linux-kernel - Re: [PATCH v4 06/10] KVM: MMU: fast path of handling guest page fault

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Date:	Wed, 02 May 2012 13:39:51 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>, Avi Kivity <avi@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v4 06/10] KVM: MMU: fast path of handling guest page fault

On 04/29/2012 04:50 PM, Takuya Yoshikawa wrote:

> On Fri, 27 Apr 2012 11:52:13 -0300
> Marcelo Tosatti <mtosatti@...hat.com> wrote:
> 
>> Yes but the objective you are aiming for is to read and write sptes
>> without mmu_lock. That is, i am not talking about this patch. 
>> Please read carefully the two examples i gave (separated by "example)").
> 
> The real objective is not still clear.
> 
> The ~10% improvement reported before was on macro benchmarks during live
> migration.  At least, that optimization was the initial objective.
> 
> But at some point, the objective suddenly changed to "lock-less" without
> understanding what introduced the original improvement.
> 
> Was the problem really mmu_lock contention?
> 


Takuya, i am so tired to argue the advantage of lockless write-protect
and lockless O(1) dirty-log again and again.

> If the path being introduced by this patch is really fast, isn't it
> possible to achieve the same improvement still using mmu_lock?
> 
> 
> Note: During live migration, the fact that the guest gets faulted is
> itself a limitation.  We could easily see noticeable slowdown of a
> program even if it runs only between two GET_DIRTY_LOGs.
> 


Obviously no.

It depends on what the guest is doing, from my autotest test, it very
easily to see that, the huge improvement is on bench-migration not
pure-migration.

> 
>> The rules for code under mmu_lock should be:
>>
>> 1) Spte updates under mmu lock must always be atomic and 
>> with locked instructions.
>> 2) Spte values must be read once, and appropriate action
>> must be taken when writing them back in case their value 
>> has changed (remote TLB flush might be required).
> 
> Although I am not certain about what will be really needed in the
> final form, if this kind of maybe-needed-overhead is going to be
> added little by little, I worry about possible regression.


Well, will you suggest Linus to reject all patches and stop
all discussion for the "possible regression" reason?

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