Re: [PATCH 2/3] KVM: MMU: Introduce a helper to read guest pte

["Roedel, Joerg" <Joerg.Roedel@amd.com>]

On Wed, Apr 20, 2011 at 05:07:12AM -0400, Avi Kivity wrote:
> On 04/18/2011 09:34 PM, Takuya Yoshikawa wrote:
> > From: Takuya Yoshikawa<yoshikawa.takuya@oss.ntt.co.jp>
> >
> > This will be optimized later.
> >
> > Signed-off-by: Takuya Yoshikawa<yoshikawa.takuya@oss.ntt.co.jp>
> > ---
> >   arch/x86/kvm/paging_tmpl.h |   12 +++++++++---
> >   1 files changed, 9 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
> > index 74f8567..109939a 100644
> > --- a/arch/x86/kvm/paging_tmpl.h
> > +++ b/arch/x86/kvm/paging_tmpl.h
> > @@ -109,6 +109,14 @@ static unsigned FNAME(gpte_access)(struct kvm_vcpu *vcpu, pt_element_t gpte)
> >   	return access;
> >   }
> >
> > +static int FNAME(read_guest_pte)(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
> > +				 gfn_t table_gfn, int offset, pt_element_t *ptep)
> > +{
> > +	return kvm_read_guest_page_mmu(vcpu, mmu, table_gfn, ptep,
> > +				       offset, sizeof(*ptep),
> > +				       PFERR_USER_MASK | PFERR_WRITE_MASK);
> > +}
> > +
> >   /*
> >    * Fetch a guest pte for a guest virtual address
> >    */
> > @@ -160,9 +168,7 @@ walk:
> >   		walker->table_gfn[walker->level - 1] = table_gfn;
> >   		walker->pte_gpa[walker->level - 1] = pte_gpa;
> >
> > -		if (kvm_read_guest_page_mmu(vcpu, mmu, table_gfn,&pte,
> > -					    offset, sizeof(pte),
> > -					    PFERR_USER_MASK|PFERR_WRITE_MASK)) {
> > +		if (FNAME(read_guest_pte)(vcpu, mmu, table_gfn, offset,&pte)) {
> >   			present = false;
> >   			break;
> >   		}
> 
> 
> I think it's better to avoid a separate function for this.  The reason 
> is I'd like to use ptep_user for cmpxchg_gpte() later on in 
> walk_addr_generic(), so we use the same calculation for both read and 
> write.  So please just inline the new code in walk_addr_generic().
> 
> In fact there's probably a bug there for nested npt - we use 
> gfn_to_page(table_gfn), but table_gfn is actually an ngfn, not a gfn.  
> Joerg, am I right here?

This patch seems only to introduce another wrapper around
kvm_read_guest_page_mmu(), so I don't see a problem in this patch.

The kvm_read_guest_page_mmu takes care whether it gets a l1-gfn or
l2-gfn (by calling mmu->translate_gpa).

Regards,

	Joerg

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