linux-kernel - Re: [PATCH 3/4] KVM: Switch to srcu-less get_dirty

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Message-ID: <4F62C9B4.7060804@linux.vnet.ibm.com>
Date:	Fri, 16 Mar 2012 13:03:48 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>
CC:	avi@...hat.com, mtosatti@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] KVM: Switch to srcu-less get_dirty_log()

On 03/01/2012 06:33 PM, Takuya Yoshikawa wrote:


> +	spin_lock(&kvm->mmu_lock);
> 
> -		r = -ENOMEM;
> -		slots = kmemdup(kvm->memslots, sizeof(*kvm->memslots), GFP_KERNEL);
> -		if (!slots)
> -			goto out;
> +	for (i = 0; i < n / sizeof(long); i++) {
> +		unsigned long mask;
> +		gfn_t offset;
> 
> -		memslot = id_to_memslot(slots, log->slot);
> -		memslot->nr_dirty_pages = 0;
> -		memslot->dirty_bitmap = dirty_bitmap_head;
> -		update_memslots(slots, NULL);
> +		if (!dirty_bitmap[i])
> +			continue;
> 
> -		old_slots = kvm->memslots;
> -		rcu_assign_pointer(kvm->memslots, slots);
> -		synchronize_srcu_expedited(&kvm->srcu);
> -		kfree(old_slots);


For my quickly review, mmu_lock can not protect everything, if the guest page
is written out of the shadow page/ept table, dirty page will be lost.

There is a example:

             CPU A                                   CPU  B
guest page is written by write-emulation

                                                  hold mmu-lock and see dirty-bitmap
                                                  is not be changed, then migration is
                                                  completed.

call mark_page_dirty() to set dirty_bit map


Right?

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