Re: [Xen-devel] allocate_empty_lowmem_region hypervisor function

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> > It's only used in the backend drivers, so I guess the build is trying to
> > omit that as dead code as far as a domU is concerned.
>
> Could it be added to DomU, otherwise there is no way to map foreign
> pages using the grant tables in unprivileged domains.

> I thought the 
> point of grant tables was to allow inter domain communication between
> non-privileged domains?

Yep.  The existing uses of grant tables (Netif, XenFS) all used page exchange, 
so they would have had somewhere to map incoming pages (i.e. empty slots left 
by frames they'd flipped to the server).  If you're running a backend then 
you really do need this function to make some space to map with.

> I could easily supply a patch if people think 
> this should be possible.

The lightest-weight solution (minimising dead code) would be to have this only 
built if you are building a backend driver.  Then again, building it 
unconditionally won't add much code and is a rather smaller patch.

In any case, I think we should enable backends without physdev access so a 
patch would be good to see.

> > IIRC the function is pure Linux-isms with no Xen-specifics, so it should
> > be fine to build into a domU (or even x86 Linux).
>
> It uses the balloon driver and hypervisor calls, so I don't think it
> would work in x86 Linux, but it works fine in DomU.

Oops, I was thinking of a different, related call in the backend :-)  But yes, 
allocate_empty_lowmem will work fine in domU.

Cheers,
Mark

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