Re: [Xen-devel] One question about the hypercall to translate gfn to mfn.

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2014-12-09 at 11:05 +0000, Paul Durrant wrote:
> > -----Original Message-----
> > From: Tim Deegan [mailto:tim@xxxxxxx]
> > Sent: 09 December 2014 10:47
> > To: Yu, Zhang
> > Cc: Paul Durrant; Keir (Xen.org); JBeulich@xxxxxxxx; Kevin Tian; Xen-
> > devel@xxxxxxxxxxxxx
> > Subject: Re: One question about the hypercall to translate gfn to mfn.
> > 
> > At 18:10 +0800 on 09 Dec (1418145055), Yu, Zhang wrote:
> > > Hi all,
> > >
> > >    As you can see, we are pushing our XenGT patches to the upstream. One
> > > feature we need in xen is to translate guests' gfn to mfn in XenGT dom0
> > > device model.
> > >
> > >    Here we may have 2 similar solutions:
> > >    1> Paul told me(and thank you, Paul :)) that there used to be a
> > > hypercall, XENMEM_translate_gpfn_list, which was removed by Keir in
> > > commit 2d2f7977a052e655db6748be5dabf5a58f5c5e32, because there was
> > no
> > > usage at that time.
> > 
> > It's been suggested before that we should revive this hypercall, and I
> > don't think it's a good idea.  Whenever a domain needs to know the
> > actual MFN of another domain's memory it's usually because the
> > security model is problematic.  In particular, finding the MFN is
> > usually followed by a brute-force mapping from a dom0 process, or by
> > passing the MFN to a device for unprotected DMA.
> > 
> > These days DMA access should be protected by IOMMUs, or else
> > the device drivers (and associated tools) are effectively inside the
> > hypervisor's TCB.  Luckily on x86 IOMMUs are widely available (and
> > presumably present on anything new enough to run XenGT?).
> > 
> > So I think the interface we need here is a please-map-this-gfn one,
> > like the existing grant-table ops (which already do what you need by
> > returning an address suitable for DMA).  If adding a grant entry for
> > every frame of the framebuffer within the guest is too much, maybe we
> > can make a new interface for the guest to grant access to larger areas.
> > 
> 
> IIUC the in-guest driver is Xen-unaware so any grant entry would have
> to be put in the guests table by the tools, which would entail some
> form of flexibly sized reserved range of grant entries otherwise any
> PV driver that are present in the guest would merrily clobber the new
> grant entries.
> A domain can already priv map a gfn into the MMU, so I think we just
>  need an equivalent for the IOMMU.

I'm not sure I'm fully understanding what's going on here, but is a
variant of XENMEM_add_to_physmap+XENMAPSPACE_gmfn_foreign which also
returns a DMA handle a plausible solution?

Ian.


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