linux-kernel - Re: [PATCH v6 3/7] KVM: MMU: fast invalidate all pages

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Message-ID: <20130522013330.GA8583@amt.cnet>
Date:	Tue, 21 May 2013 22:33:30 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	avi.kivity@...il.com, pbonzini@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v6 3/7] KVM: MMU: fast invalidate all pages

On Tue, May 21, 2013 at 11:39:03AM +0300, Gleb Natapov wrote:
> > Any pages with stale information will be zapped by kvm_mmu_zap_all().
> > When that happens, page faults will take place which will automatically 
> > use the new generation number.
> > 
> > So still not clear why is this necessary.
> > 
> This is not, strictly speaking, necessary, but it is the sane thing to do.
> You cannot update page's generation number to prevent it from been
> destroyed since after kvm_mmu_zap_all() completes stale ptes in the
> shadow page may point to now deleted memslot. So why build shadow page
> table with a page that is in a process of been destroyed?

OK, can this be introduced separately, in a later patch, with separate
justification, then?

Xiao please have the first patches of the patchset focus on the problem
at hand: fix long mmu_lock hold times.

> Not sure what you mean again. We flush TLB once before entering this function.
> kvm_reload_remote_mmus() does this for us, no?

kvm_reload_remote_mmus() is used as an optimization, its separate from the
problem solution.

> > 
> > What was suggested was... go to phrase which starts with "The only purpose
> > of the generation number should be to".
> > 
> > The comment quoted here does not match that description.
> > 
> The comment describes what code does and in this it is correct.
> 
> You propose to not reload roots right away and do it only when root sp
> is encountered, right? So my question is what's the point? There are,
> obviously, root sps with invalid generation number at this point, so
> reload will happen regardless in kvm_mmu_prepare_zap_page(). So why not
> do it here right away and avoid it in kvm_mmu_prepare_zap_page() for
> invalid and obsolete sps as I proposed in one of my email?

Sure. But Xiao please introduce that TLB collapsing optimization as a
later patch, so we can reason about it in a more organized fashion.

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