From mboxrd@z Thu Jan  1 00:00:00 1970
From: Xiao Guangrong <xiaoguangrong.eric@gmail.com>
Subject: Re: [PATCH v3 4/9] KVM: MMU: introduce SPTE_ALLOW_WRITE bit
Date: Sat, 21 Apr 2012 11:30:55 +0800
Message-ID: <4F9229EF.4010506@gmail.com>
References: <4F911B74.4040305@linux.vnet.ibm.com> <4F911BE7.30206@linux.vnet.ibm.com> <20120420213925.GB13817@amt.cnet>
Mime-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Cc: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>,
	Avi Kivity <avi@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>, KVM <kvm@vger.kernel.org>
To: Marcelo Tosatti <mtosatti@redhat.com>
Return-path: <kvm-owner@vger.kernel.org>
Received: from mail-pz0-f52.google.com ([209.85.210.52]:60797 "EHLO
	mail-pz0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751138Ab2DUDa7 (ORCPT <rfc822;kvm@vger.kernel.org>);
	Fri, 20 Apr 2012 23:30:59 -0400
In-Reply-To: <20120420213925.GB13817@amt.cnet>
Sender: kvm-owner@vger.kernel.org
List-ID: <kvm.vger.kernel.org>

On 04/21/2012 05:39 AM, Marcelo Tosatti wrote:


>> @@ -1177,9 +1178,8 @@ static int kvm_set_pte_rmapp(struct kvm *kvm, unsigned long *rmapp,
>>  			new_spte = *sptep & ~PT64_BASE_ADDR_MASK;
>>  			new_spte |= (u64)new_pfn << PAGE_SHIFT;
>>
>> -			new_spte &= ~PT_WRITABLE_MASK;
>> -			new_spte &= ~SPTE_HOST_WRITEABLE;
>> -			new_spte &= ~shadow_accessed_mask;
>> +			new_spte &= ~(PT_WRITABLE_MASK | SPTE_HOST_WRITEABLE |
>> +				      shadow_accessed_mask | SPTE_ALLOW_WRITE);
> 
> Each bit should have a distinct meaning. Here the host pte is being
> write-protected, which means only the SPTE_HOST_WRITEABLE bit
> should be cleared.


Hmm, it is no problem if SPTE_ALLOW_WRITE is not cleared.

But the meaning of SPTE_ALLOW_WRITE will become strange: we will see a
spte with spte.SPTE_ALLOW_WRITE = 1 (means the spte is writable on host
and guest) and spte.SPTE_HOST_WRITEABLE = 0 (means the spte is read-only
on host).