Re: [Xen-devel] [PATCH] nestedhvm: do not translate INVALID_GFN

[Tim Deegan <tim@xxxxxxx>]

At 13:28 +0200 on 02 Aug (1343914136), Christoph Egger wrote:
> On 08/02/12 13:19, Tim Deegan wrote:
> 
> > Hi,
> > 
> > At 17:37 +0200 on 31 Jul (1343756240), Christoph Egger wrote:
> >> Do not translate INVALID_GFN as l2 guest gfn into l1 guest gfn.
> > 
> > Why not?  l2 gfns don't have any special meaning that we can
> > dictate from inside Xen.
> > 
> >> Pass correct pfec for translation into l1 guest gfn.
> > 
> > This seems like a good idea, but probably should happen for all
> > entries, not just INVALID_GFN ones -- we shouldn't be returning a PFEC
> > to the guest that comes from translations outside his control.
> > 
> > How about this:
> > 
> > diff -r fdd4b7b36959 xen/arch/x86/mm/p2m.c
> > --- a/xen/arch/x86/mm/p2m.c Thu Aug 02 12:04:31 2012 +0100
> > +++ b/xen/arch/x86/mm/p2m.c Thu Aug 02 12:17:48 2012 +0100
> > @@ -1581,6 +1581,7 @@ unsigned long paging_gva_to_gfn(struct v
> >          unsigned long gfn;
> >          struct p2m_domain *p2m;
> >          const struct paging_mode *mode;
> > +        uint32_t pfec_21 = *pfec;
> >          uint64_t ncr3 = nhvm_vcpu_hostcr3(v);
> >  
> >          /* translate l2 guest va into l2 guest gfn */
> > @@ -1590,7 +1591,7 @@ unsigned long paging_gva_to_gfn(struct v
> >  
> >          /* translate l2 guest gfn into l1 guest gfn */
> >          return hostmode->p2m_ga_to_gfn(v, hostp2m, ncr3,
> > -                                       gfn << PAGE_SHIFT, pfec, NULL);
> > +                                       gfn << PAGE_SHIFT, &pfec_21, NULL);
> 
> 
> The caller will see the return value of pfec and not from pfec_21.
> If this is what the caller expects then this is fine with me.

Yes, I think that is what the caller expects -- the error code is made
up from the pagetable walk rather than from the p2m table.

Can I take that as an ack?

And more importantly, does it fix the Hyper-V problem you encountered?

Cheers,

Tim

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