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx>
>wrote:
>> On Thu, Jun 26, 2014 at 12:56 PM, Andi Kleen <andi@xxxxxxxxxxxxxx>
>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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