Re: [PATCH v5 20/21] tools/testing/nvdimm: manufactured NFITs for interface development

Dan Williams
2015-06-11T20:12:38Z

On Mon, Jun 8, 2015 at 11:48 PM, Christoph Hellwig <hch@lst.de> wrote:
> The description fails to mention the hacks how this is archived. I don't
> think the include hackery and otherwise needed exports are acceptable
> if with an explanation, though.

I've killed the include hackery.

> Really - get your fake hardware into qemu right now and don't bother
> with trying to hack the subsystem like this.

Rest assured that NFIT enabling for QEMU/KVM is well under way, but I
disagree that QEMU is the right place to enable unit tests. Here is
the updated changelog that appears in "[PATCH v6 20/21]
tools/testing/nvdimm: libnvdimm unit test infrastructure":

---

tools/testing/nvdimm: libnvdimm unit test infrastructure

'libnvdimm' is the first driver sub-system in the kernel to implement
mocking for unit test coverage. The nfit_test module gets built as an
external module and arranges for external module replacements of nfit,
libnvdimm, nd_pmem, and nd_blk. These replacements use the linker
--wrap option to redirect calls to ioremap() + request_mem_region() to
custom defined unit test resources. The end result is a fully
functional nvdimm_bus, as far as userspace is concerned, but with the
capability to perform otherwise destructive tests on emulated
resources.

Q: Why not use QEMU for this emulation?
QEMU is not suitable for unit testing. QEMU's role is to faithfully
emulate the platform. A unit test's role is to unfaithfully implement
the platform with the goal of triggering bugs in the corners of the
sub-system implementation. As bugs are discovered in platforms, or
the sub-system itself, the unit tests are extended to backstop a fix
with a reproducer unit test.

Another problem with QEMU is that it would require coordination of 3
software projects instead of 2 (kernel + libndctl [1]) to maintain and
execute the tests. The chances for bit rot and the difficulty of
getting the tests running goes up non-linearly the more components
involved.

Q: Why submit this to the kernel tree instead of external modules in libndctl?
Simple, to alleviate the same risk that out-of-tree external modules
face. Updates to drivers/nvdimm/ can be immediately evaluated to see
if they have any impact on tools/testing/nvdimm/.

[1]: https://github.com/pmem/ndctl