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

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Message-ID: <20120502211031.GB12604@amt.cnet>
Date:	Wed, 2 May 2012 18:10:31 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	Takuya Yoshikawa <takuya.yoshikawa@...il.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 Wed, May 02, 2012 at 01:39:51PM +0800, Xiao Guangrong wrote:
> 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.

His point is valid: there is a lack of understanding on the details of
the improvement.

Did you see the pahole output on struct kvm? Apparently mmu_lock is
sharing a cacheline with read-intensive memslots pointer. It would be 
interesting to see what are the effects of cacheline aligning mmu_lock.

> > 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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