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

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Date:	Tue, 19 Mar 2013 23:37:38 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Marcelo Tosatti <mtosatti@...hat.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 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

> Well, maybe resend patchset with bug fixes / improvements and 
> we go from there.

I agree. Thanks for your time, Marcelo!


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