linux-kernel - Re: Performance overhead of paravirt

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Message-ID: <4A161E55.4040401@zytor.com>
Date:	Thu, 21 May 2009 20:39:01 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Xin, Xiaohui" <xiaohui.xin@...el.com>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Chuck Ebbert <cebbert@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	"Li, Xin" <xin.li@...el.com>,
	"Nakajima, Jun" <jun.nakajima@...el.com>,
	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Xen-devel <xen-devel@...ts.xensource.com>
Subject: Re: Performance overhead of paravirt_ops on native identified

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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