linux-kernel - Re: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravirt ops callsites to make them patchable

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Message-ID: <46003BF1.4090601@vmware.com>
Date:	Tue, 20 Mar 2007 11:54:25 -0800
From:	Zachary Amsden <zach@...are.com>
To:	Andi Kleen <ak@....de>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	David Miller <davem@...emloft.net>, mingo@...e.hu,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.osdl.org, xen-devel@...ts.xensource.com,
	chrisw@...s-sol.org, anthony@...emonkey.ws, netdev@...r.kernel.org
Subject: Re: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravirt ops
 callsites to make them patchable

Andi Kleen wrote:
> One thing I was pondering was to replace the expensive popfs with
>
> bt  $IF,(%rsp) 
> jnc 1f 
> sti
> 1: 
>
> But would be mostly a P4 optimization again and I'm not 100% sure it is
> worth it.
>   

Worth it on 32-bit.  On AMD64, probably not.  On Intel 64-bit, maybe, 
but less important than in P4 days.

This could change character completely if used at the tail of a function 
where you now have

sti; 1: ret

Which generates an interrupt holdoff on the ret, an unusual thing to do.

Zach
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