linux-kernel - RE: [patch 14/21] Xen-paravirt: Add XEN config options and disableunsupported config options.

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Message-ID: <BBC437B5B077FF46935BA1D8C9A2172A01EBEBF4@exch2.ad.xensource.com>
Date:	Fri, 16 Feb 2007 02:00:39 -0800
From:	"Christian Limpach" <Christian.Limpach@...source.com>
To:	"Jeremy Fitzhardinge" <jeremy@...p.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>
Cc:	"Chris Wright" <chrisw@...s-sol.org>, "Andi Kleen" <ak@....de>,
	"Keir Fraser" <Keir.Fraser@...cam.ac.uk>,
	<xen-devel@...ts.xensource.com>,
	"Ian Pratt" <Ian.Pratt@...source.com>,
	<virtualization@...ts.osdl.org>,
	"Steven Hand" <steven.hand@...cam.ac.uk>,
	<linux-kernel@...r.kernel.org>
Subject: RE: [patch 14/21] Xen-paravirt: Add XEN config options and disableunsupported config options.

Jeremy Fitzhardinge wrote:
> Andrew Morton wrote:
> > On Thu, 15 Feb 2007 22:14:45 -0800 Dan Hecht 
> <dhecht@...are.com> wrote:
> >
> >   
> >>>  config PREEMPT
> >>>  	bool "Preemptible Kernel (Low-Latency Desktop)"
> >>> +	depends on !XEN
> >>>  	help
> >>>  	  This option reduces the latency of the kernel by making
> >>>  	  all kernel code (that is not executing in a critical section)
> >>>
> >>>       
> >
> > Oh, so that's why it doesn't break when CONFIG_PREEMPT=y.  
> In which case
> > that preempt_disable() I spotted is wrong-and-unneeded.
> >
> > Why doesn't Xen work with preemption??
> >   
> 
> I've forgotten the details.  Ian?  Keir?  Steven?  Maybe it 
> can be done.

With CONFIG_PREEMPT, we can have preempted threads reference machine
addresses across save/restore.  After restore, these machine addresses
will be incorrect.

    Christian
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