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

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Date:	Thu, 11 Jun 2015 13:12:32 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Christoph Hellwig <hch@....de>
Cc:	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	Jens Axboe <axboe@...nel.dk>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Neil Brown <neilb@...e.de>,
	Greg KH <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linux ACPI <linux-acpi@...r.kernel.org>,
	jmoyer <jmoyer@...hat.com>, linux-api@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v5 20/21] tools/testing/nvdimm: manufactured NFITs for
 interface development

On Mon, Jun 8, 2015 at 11:48 PM, Christoph Hellwig <hch@....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
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