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

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Message-Id: <20070216025452.d25e3545.akpm@linux-foundation.org>
Date:	Fri, 16 Feb 2007 02:54:52 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Keir Fraser <keir@...source.com>
Cc:	Christian Limpach <Christian.Limpach@...source.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Chris Wright <chrisw@...s-sol.org>, Andi Kleen <ak@....de>,
	<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.

On Fri, 16 Feb 2007 10:47:11 +0000 Keir Fraser <keir@...source.com> wrote:

> On 16/2/07 10:09, "Andrew Morton" <akpm@...ux-foundation.org> wrote:
> 
> > Are the places where the domU code references machine addresses splattered
> > all over the code?  If not, they can just be wrapped with
> > preempt_disable/preempt_enable?
> 
> The main places where machine addresses are 'visible' are any code that
> holds a pte_t,pmd_t,pud_t,pgd_t. We hide the machine-to-pseudophysical and
> pseudophysical-to-machine translations inside e.g., pte_val() and __pte()
> (i.e., constructors and extractors for page table entries). Obviously the
> users of these macros are open coded all over the place, quite apart from
> the performance cost of sprinkling preempt_{enable,disable} so liberally.

OK, you're screwed.  I agree that the process freezer is the way out of that one.

Ingo said that he's clocked the freezer at a few milliseconds.  But if it's
any higher than that it'll need to get sped up once we convert cpu hotplug
to use it.
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