linux-kernel - Re: [PATCH] kvm cleanup: Introduce sibling_pte and do cleanup for reverse map and parent

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Message-ID: <4C57DA5D.9060700@cn.fujitsu.com>
Date:	Tue, 03 Aug 2010 16:59:09 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org
Subject: Re: [PATCH] kvm cleanup: Introduce sibling_pte and do cleanup for
 reverse map and parent_pte

On 08/03/2010 02:51 PM, Avi Kivity wrote:
>  On 08/03/2010 05:30 AM, Lai Jiangshan wrote:
>> This patch is just a big cleanup. it reduces 220 lines of code.
>>
>> It introduces sibling_pte array for tracking identical sptes, so the
>> identical sptes can be linked as a single linked list by their
>> corresponding sibling_pte. A reverse map or a parent_pte points at
>> the head of this single linked list. So we can do cleanup for
>> reverse map and parent_pte VERY LARGELY.
>>
>> BAD:
>>    If most rmap have only one entry or most sp have only one parent,
>>    this patch may use more memory than before.
> 
> That is the case with NPT and EPT.  Each page has exactly one spte
> (except a few vga pages), and each sp has exactly one parent_pte (except
> the root pages).
> 
>> GOOD:
>>    1) Reduce a lot of code, The functions which are in hot path becomes
>>       very very simple and terrifically fast.
>>    2) rmap_next(): O(N) ->  O(1). traveling a ramp: O(N*N) ->  O(N)
> 
> The existing rmap_next() is not O(N), it's O(RMAP_EXT), which is 4.  The
> data structure was chosen over a simple linked list to avoid extra cache
> misses.
> 
>>    3) Remove the ugly interlayer: struct kvm_rmap_desc, struct
>> kvm_pte_chain
> 
> kvm_rmap_desc and kvm_pte_chain are indeed ugly, but they do save a lot
> of memory and cache misses.
> 
>>    4) We don't need to allocate any thing when we change the mappings.
>>       So we can avoid allocation when we have held kvm mmu spin lock.
>>       (this feature is very helpful in future).
>>    5) better readability.
> 
> I agree the new code is more readable.  Unfortunately it uses more
> memory and is likely to be slower.  You add a cache miss for every spte,
> while kvm_rmap_desc amortizes the cache miss among 4 sptes, and special
> cases 1 spte to have no cache misses (or extra memory requirements).
> 

You are right, please omit this patch

thanks, lai.
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