[LLVMdev] A new mechanism to compiler kernel modules using llvm: Defer type evaluation in clang?

[Jovi Zhang]

On Tue, Apr 30, 2013 at 12:31 AM, John Criswell <criswell at illinois.edu> wrote:
> On 4/28/13 11:42 AM, Jovi Zhang wrote:
>>
>> Hi,
>>
>> First of all, I didn't study on compiler too much, I'm a Linux kernel
>> developer,
>> Now I have one idea about compile kernel module by using llvm framework.
>>
>> In Linux world, traditionally compile Linux kernel module only have one
>> way:
>> compile it to machine code directly, deploy to target machine, then
>> run it in target machine.
>>
>> This have some problem from long term view, especially kernel module
>> compatibility issue,
>> we know that Linux kernel change fast, when kernel change the data
>> structure exposed to external
>> kernel modules, then all ko depend on that data structure will need
>> recompile, one case is
>> kernel upgrade.
>> This is really a problem, we already suffered it many years, in whole
>> Linux world.
>> Linux distribution suffer it, third party kernel modules developers suffer
>> it.
>> hardware driver developers also suffer it, we cannot avoid re-compile
>> kernel modules whenever new kernel
>> release if the structure changed, why? because kernel modules depend
>> on kernel header data structure, this is the point.
>
>
> The basic idea seems feasible, but I think this is a "devil in the details"
> sort of project.  As Joshua has pointed out, there are certain constructs
> you can't use (such as sizeof() in preprocessor #if statements), and I think
> you'd need to enhance the LLVM IR to represent unknown structure sizes
> symbolically.  There is also the fact that the Clang frontend generates
> architecture-specific code and that there are LLVM optimizations that use
> the size of structures for optimization.  That is all stuff that your
> project would have to "fix."  You'd be swimming against a strong current, so
> to speak.

Thanks.

>From LLVM's point of view, the design I described could be interpreted as:
     "Implementing Portable sizeof, offsetof and Variable Sized
Structures in LLVM"

It's a random LLVM notes at:
    http://nondot.org/sabre/LLVMNotes/
    http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt

Basically the note wrote in there will solve main part of my problem,
only another problem
is structure field name need pass into IR file, I don't think it's a
hard technical problem, we have choose
many way to solve this, even put structure field name as comments into IR file.

Look, it's not that complicated, what we need is implementing a
portable sizeof/offsetof in LLVM,
and most likely my design is only another use case for this "portable
sizeof/offset" feature.

>
> More importantly, while you raise a number of limitations of linking native
> code for kernel modules, I don't think the Linux community sees this is a
> disadvantage.  Rather, they see it as a way of encouraging developers to
> open-source their drivers and have them included in the Linux kernel tree.
> I'm not convinced that you'd get buy-in from the Linux community.

Your opinion is same as Karen Shaeffer.
We cannot upstream all kernel modules source code into mainline, this
is not simply a
technical problem or community problem, I believe that every company
have its own
kernel module which cannot upstream, or not worthwhile, if the company
is working
on kernel module development. upstream kernel modules just is a small
part of all kernel
modules in Linux world, this is the truth we need admit it.

>
> If this sort of project interests you, then I think you should aim for
> something smaller and that has more interest.  For example, I think the NaCL
> developers at Google were looking at ways to make the LLVM IR more suitable
> as a format for shipping NaCL plugins.  If that hasn't been done yet, it's
> an easier problem to tackle, and there is a group that is already interested
> in having it.
>
> Finally, as a shameless plug, your project idea reminds me of the LLVA work
> that was done here at Illinois (which became the foundation for my own
> research on the Secure Virtual Architecture). That work aimed to replace the
> processor native instruction set with the LLVM IR to allow the processor to
> adapt without requiring manual recompilation of software.  You can find
> those papers on the llvm.org publications page if you're interested.
>
> -- John T.
>
>