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[Xen-devel] Re: Performance overhead of paravirt_ops on native identifie

To: "Xin, Xiaohui" <xiaohui.xin@xxxxxxxxx>
Subject: [Xen-devel] Re: Performance overhead of paravirt_ops on native identified
From: "H. Peter Anvin" <hpa@xxxxxxxxx>
Date: Thu, 21 May 2009 20:39:01 -0700
Cc: Nick Piggin <npiggin@xxxxxxx>, Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Chuck Ebbert <cebbert@xxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, "Li, Xin" <xin.li@xxxxxxxxx>, "Nakajima, Jun" <jun.nakajima@xxxxxxxxx>, Ingo Molnar <mingo@xxxxxxx>
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Xin, Xiaohui wrote:
> Remember we have done one experiment with "jump", the result shows seems the 
> overhead is even more than the call.

I didn't, no.  That seems extremely weird to me.

(Unbalancing the call/ret stack is known to suck royally, of course.)

>>>
>>>     
>> Can't those calls be changed to jumps?
>>   
> 
> In this specific instance of this example, yes.  But if you start 
> enabling various spinlock debug options then there'll be code following 
> the call.  It would be hard for the runtime patching machinery to know 
> when it would be safe to do the substitution.
> 

When there is code after the call, it's rather obviously not safe.

        -hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.


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