Re: [Xen-users] Reading a DomU's memory from Dom0 or Xen hypervisor

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Oct 09, 2017 at 10:54:18PM +0000, Brett Stahlman wrote:
> On Mon, Oct 9, 2017 at 4:35 AM, Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:
> > On Fri, Oct 06, 2017 at 04:01:29PM +0000, Brett Stahlman wrote:
> >> On Fri, Oct 6, 2017 at 3:51 AM, Roger Pau Monné <roger.pau@xxxxxxxxxx> 
> >> wrote:
> >> > On Thu, Oct 05, 2017 at 05:51:39PM +0000, Brett Stahlman wrote:
> >> >> I'd like to be able to read specific memory locations within a DomU,
> >> >> either by guest-physical or guest-virtual address, and I'm wondering
> >> >> what the Xen-provided options in this area are. I've seen
> >> >> documentation on "grant tables", but that mechanism seems tailored
> >> >> towards communication between front and back-end drivers. All I want
> >> >> to do is allow code in either Dom0 or the hypervisor itself to read
> >> >> specific memory ranges within a specific DomU. I notice there's a
> >> >> raw_copy_from_guest() function (guestcopy.c), which looks like it
> >> >> could be useful. However, the hardcoding of "current->domain" in the
> >> >> call to get_page_from_gva() leads me to suspect that it's intended for
> >> >> use only in a hypercall. Is there a standard way to read memory from
> >> >> an *arbitrary* guest, either using guest-physical or guest-virtual
> >> >> addresses? If so, is it possible to do it from Dom0 without modifying
> >> >> the Xen hypervisor?
> >> >>
> >> >> Links to relevant documentation or example source code would be
> >> >> greatly appreciated...
> >> >
> >> > AFAICT you want to take a look at the foreignmemory library. It allows
> >> > you to map DomU memory from Dom0 user-space [0].
> >>
> >> Hmm... Just noticed that the foreignmemory mapping code was factored
> >> out of libxc and into libxenforeignmemory in commit 324869 (Xen 4.7).
> >> So it looks like there was no alternative to the libxc functions until
> >> fairly recently. I'll probably try using the libxenforeignmemory
> >> functions (tools/libs/foreignmemory) directly, unless someone can give
> >> me a reason to prefer the libxc wrapper...
> >
> > libxc doesn't have a stable interface, libxenforeignmemory OTOH is
> > stable, so you should aim at using the later, or else you
> > implementation might be tied to a specific Xen version.
> 
> Can you point me to documentation on building custom userspace apps
> (and/or LKMs) against Xen libraries such as libxenforeignmemory?
> (Alternatively, a link to an open source project that does it would be
> helpful.) I'm thinking macros like CFLAGS_libxenforeignmemory (defined
> in tools/Rules.mk) would be useful, but my initial attempt to include
> Rules.mk from a makefile outside the Xen source tree resulted in an
> error that led me to suspect that the file was meant to be used only
> within the Xen build framework. Considered adding a C file under
> components/apps/xen/xen-src/tools/misc and updating
> tools/misc/Makefile accordingly, but monkeying around in the Xen
> source tree didn't seem right...

The main user of libxenforeignmemory is QEMU. No, you shouldn't use
any of the Xen Makefiles, that's for internal use only.

I would expect that you need to include the header and link against
the library during the build process, but I'm no expert in user-space
libraries. There are .pc files for the libraries, so pkgconfig [0] can
probably help.

Roger.

[0] https://www.freedesktop.org/wiki/Software/pkg-config/

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