• Dan Williams's avatar
    tools/testing/nvdimm: libnvdimm unit test infrastructure · 6bc75619
    Dan Williams authored
    '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/.
    
    
    Q: What are the negative implications of merging this?
    It is a unique maintenance burden because the purpose of mocking an
    interface to enable a unit test is to purposefully short circuit the
    semantics of a routine to enable testing.  For example
    __wrap_ioremap_cache() fakes the pmem driver into "ioremap()'ing" a test
    resource buffer allocated by dma_alloc_coherent().  The future
    maintenance burden hits when someone changes the semantics of
    ioremap_cache() and wonders what the implications are for the unit test.
    
    [1]: https://github.com/pmem/ndctl
    
    
    
    Cc: <linux-acpi@vger.kernel.org>
    Cc: Lv Zheng <lv.zheng@intel.com>
    Cc: Robert Moore <robert.moore@intel.com>
    Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
    Cc: Christoph Hellwig <hch@lst.de>
    Signed-off-by: default avatarDan Williams <dan.j.williams@intel.com>
    6bc75619