Re: [Xen-devel] get_page_from_gfn() for foreign pages

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 01 March 2018 14:00
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: xen-devel (xen-devel@xxxxxxxxxxxxxxxxxxxx) <xen-
> devel@xxxxxxxxxxxxxxxxxxxx>
> Subject: Re: [Xen-devel] get_page_from_gfn() for foreign pages
> 
> >>> On 01.03.18 at 12:39, <Paul.Durrant@xxxxxxxxxx> wrote:
> > Whilst debugging my PV-IOMMU code I ran into an issue with
> > get_page_from_gfn(): It is failing when the gfn in question is actually a
> > grant map from another domain, and the reason for this is that
> get_page(page,
> > domain) explicitly fails if the page owner is not the domain specified to 
> > the
> > call. However, I find that ARM implementation of get_page_from_gfn() has
> a
> > test for foreign p2m entries and completes successfully (by avoiding a call
> > to get_page() and instead calling page_get_owner_and_reference()
> directly).
> >
> > I can't think of any particular reason why, on x86, get_page_from_gfn()
> > should not work for foreign mapped or grant mapped pages. Does anyone
> know
> > why the restriction is there?
> 
> What part of {,p2m_}get_page_from_gfn() are you referring to?
> p2m_get_page_from_gfn()'s fast path does what you want; it looks
> like the slow path was wrongly ignored in 50fe6e7370 ("pvh dom0:
> add and remove foreign pages"). Or are you thinking about this for
> a PV domain (and hence you look at the second half of
> get_page_from_gfn())?

Yes, it's the PV case I'm hitting, i.e:

page = __mfn_to_page(gfn);
return mfn_valid(_mfn(gfn)) && get_page(page, d) ? page : NULL;

In my particular case I have an mfn but it represents a page that dom0 has 
grant mapped. When I come to look it up as a GFN with DOMID_SELF I get the 
error.

> 
> For grant mapped pages the situation may be a little less clear:
> How would you deal with a grant being revoked when you have
> random uses of the page in another domain?
> 

I suppose it comes down to whether we want to be able to have domA (which has 
priv mapping rights over domB) to be able to take a ref on a page from domC 
that was granted to domB. If, for example, you have a stubdom with a PV backend 
for a framebuffer which is then exported via a VNC server running in dom0 then 
that's exactly what you'd probably want to set up. So I don't see why it should 
be disallowed. In my case domA == domB and it certainly seems reasonable to me 
that a domain be able to page references on GFNs in its own P2M regardless of 
the actual page owner.

  Paul

> Jan


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