linux-kernel - Re: [PATCH 09/12] KVM: MMU: coalesce zapping page after mmu_sync

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Date:	Thu, 25 Feb 2016 17:10:46 +0800
From:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	Takuya Yoshikawa <yoshikawa_takuya_b1@....ntt.co.jp>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:	mtosatti@...hat.com
Subject: Re: [PATCH 09/12] KVM: MMU: coalesce zapping page after
 mmu_sync_children



On 02/25/2016 04:49 PM, Paolo Bonzini wrote:
>
>
> On 25/02/2016 08:35, Xiao Guangrong wrote:
>>> This may release the mmu_lock before committing the zapping.
>>> Is it safe?  If so, we may want to see the reason in the changelog.
>>
>> It is unsafe indeed, please do not do it.
>
> Can you explain why?  kvm_zap_obsolete_pages does the same.

It's not the same, please see the comment in  kvm_mmu_invalidate_zap_all_pages:
	/*
	 * Notify all vcpus to reload its shadow page table
	 * and flush TLB. Then all vcpus will switch to new
	 * shadow page table with the new mmu_valid_gen.
	 *
	 * Note: we should do this under the protection of
	 * mmu-lock, otherwise, vcpu would purge shadow page
	 * but miss tlb flush.
	 */
	kvm_reload_remote_mmus(kvm);

That means the tlb is flushed before releasing mmu-lock.

A example is in rmap_write_protect(), when KVM creates a shadow page table for
the the guest, it detects no spte pointing to the gfn, so tlb is not flushed so
that guest can freely updates its pte.