Re: [RFC v3 11/19] kunit: add Python libraries for handing KUnit config and kernel

[Matthew Wilcox]

On Thu, Dec 06, 2018 at 12:32:47PM +0000, Kieran Bingham wrote:
> On 04/12/2018 20:47, Luis Chamberlain wrote:
> > On Mon, Dec 03, 2018 at 03:48:15PM -0800, Brendan Higgins wrote:
> >> On Thu, Nov 29, 2018 at 5:54 AM Kieran Bingham
> >> <kieran.bingham@xxxxxxxxxxxxxxxx> wrote:
> >>>
> >>> Hi Brendan,
> >>>
> >>> Thanks again for this series!
> >>>
> >>> On 28/11/2018 19:36, Brendan Higgins wrote:
> >>>> The ultimate goal is to create minimal isolated test binaries; in the
> >>>> meantime we are using UML to provide the infrastructure to run tests, so
> >>>> define an abstract way to configure and run tests that allow us to
> >>>> change the context in which tests are built without affecting the user.
> >>>> This also makes pretty and dynamic error reporting, and a lot of other
> >>>> nice features easier.
> >>>
> >>>
> >>> I wonder if we could somehow generate a shared library object
> >>> 'libkernel' or 'libumlinux' from a UM configured set of headers and
> >>> objects so that we could create binary targets directly ?
> >>
> >> That's an interesting idea. I think it would be difficult to figure
> >> out exactly where to draw the line of what goes in there and what
> >> needs to be built specific to a test a priori. Of course, that leads
> >> into the biggest problem in general, needed to know what I need to
> >> build to test the thing that I want to test.
> >>
> >> Nevertheless, I could definitely imagine that being useful in a lot of cases.
> > 
> > Whether or not we can abstract away the kernel into such a mechanism
> > with uml libraries is a good question worth exploring.
> > 
> > Developers working upstream do modify their kernels a lot, so we'd have
> > to update such libraries quite a bit, but I think that's fine too. The
> > *real* value I think from the above suggestion would be enterprise /
> > mobile distros or stable kernel maintainers which have a static kernel
> > they need to support for a relatively *long time*, consider a 10 year
> > time frame. Running unit tests without qemu with uml and libraries for
> > respective kernels seems real worthy.
> 
> I think any such library might be something generated by the kernel
> build system, so if someone makes substantial changes to a core
> component provided by the library - it can be up to them to build a
> corresponding userspace library as well.
> 
> We could also consider to only provide *static* libraries rather than
> dynamic. So any one building some userspace tool / test with this would
> be required to compile against (the version of) the kernel they expect
> perhaps... - much like we expect modules to be compiled currently.
> 
> And then the userspace binary would be sufficiently able to live it's
> life on it's own :)
> 
> > The overhead for testing a unit test for said targets, *ideally*, would
> > just be to to reboot into the system with such libraries available, a
> > unit test would just look for the respective uname -r library and mimic
> > that kernel, much the same way enterprise distributions today rely on
> > having debugging symbols available to run against crash / gdb. Having
> > debug modules / kernel for crash requires such effort already, so this
> > would just be an extra layer of other prospect tests.
> 
> Oh - although, yes - there are some good concepts there - but I'm a bit
> weary of how easy it would be to 'run' the said test against multiple
> kernel version libraries... there would be a lot of possible ABI
> conflicts perhaps.
> 
> My main initial idea for a libumlinux is to provide infrastructure such
> as our linked-lists and other kernel formatting so that we can take
> kernel code directly to userspace for test and debug (assuming that
> there are no hardware dependencies or things that we can't mock out)
> 
> I think all of this could complement kunit of course - this isn't
> suggesting an alternative implementation :-)

I suspect the reason Luis cc'd me on this is that we already have some
artisinally-crafted userspace kernel-mocking interfaces under tools/.
The tools/testing/radix-tree directory is the source of some of this,
but I've been moving pieces out into tools/ more generally where it
makes sense to.

We have liburcu already, which is good.  The main sticking points are:

 - No emulation of kernel thread interfaces
 - The kernel does not provide the ability to aggressively fail memory
   allocations (which is useful when trying to exercise the memory failure
   paths).
 - printk has started adding a lot of %pX enhancements which printf
   obviously doesn't know about.
 - No global pseudo-random number generator in the kernel.  Probably
   we should steal the i915 one.

I know Dan Williams has also done a lot of working mocking kernel
interfaces for libnvdimm.