Hi Matthew, On 06/12/2018 15:37, Matthew Wilcox wrote: > 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/. Aha - excellent - I had hoped to grab you at Plumbers to talk about this, after hearing you mention something at your Xarray talk - but didn't seem to find a suitable time. > 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. Sounds like we already have a starting point then. > We have liburcu already, which is good. The main sticking points are: > > - No emulation of kernel thread interfaces Scheduling finesse aside, This shouldn't be too hard to emulate/wrap with pthreads? > - The kernel does not provide the ability to aggressively fail memory > allocations (which is useful when trying to exercise the memory failure > paths). Fault injection throughout would certainly be a valuable addition to any unit-testing. Wrapping tests into a single userspace binary could facilitate further memory leak checking or other valgrind facilities too. > - printk has started adding a lot of %pX enhancements which printf > obviously doesn't know about. Wrapping through User-mode linux essentially provides this already though. In fact I guess that goes for the thread interfaces topic above too. > - 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. Thanks for the references - more to investigate. -- Regards -- Kieran
