A proposal - binary

Zachary Amsden
2006-08-03T10:14:23Z

I would like to propose an interface for linking a GPL kernel,
specifically,
Linux, against binary blobs. Stop. Before you condemn this as evil,
let me
explain in precise detail what a binary blob is.

First, there are two kinds of binary blobs. There are the evil, malignant
kind, that use manipulative and despicable techniques to subvert the GPL by
copying code into Linux and, the most evil kind, which uses a GPL
wrapper to
export GPL-only symbols to the binary blob. This is unconditionally
wrong. I
do not support this kind of use. These evil blobs are used to lock
people into
a particular type of protocol or proprietary hardware interface. In my
personal opinion, they should be unconditionally banned, or at least
phased out
rapidly from any GPL kernel. I have been frustrated by this in the
past, where
binary filesystem modules would not allow me access to AFS when
upgrading to a
new kernel version. I do not wish that on anyone.

But there is also another kind of binary blob. These are not the evil,
nasty
subversion type blobs, but a benign kind that actually exists in binary
form
not to close off choice, but to open it. This is exactly the kind of
binary
interface we are proposing with the VMI design. Only with a specific
ABI can you guarantee future compatibility. This is exactly the same thing
I believe some hardware vendors are trying to do. When you have a fairly
complex interaction with the hardware layer, you have a lot of code which
suddenly becomes hardware dependent. When that hardware is actually
changing
rapidly, you have a serious problem keeping released drivers for that
hardware
in sync. Software release cycles are becoming much longer, and
delivering new
capabilities to average consumers outside of that software release cycle
is a
very difficult problem to solve. As a result, vendors build some smarts
into
the hardware - a firmware piece that can be loaded and actually run on the
processor. This firmware allows the same driver to be used for many
different
versions of the hardware, allowing a single software release to support
multiple versions of yet to be released hardware. It is only natural to
run
this privileged software inside a privileged context - the kernel.

In our case, this "hardware" is actually the virtual machine. We must deal
with changes to the underlying hardware, as they are happening rapidly,
and we
must support future compatibility for customers that decide to start
using a
virtual machine in 2006 - it is a virtual machine, after all, and it should
continue running in 2016, no matter what the underlying hardware at that
time
will look like. In this sense, we have an even larger future compatibility
problem to solve than most hardware vendors. So it is critical to get an
interface that works now.

The essence of our interface is a separation between the kernel, and the
hypervisor compatibility layer, which we call the VMI. This layer is
completely separate from the kernel, and absolutely cannot be compiled
into the
kernel. Why? Because doing so negates all of the benefits this layer is
supposed to provide. It is separate from the kernel not to lock anyone
into a
proprietary design or prevent anyone from distributing a working
kernel. It is
separate to allow the hypervisor backend to change radically without
introducing any changes whatsoever into the kernel. This is absolutely
required for future compatibility - with new versions of each hypervisor
being
released regularly, and new competing hypervisors emerging, it is a
necessity.
This allows the hypervisor development process, as well as the Linux kernel
development process, to continue unimpeded in the face of rapid change
on each
side. Having an open binary interface encourages growth and competition in
this area, rather than locking anyone into a proprietary design. It
also does
not stop anyone from distributing a working, fully source compiled
kernel in
any circumstance I can imagine. If you don't have the firmware for a
VE-10TB
network card compiled into your kernel, but also don't have a VE-10TB
network
card, you haven't been deprived of anything, and have very little to
complain
about. Provided, of course, that when you do buy a VE-10TB network
card, it
happily provides the required firmware to you at boot time.

On the other hand, the GPL is not friendly to this type of linking against
binary fragments that come from firmware. But they really, absolutely,
must be
separate from the kernel. There is no argument against this from a feature
point of view. But there is also no reason that they must be
binary-only. The
interface between the two components surely must be binary - just as the
interface between userspace and the kernel, or between the apps and
glibc must
be binary. This means the code from one layer is visible to the other
purely
as a binary "blob". But not an evil one. And by NO circumstances, is it
required to be a CLOSED source binary blob. In fact, why can't it be
open? In
the event of a firmware bug, in fact, it is very desirable to have this
software be open so that it can be fixed - and reflashed onto the card,
where
the firmware belongs.

Let me illustrate the major differences between an "evil" binary blob, a
typical vendor designed hardware support layer, a well designed, open
binary
interface, and a traditional ROM or extensible firmware layer. I think you
will see why our VMI layer is quite similar to a traditional ROM, and very
dissimilar to an evil GPL-circumvention device. I can't truly speak for
the
video card vendors who have large binary modules in the kernel, but I would
imagine I'm not far off in my guesses, and they can correct me if I am
wrong.

EVIL VENDOR VMI ROM
Module runs at kernel privilege level: YES YES YES
MAYBE (*)
Module is copied into the kernel: YES MAYBE NO NO
Module is part of kernel address space: YES YES NO(+) ??
Module has hooks back into kernel: YES MAYBE NO NO
Kernel has hooks into module: YES YES YES YES
Module has proprietary 3rd party code: MAYBE MAYBE(?) NO YES
Module has significant software IP: YES MAYBE(?) NO
MAYBE (?)
Module is open source: NO MAYBE MAYBE NO

(*) In the ROM case, sometimes these are run in V8086 mode, not at full
hardware privilege level, and whether the < 1MB physical ROM region is
part of
the "kernel" address space proper, or just happens to appear in kernel
address
space as a result of linear mapping of physical space is a debatable
matter.

(+) The VMI layer is not technically part of the kernel address space.
It is
never mapped by the kernel, and merely exists magically hanging out in
virtual
address space above the top of the fixmap, in hypervisor address space.
But it
can be read and called into by the kernel, so whether this constitutes
being
part of the same address space is a dicey matter of precise definition.
I would
propose that only supervisor level pages that are allocated, mapped and
controlled by the kernel constitute the kernel address space, or
alternately,
the kernel address space consists of the linear range of virtual address
space
for which it can create supervisor-level mappings.

(?) There are only two reasonable objections I can see to open sourcing the
binary layer. One is revealing IP by letting people see the code. This is
really a selfish concern, not a justification for keeping the code
binary only,
while still allowing it the privilege of running in the kernel address
space.
The other objection I see is if that code has 3rd party pieces in it
that are
unable to be licensed under an open software license. This really is a
hard
stopper for open sourcing the code, as the vendor doesn't actually own the
copyright, and thus can't redistribute that code under a different
license. We
don't have any such restrictions, as we wrote all our code ourselves,
but many
ROMs and firmware layers do have such problems. ROMs might also have
some IP
in the form of trade secrets protecting power management or other
features run
in SMM mode - but this is just a guess. And IANAL - so take all this
with a
grain of salt, it is purely my uninformed personal opinion.

This brings me to the essence of my proposal - why not allow binary
"blobs" to
be linked inside Linux, in exactly the same way modules are linked? If
these
binary modules are actually open sourced, but separate from the kernel, is
there no reason they can't actually link against GPL symbols within Linux?
What if these modules exposed an ELF header which had exactly the same
information as a typical module? In this case, kernel version is not
relevant,
as these modules are truly kernel independent, but device/module version
is.
The biggest issue is that the source and build environment to these
modules is
not the standard environment -- in fact many of these binary modules
might have
extremely bizarre build requirements. We certainly do. But still there
remains no reason that a well encapsulated build environment and open
source
distribution of these modules cannot exist. We are actively working on
this for
our VMI layer. Perhaps a good solution to this problem would be to
provide a
link embedded in the binary which points to a URL where this environment
can be
downloaded and built - or even fully buildable compressed source within the
binary itself for most friendly binaries with plenty of space to burn.
There may be other issues which I may not be aware of on our end, but that
has no bearing on finding out what the Linux and open source community
wants.

I propose this as a solution because I would like to see binary (only)
blobs go
away, and I would never again like to see hardware vendors design stupid
code
which relies on firmware in the operating system to initialize a hardware
device (can I say WinModem?) which is not published and open code. The
point
of an interface like this is to open and standardize things in a way that
vendors can benefit from a hardware abstraction layer, and to make sure
that
the GPL is not violated in the process of doing so. I would very much
like to
see Linux come up with a long term proposal that can accommodate open
firmware
which actually runs in kernel mode, while at the same time assuring that
this
code is authorized within the license boundaries afforded to it and
available
for use by any operating system.

Thank you very much for your patience - I can get verbose, and I've already
gone too long. This is still a very controversial issue, and I wanted to
clarify several points that merit attention before dooming myself to
enduring
the yet to come flames. Again, I must say IANAL, and I can't promise
that VMware or any other hardware vendor that wants to use a binary
interface will agree to everything I have proposed. But I would like to
open the issue for discussion and get feedback from the open source
community
on this issue, which I think will become more important in the future.

Zach