linux-kernel - Re: [PATCH v4 4/6] KVM: MMU: fast invalid all shadow pages

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Message-ID: <20130508104100.GU12349@redhat.com>
Date:	Wed, 8 May 2013 13:41:00 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	avi.kivity@...il.com, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, takuya.yoshikawa@...il.com
Subject: Re: [PATCH v4 4/6] KVM: MMU: fast invalid all shadow pages

On Tue, May 07, 2013 at 12:09:29PM -0300, Marcelo Tosatti wrote:
> On Tue, May 07, 2013 at 05:56:08PM +0300, Gleb Natapov wrote:
> > > > Yes, I am missing what Marcelo means there too. We cannot free memslot
> > > > until we unmap its rmap one way or the other.
> > > 
> > > I do not understand what are you optimizing for, given the four possible
> > > cases we discussed at
> > > 
> > > https://lkml.org/lkml/2013/4/18/280
> > > 
> > We are optimizing mmu_lock holding time for all of those cases.
> > 
> > But you cannot just "zap roots + sp gen number increase." on slot
> > deletion because you need to transfer access/dirty information from rmap
> > that is going to be deleted to actual page before
> > kvm_set_memory_region() returns to a caller.
> > 
> > > That is, why a simple for_each_all_shadow_page(zap_page) is not sufficient.
> > With a lock break? It is. We tried to optimize that by zapping only pages
> > that reference memslot that is going to be deleted and zap all other
> > later when recycling old sps, but if you think this is premature
> > optimization I am fine with it.
> 
> If it can be shown that its not premature optimization, I am fine with
> it.
> 
> AFAICS all cases are 1) rare and 2) not latency sensitive (as in there
> is no requirement for those cases to finish in a short period of time).
OK, lets start from a simple version. The one that goes through rmap
turned out to be more complicated that we expected.

--
			Gleb.
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