Re: [Xen-devel] Why does xc_map_foreign_range() refuse to map pfns below 1M from a domU

[Tomasz Wroblewski <tomasz.wroblewski@xxxxxxxxxx>]

On 12/04/2013 12:04 PM, Ian Campbell wrote:
> On Wed, 2013-12-04 at 10:54 +0000, Jan Beulich wrote:
> > > > > On 04.12.13 at 11:45, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> > > Correct. The check for mapping domain 0's 1:1 map is overly broad I
> > > think, and erroneously prevents a domU from mapping a foreign PFN < 1M.
> > But that's the source of my not understanding: xen_make_pte()
> > derives addr from the passed in pte, and that pte can - for a
> > foreign domain's page - hardly hold a PFN. Otherwise how would
> > the translation to MFN be supposed to happen? Yet, if it's a
> > machine address that's coming in, it can't point into the low 1Mb.
> Isn't it a foreign gpfn at this point, which for an HVM guest is
> actually a PFN not an MFN?
>
> You are making me think I might be talking out my a**e though, because
> what is a foreign mapping even doing in xen_make_pte -- those need to be
> instantiated in a special way.
I believe the callpath for this is

xen_remap_domain_range() (mmu.c)
 |
 v
remap_area_pfn_pte() (mmu.c)
 |
 v
pfn_pte() (somewhere, one of the pgtable.h hdrs)
 |
 v
__pte() (paravirt.h)
 |
 v
xen_make_pte (mmu.c) via pv_mmu_ops.make_pte

Sorry, can't offer much insight as to why addr in pte holds the hvm's PFN, but 
it seems the case.


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