linux-kernel - Re: [PATCH v3 4/9] KVM: MMU: introduce SPTE_ALLOW

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Message-ID: <4F941FF7.10808@redhat.com>
Date:	Sun, 22 Apr 2012 18:12:55 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
CC:	Xiao Guangrong <xiaoguangrong.eric@...il.com>,
	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v3 4/9] KVM: MMU: introduce SPTE_ALLOW_WRITE bit

On 04/21/2012 07:22 AM, Marcelo Tosatti wrote:
> On Sat, Apr 21, 2012 at 11:30:55AM +0800, Xiao Guangrong wrote:
> > 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).
>  
> You are combining gpte writable bit, and host pte writable bit (which
> are separate and independent of each other) into one bit. 
>
> SPTE_HOST_WRITEABLE already indicates whether the host pte is writable 
> or not.

Maybe we should rename SPTE_ALLOW_WRITE to SPTE_NOT_SHADOWED (or
SPTE_SHADOWED with the opposite meaning).

Alternatively, SPTE_MMU_WRITEABLE (complements SPTE_HOST_WRITEABLE).

-- 
error compiling committee.c: too many arguments to function

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