linux-kernel - Re: [PATCH v2 3/10] KVM: MMU: fix direct sp's access corruptted

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Message-ID: <4C29A49C.8040804@cn.fujitsu.com>
Date:	Tue, 29 Jun 2010 15:45:32 +0800
From:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>
Subject: Re: [PATCH v2 3/10] KVM: MMU: fix direct sp's access corruptted



Avi Kivity wrote:

> 
> Note:
> 
> - modifying walk_addr() to call kvm_mmu_pte_write() is probably not so
> bad.  It's rare that a large pte walk sets the dirty bit, and it's
> probably rare to share those large ptes.  Still, I think the fetch()
> change is better since it's more local.
> 
> - there was once talk that instead of folding pt_access and pte_access
> together into the leaf sp->role.access, each sp level would have its own
> access permissions.  In this case we don't even have to get a new direct
> sp, only change the PT_DIRECTORY_LEVEL spte to add write permissions
> (all direct sp's would be writeable and permissions would be controlled
> at their parent_pte level).  Of course that's a much bigger change than
> this bug fix.
> 

Yeah, i have considered this way, but it will change the shadow page's mapping
way: it control the access at the upper level, but in the current code, we allow
the upper level have the ALL_ACCESS and control the access right at the last level.
It will break many things, such as write-protected...
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