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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 04.12.13 at 13:23, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> On Wed, 2013-12-04 at 12:14 +0000, Jan Beulich wrote:
>> >>> On 04.12.13 at 13:01, Tomasz Wroblewski <tomasz.wroblewski@xxxxxxxxxx> 
>> >>> wrote:
>> > On 12/04/2013 12:36 PM, Jan Beulich wrote:
>> >> 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,
>> 
>> That hypercall does translation only for auto-translated guests,
>> which a normal PV one clearly isn't.
> 
> When mapping a foreign owned page it is the remote owners mode which
> matters though, isn't it?

Oh, right. Which - for the code at hand - makes it even more
difficult to do the right thing (refuse PV DomU mappings of MFNs
below 1Mb, but allow translated DomU mappings of PFNs in that
range). I.e. we're back to why execution goes that route in the
first place for foreign mappings and doesn't - like on XenoLinux -
bypass the normal PTE construction code.

Jan


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