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

[John Criswell]

On 4/30/13 1:10 AM, Jovi Zhang wrote:
> 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.

Good point.  What I'd recommend you do is write a proposal for 
supporting those features and list several applications of that feature 
(portable kernel modules being one such application).  That will give 
your proposal a better chance at being accepted; the more potential uses 
of your work there are, the more likely a reviewer will find one use 
which he or she cares about and think favorably of the proposal.

>
>> 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.

I don't agree with the idea that the Linux kernel should change its 
kernel module API arbitrarily, but that is the policy and intent that 
the Linux kernel developers appear to have.  If your proposal requires 
changes to the Linux kernel to get this to work, then I think your 
project has a serious adoption problem because the kernel developers may 
not want to upstream the changes necessary to make your solution work.

If your proposed solution does not require any Linux kernel changes, 
then you should state that explicitly in your proposal.

The reason that people here are concerned with the Linux community's 
philosophy is that they want to see a realistic plan for your work to 
get adopted.  If some Linux kernel development policy/philosophy is 
going to complicate adoption, it hurts your proposal.

In summary, I think you can strengthen your proposal by doing the following:

1) Make the focus of your proposal be the variable offset and sizeof 
features in LLVM IR.

2) Argue that variable offset and sizeof features have several 
interesting uses, including portable kernel modules for Linux (and other 
operating systems) and any other applications you can think of.

3) Describe how you will address the technical challenges.  How will you 
handle all the optimizations that use DataLayout to make non-portable 
LLVM IR transformations?  Will you need to add support to the Clang 
frontend, and if so, what?  Will you handle the use of sizeof() in 
preprocessor macros, or is that feature so seldom used that you can 
ignore it?

I think doing those things will give you a better chance of getting your 
proposal accepted.

-- John T.