linux-kernel - Re: [PATCH v3 07/15] KVM: MMU: introduce nulls desc

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Message-ID: <5294111B.5040905@linux.vnet.ibm.com>
Date:	Tue, 26 Nov 2013 11:10:19 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>,
	Avi Kivity <avi.kivity@...il.com>
CC:	Gleb Natapov <gleb@...hat.com>,
	"pbonzini@...hat.com Bonzini" <pbonzini@...hat.com>,
	linux-kernel@...r.kernel.org, kvm <kvm@...r.kernel.org>,
	Eric Dumazet <dada1@...mosbay.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 07/15] KVM: MMU: introduce nulls desc

On 11/25/2013 10:23 PM, Marcelo Tosatti wrote:
> On Mon, Nov 25, 2013 at 02:48:37PM +0200, Avi Kivity wrote:
>> On Mon, Nov 25, 2013 at 8:11 AM, Xiao Guangrong
>> <xiaoguangrong@...ux.vnet.ibm.com> wrote:
>>>
>>> On Nov 23, 2013, at 3:14 AM, Marcelo Tosatti <mtosatti@...hat.com> wrote:
>>
>> <snip complicated stuff about parent_pte>
>>
>> I'm not really following, but note that parent_pte predates EPT (and
>> the use of rcu in kvm), so all the complexity that is the result of
>> trying to pack as many list entries into a cache line can be dropped.
>> Most setups now would have exactly one list entry, which is handled
>> specially antyway.
>>
>> Alternatively, the trick of storing multiple entries in one list entry
>> can be moved to generic code, it may be useful to others.
> 
> Yes, can the lockless list walking code be transformed into generic
> single-linked list walking? So the correctness can be verified
> independently, and KVM becomes a simple user of that interface.

I'am afraid the signle-entry list is not so good as we expected. In my
experience, there're too many entries on rmap, more than 300 sometimes.
(consider a case that a lib shared by all processes).

> 
> The simpler version is to maintain lockless walk on depth-1 rmap entries
> (and grab the lock once depth-2 entry is found).

I still think rmap-lockless is more graceful: soft mmu can get benefit
from it also it is promising to be used in some mmu-notify functions. :)


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