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 12:36 PM, Jan Beulich wrote:
> > > > On 04.12.13 at 12:23, Tomasz Wroblewski <tomasz.wroblewski@xxxxxxxxxx> wrote:
> > 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.
> But that's a fundamental thing to explain. As Ian says - foreign PFNs
> shouldn't make it here, or else how do you know how to translate
> them to MFNs (as you can't consult the local P2M table to do so)?
I was under the impression that the translation is done inside in xen inside HYPERVISOR_mmu_update, which gets called from 
xen_remap_domain_mfn_range shortly after setting up the ptes via xen_make_pte:
int xen_remap_domain_mfn_range(struct vm_area_struct *vma,
                               unsigned long addr,
                               xen_pfn_t mfn, int nr,
                               pgprot_t prot, unsigned domid,
                               struct page **pages)
...
                err = apply_to_page_range(vma->vm_mm, addr, range,
                                          remap_area_mfn_pte_fn, &rmd);
^^^ this calls xen_make_pte via the callpath I quoted in previous post
                if (err)
                        goto out;

                err = HYPERVISOR_mmu_update(mmu_update, batch, NULL, domid);
^^^ this goes into xen and does p2m translation and mmu setup etc

                if (err < 0)
                        goto out;
...


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