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

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Message-Id: <20120429175004.b54d8c095a60d98c8cdbc942@gmail.com>
Date:	Sun, 29 Apr 2012 17:50:04 +0900
From:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.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 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?

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.

> 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.

Thanks,
	Takuya
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