linux-kernel - Re: [PATCH 16/22] KVM: MMU: Track page fault data in struct vcpu

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Message-ID: <20100504120039.GH28950@amd.com>
Date:	Tue, 4 May 2010 14:00:39 +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 16/22] KVM: MMU: Track page fault data in struct vcpu

On Tue, May 04, 2010 at 05:50:50AM -0400, Avi Kivity wrote:
> On 05/04/2010 12:45 PM, Avi Kivity wrote:
> >
> >
> > Just a silly bug.  kvm_pdptr_read() can cause a guest memory read on 
> > svm, in this case with the mmu lock taken.  I'll post something to fix 
> > it.
> 
> I guess this was not reported because most svm machines have npt, and 
> this requires npt=0 to trigger.  Nonpae paging disables npt, so you were 
> hit.  Interestingly, nsvm makes it more likely to appear, since npt on 
> i386+pae will need the pdptrs.

Hmm, actually it happened on 32 bit with npt enabled. I think this
can trigger when mmu_alloc_roots is called for an pae guest because it
accidentially tries read the root_gfn from the guest before it figures
out that it runs with tdp and omits the gfn read from the guest.
I need to touch this for nested-npt and will look into a way improving
this.

	Joerg


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