linux-kernel - Re: [PATCH 1/3] KVM: MMU: Fix 32 bit legacy paging with NPT

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Message-ID: <20100902163220.GE1964@amd.com>
Date:	Thu, 2 Sep 2010 18:32:20 +0200
From:	"Roedel, Joerg" <Joerg.Roedel@....com>
To:	Avi Kivity <avi@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] KVM: MMU: Fix 32 bit legacy paging with NPT

On Thu, Sep 02, 2010 at 11:56:10AM -0400, Avi Kivity wrote:
>   On 09/02/2010 06:29 PM, Joerg Roedel wrote:
> > This patch fixes 32 bit legacy paging with NPT enabled. The
> > mmu_check_root call on the top-level of the loop causes
> > root_gfn to take values (in the tdp_enabled path) which are
> > outside of guest memory. So the mmu_check_root call fails at
> > some point in the loop interation causing the guest to
> > tiple-fault.
> > This patch changes the mmu_check_root calls to the places
> > where they are really necessary. As a side-effect it
> > introduces a check for the root of a pae page table too.
> >
> >
> > @@ -2387,6 +2387,10 @@ static int mmu_alloc_roots(struct kvm_vcpu *vcpu)
> >   		return 0;
> >   	}
> >   	direct = !is_paging(vcpu);
> > +
> > +	if (mmu_check_root(vcpu, root_gfn))
> > +		return 1;
> > +
> >   	for (i = 0; i<  4; ++i) {
> >   		hpa_t root = vcpu->arch.mmu.pae_root[i];
> >
> > @@ -2398,10 +2402,10 @@ static int mmu_alloc_roots(struct kvm_vcpu *vcpu)
> >   				continue;
> >   			}
> >   			root_gfn = pdptr>>  PAGE_SHIFT;
> > +			if (mmu_check_root(vcpu, root_gfn))
> > +				return 1;
> >   		} else if (vcpu->arch.mmu.root_level == 0)
> >   			root_gfn = 0;
> > -		if (mmu_check_root(vcpu, root_gfn))
> > -			return 1;
> >   		if (tdp_enabled) {
> >   			direct = 1;
> >   			root_gfn = i<<  30;
> 
> The overloading of root_gfn is pretty bad.  Also, we don't really need 
> to check root_gfn for the direct case (the guest can easily switch cr3 
> later to one that would fail the check).
> 
> However, I'll apply the patch since it fixes the direct problem.  More 
> involved fixes can come later (esp. after the nnpt patches land).

Ok, great. I will rebase my nnpt patches on-top of this and do some more
final testing before I post them.

Thanks,

	Joerg

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