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

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Message-ID: <20130522063413.GL14287@redhat.com>
Date:	Wed, 22 May 2013 09:34:13 +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, 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 10:33:30PM -0300, Marcelo Tosatti wrote:
> 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.

So, if I understand correctly, you are asking to move is_obsolete_sp()
check from kvm_mmu_get_page() and kvm_reload_remote_mmus() from
kvm_mmu_invalidate_all_pages() to a separate patch. Fine by me, but if
we drop kvm_reload_remote_mmus() from kvm_mmu_invalidate_all_pages() the
call to kvm_mmu_invalidate_all_pages() in emulator_fix_hypercall() will
become nop. But I question the need to zap all shadow pages tables there
in the first place, why kvm_flush_remote_tlbs() is not enough?

--
			Gleb.
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