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

On Tue, Oct 10, 2017 at 2:23 AM, Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:
> 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.

Ok. I had actually tried this approach, but I guess there's some sort
of conflict between the Xen headers (included by xenforeignmemory.h)
and my cross-compiler toolchain. I'm attempting to build Xen as part
of a larger system that uses Petalinux, which likely introduces its
own set of idiosyncrasies. Will keep digging, and will see whether
anyone on a more petalinux-focused mailing list has any suggestions...

Brett S.

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

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