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

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Date:	Wed, 14 Mar 2007 12:23:43 -0800
From:	Zachary Amsden <zach@...are.com>
To:	Benjamin LaHaise <bcrl@...ck.org>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>, 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>,
	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:
>   
>> +/*
>> + * We aggressively remove defunct pgd from cr3. We execute unmap_vmas()
>> + * *much* faster this way.
>> + */
>>     
>
> 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.
>   

I tried this experiment on both x86 and x86-64 and I recall not seeing 
any effect.  The mmu_gather optimization does most of this already.

Zach
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