linux-kernel - Re: [patch 28/34] Xen-pv_ops: Xen SMP guest support

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Date:	Wed, 14 Mar 2007 08:05:28 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Benjamin LaHaise <bcrl@...ck.org>
CC:	Andi Kleen <ak@....de>, Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, virtualization@...ts.osdl.org,
	xen-devel@...ts.xensource.com, Chris Wright <chrisw@...s-sol.org>,
	Zachary Amsden <zach@...are.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Ingo Molnar <mingo@...hat.com>, Andi Kleen <ak@...e.de>
Subject: Re: [patch 28/34] Xen-pv_ops: Xen SMP guest support

Benjamin LaHaise wrote:
> Are you switching mms in order to avoid all of the overhead associated with 
> flushing an active mm during task exit?  If so, it makes much more sense to 
> do this in the generic helper, as it should be a win on x86 and x86-64 
> where switching to init_mm is relatively low cost given the support for 
> global pages.  I'll run some tests on a couple of systems this evening and 
> post the results.

Under Xen, the pagetables are mapped read-only to the guest, so all
pagetable updates need to go via the hypervisor.  If we switch to
init_mm and unpin (make all the pages RW) before pulling the pagetable
apart, then we can just traverse and modify the pagetable memory as
normal RW memory.  It avoids a heap of traps into the hypervisor.

    J

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