Date:   Thu, 6 Dec 2018 07:37:18 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Kieran Bingham <kieran.bingham@...asonboard.com>
Cc:     Luis Chamberlain <mcgrof@...nel.org>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        Kent Overstreet <kent.overstreet@...il.com>,
        Eryu Guan <guaneryu@...il.com>,
        Eric Sandeen <sandeen@...deen.net>, jeffm@...e.com,
        Sasha Levin <Alexander.Levin@...rosoft.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...gle.com>, shuah@...nel.org,
        Joel Stanley <joel@....id.au>, mpe@...erman.id.au,
        joe@...ches.com, brakmo@...com, rostedt@...dmis.org,
        Tim.Bird@...y.com, khilman@...libre.com,
        Julia Lawall <julia.lawall@...6.fr>,
        linux-kselftest@...r.kernel.org, kunit-dev@...glegroups.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        jdike@...toit.com, richard@....at, linux-um@...ts.infradead.org,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org, Rob Herring <robh@...nel.org>,
        dan.j.williams@...el.com, linux-nvdimm@...ts.01.org,
        Frank Rowand <frowand.list@...il.com>,
        Knut Omang <knut.omang@...cle.com>,
        Felix Guo <felixguoxiuping@...il.com>,
        Petr Mladek <pmladek@...e.com>, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC v3 11/19] kunit: add Python libraries for handing KUnit
 config and kernel

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@...asonboard.com> 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.

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