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

To: Benjamin LaHaise <bcrl@xxxxxxxxx>
Subject: [Xen-devel] Re: [patch 28/34] Xen-pv_ops: Xen SMP guest support
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Wed, 14 Mar 2007 08:05:28 -0700
Cc: Zachary Amsden <zach@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, virtualization@xxxxxxxxxxxxxx, Rusty Russell <rusty@xxxxxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, Andi Kleen <ak@xxxxxxx>, Chris Wright <chrisw@xxxxxxxxxxxx>, Andi Kleen <ak@xxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
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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.


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