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

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Date:	Tue, 19 Mar 2013 19:37:37 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	Gleb Natapov <gleb@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 6/6] KVM: MMU: fast zap all shadow pages

On Tue, Mar 19, 2013 at 11:37:38PM +0800, Xiao Guangrong wrote:
> On 03/19/2013 10:40 PM, Marcelo Tosatti wrote:
> 
> > 
> > I misunderstood the benefit of your idea (now i got it: zap root
> > and flush TLB guarantees vcpus will refault). What i'd like to avoid is
> > 
> > memset(cache, 0, sizeof(*cache));
> > kvm_mmu_init(kvm);
> > 
> > I'd prefer normal operations on those data structures (in mmu_cache).
> > And also the page accounting is a problem.
> > 
> > Perhaps you can use a generation number to consider whether shadow pages
> > are valid? So: 
> > 
> > find_sp(gfn_t gfn)
> > lookup hash
> > if sp->generation_number != mmu->current_generation_number
> > 	initialize page as if it were just allocated (but keep it in the hash list)
> > 
> > And on kvm_mmu_zap_all()
> > spin_lock(mmu_lock)
> > for each page
> > if page->root_count
> > 	zero sp->spt[]
> > 
> > flush TLB
> > mmu->current_generation_number++
> > spin_unlock(mmu_lock)
> > 
> > Then have kvm_mmu_free_all() that actually frees all data.
> > 
> > Hum, not sure if thats any better than your current patchset.
> 
> I also got the idea of generation number like yours which i mentioned
> it in the [PATCH 0/6]:
> 
> * TODO
> Avoiding Marcelo beat me :), they are some works not attached to make the
> patchset more smaller:
> (1): batch kvm_reload_remote_mmus for zapping root shadow pages
> (2): free shadow pages by using generation-number
> (3): remove unneeded kvm_reload_remote_mmus after kvm_mmu_zap_all
> (4): drop unnecessary @npages from kvm_arch_create_memslot
> (5): rename init_kvm_mmu to init_vcpu_mmu

Long patch sets are OK. Problem are unrelated changes in a patchset.

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