linux-kernel - Re: [PATCH 04/12] KVM: MMU: log dirty page after marking spte writable

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Date:	Tue, 6 Aug 2013 22:48:29 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	gleb@...hat.com, avi.kivity@...il.com, pbonzini@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 04/12] KVM: MMU: log dirty page after marking spte
 writable

On Tue, Jul 30, 2013 at 09:02:02PM +0800, Xiao Guangrong wrote:
> Make sure we can see the writable spte before the dirt bitmap is visible
> 
> We do this is for kvm_vm_ioctl_get_dirty_log() write-protects the spte based
> on the dirty bitmap, we should ensure the writable spte can be found in rmap
> before the dirty bitmap is visible. Otherwise, we cleared the dirty bitmap and
> failed to write-protect the page
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
> ---
>  arch/x86/kvm/mmu.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)

Can you explain why this is safe, with regard to the rule 
at edde99ce05290e50 ?

"The rule is that all pages are either dirty in the current bitmap,
or write-protected, which is violated here."

Overall, please document what is the required order of operations for
both set_spte and get_dirty_log and why this order is safe (right on top
of the code).

> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 35d4b50..0fe56ad 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -2486,12 +2486,12 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
>  		}
>  	}
>  
> -	if (pte_access & ACC_WRITE_MASK)
> -		mark_page_dirty(vcpu->kvm, gfn);
> -
>  set_pte:
>  	if (mmu_spte_update(sptep, spte))
>  		kvm_flush_remote_tlbs(vcpu->kvm);

Here, there is a modified guest page without dirty log bit set (think
another vcpu writing to the page via this spte).

> +
> +	if (pte_access & ACC_WRITE_MASK)
> +		mark_page_dirty(vcpu->kvm, gfn);
>  done:
>  	return ret;
>  }
> -- 
> 1.8.1.4
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