On 26/05/2014 4:39 am, Cláudio Silva wrote:
Sorry all, miss send. Returning to where I was:
Yes, Partitions are fully linked executables created "off-line". No
linking occurs in run-time. The partition elf is simply loaded in to
memory by a partition loader or by the bootloader.
Typically, you have a thin abstraction layer defining simple functions
that will trap into the partitioning kernel. Something like:
https://github.com/t-crest/ospat/blob/master/libpok/middleware/portqueueingsend.c
Thanks. This lines up with my understand.
I think RTEMS needs to view POK or any virtual environment as a black
box and we cannot assume anything about the environment other than the
ABI presented to us. We cannot look through to the host system and do
things as a result. Maintaining a strict black box view may not be
possible however we should start with the black box and move to a
position that is practical and possible.
We should not borrow (reference in the POK source tree) header files to
pick up common definitions directly or indirectly via static libraries.
Any interfaces defined by the partition's host need to be formally
exported from the project and made available. This defines the ABI we
are working against. I see POK is highly configurable so this
complicates what is exported.
If I build POK with port queueing enabled and build a guest that uses it
then I reconfigure POK and remove port queueing however I do not rebuild
the guest will POK detect the problem and handle the mismatch ?
In POK + RTEMS case, this library is being compiled by POK's compiler
and then linked to each partitions' executable. When this partition is
an RTEMS one, the remaining binaries (RTEMS+Application) were compiled
with the RTEMS' compiler. Therefore you are linking two sets of
binaries produced by different compilers.
We cannot do this. Given the type of applications expected to use POK I
would expect an audit to flag this as a fail. There are a number of
reasons why this is not a sustainable path forward, the most simple one
is having to have code in the RTEMS partition built with a compiler we
do not control. In this example the dependency is backwards, for example
will the POK project make sure its compiler can always build the
interface and work with RTEMS, ie all tests pass. I suspect the POK
maintainers will have limited or no real interest in RTEMS's development
process and I see this as normal and expected. I do not see this path as
being sustainable and I do not support it. As a hack to get things going
it has been perfectly fine.
I guess this is also true for the recently submitted Xtratum BSP.
Current Xtratum BSP is not going to happen so lets put that one to one side.
From the RTEMS point of view I see POK as no different to any
architecture we support. We support running on hard IP in a device, for
example a Blackfin, a mix of hard IP and soft IP, for example a Zynq,
soft IP, for example a NIOS2, hard IP (or mixed soft IP) and software
intervention in a virtual environment, for example POK, and full
software emulation via simulation, for example qemu.
For RTEMS to support an architecture we need separation so each part can
vary in version, the version dependencies flow down to RTEMS and not the
other direction, and we need a way to test. Testing means we have a
stable version of POK built on our test system and we build RTEMS
against that.
We have several options available:
- Let the two binaries (RTEMS + abstraction lib), compiled with
different compilers, to be linked together (how it is currently
happening)
No.
- Compile the partitioning kernel abstraction layer with the RTEMS
compiler - this will result in a complex configuration and build
procedure.
This is possible however is depends on how this is implement and where
the code resides.
If the code lives inside the POK source tree and references files that
are core to POK we have an issue. This approach would place a burden on
the POK project to have a current set of RTEMS tools for the
architectures supported and to test the builds work. It means POK
maintainers would need to monitor and track the RTEMS development
process. The dependency here is from RTEMS to POK and as I mentioned
before the dependence should be the other away around.
The code could be a separate project or sub-project of POK and both
projects need to maintain it. The POK project would need to maintain the
ABI and RTEMS would need to maintain the dependency with RTEMS tools and
RTEMS. For RTEMS how we interface to this because the next issue to solve.
- Implement the functionality of the abstraction layer completely
inside RTEMS as part of the libbsp/libcpu - This can be a good
solution for the CPU/TImer/Interrupt part of the abstraction layer.
If the POK interface is stable and the header dependency can be made
thin this works for me. In the ideal world if something changes in the
POK ABI they would publish an update and we would import it. There would
be versioning issues, ie backwards compatibility, and this is normal and
nothing new for us.
However, for other elements like the queueing and sampling port APIs
that have no match inside RTEMS...
Could you please elaborate a little more ? Are these not supported
because RTEMS application's would not use them or this is still to be done ?
Chris
-
Best Regards,
Cláudio
On Sun, May 25, 2014 at 7:22 PM, Cláudio Silva <claudiodcsi...@gmail.com> wrote:
Hello Chris,
Yes, Partitions are fully linked executables created "off-line". No
linking occurs in run-time. The partition elf is simply loaded in to
memory by a partition loader or the bootloader.
Typically, you have a thin abstraction layer defining simple functions
that will trap into the partitioning kernel. Something like:
On Sat, May 24, 2014 at 12:27 AM, Chris Johns <chr...@rtems.org> wrote:
On 23/05/2014 6:46 pm, Philipp Eppelt wrote:
On 05/21/2014 03:54 PM, Gedare Bloom wrote:> On Wed, May 21, 2014 at 4:04
AM, Christian Mauderer
<christian.maude...@embedded-brains.de> wrote:
First of all: Thanks for your comments. You will find answers below.
Am 20.05.2014 16:58, schrieb Gedare Bloom:
That is correct. It's part of XtratuM. Is there some preferred way of
marking such headers?
Not that I know of. We have discussed a similar issue with the POK
paravirtualization project. The problem is to allow external code
linking to RTEMS. The design should be considered carefully and
probably discussed in a separate thread.
To pick up this discussion, I like first to picture the problem, second
to collect possible solutions and third name some known problems with
the solutions.
Thank you for posting this email. I followed the links and looked at the
code and realised I need to ask more questions.
In a paravirtualized environment we need to make function calls from our
guest system to the host system. The implementation of these functions
rely sole on the host system.
Are they function calls ? To me a function call assumes some sort of linking
process between the guest and the host system either static or via some form
of runtime binding, ie a link editor. I also assume a guest is an RTEMS
partition executable and the host is the POK microkernel providing the
partitioned environment in some sort of protective environment.
Hence, at compile time the guest system is missing the implementation of
this host specific functions.
My understanding is partitions are loaded into an ELF image that POK loads
and executes. Is there some sort of linking happening when the partition is
written into the ELF image or when it is being loaded ?
For me this is the really important because I am wanting to first understand
the which pieces of code are compiled into object files and by which
compilers and then archived into static libraries and then linked to any
RTEMS code compiled with an RTEMS compiler. I do not understand if there are
function calls into the POK kernel or syscall type calls which I consider an
ABI, ie a trap or VM exception.
If I can understand these needs I will be in a better position to help with
the remaining topics in this email.
Chris
To provide the guest system with the function implementation there are a
couple of possible solutions.
(A) Provide a host library
The host compiles a library containing the function implementations
including all dependencies. This host library is passed to the guest
system, to resolve the `undefined reference` errors during link time.
In the case of POK+RTEMS the resulting RTEMS binary is a valid POK
partition and can run without further modifications.
The recently propose XantruM BSP seems to follow a similar approach.
(B) Use function stubs and build a jump table in the host
The guest system makes use of function stubs, written into a special
section in the binary to build. This section is analysed by the host
system to intercept function calls and replace them with the host
implementation of the calls.
This approach is used in L4Linux and I gave it a shot in L4RTEMS.
[1] shows the macro for the function stubs, [2] the hosts resolver part.
(C) add your solution
Problems with the solutions
P(A) Dependencies and foreign code
The library is host code, which has to be maintained, which can get out
of sync between BSP and host. Licensing is another question.
To my knowledge this approach won't get merged upstream.
P(B) Support by the host system
The host system needs a piece of code, loading the guest binary and
providing it with knowledge about the environment (external_resolver).
We haven't discussed this approach in detail.
I hope, this gives a good overview and we can have a fruitful discussion.
Cheers,
Philipp
[1]
https://github.com/phipse/L4RTEMS/blob/master/c/src/lib/libbsp/l4vcpu/pc386/startup/l4lib.h
[2]
https://github.com/phipse/L4RTEMS/blob/f5a32ed0121b3e5f0ac8ae4c53e636eab2257fb1/l4/pkg/RTEMS_wrapper/server/src/res.c
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