From mboxrd@z Thu Jan  1 00:00:00 1970
From: Tomasz Wroblewski <tomasz.wroblewski@citrix.com>
Subject: Re: Why does xc_map_foreign_range() refuse to map pfns
 below 1M from a domU
Date: Wed, 4 Dec 2013 12:23:11 +0100
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To: Ian Campbell <Ian.Campbell@citrix.com>
Cc: Razvan Cojocaru <rzvncj@gmail.com>, Jan Beulich <JBeulich@suse.com>, "xen-devel@lists.xen.org" <xen-devel@lists.xen.org>
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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@citrix.com> 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.