iommu/arm-smmu-v3: Make the kunit into a module

It turns out kconfig has problems ensuring the SMMU module and the KUNIT
module are consistently y/m to allow linking. It will permit KUNIT to be a
module while SMMU is built in.

Also, Fedora apparently enables kunit on production kernels.

So, put the entire kunit in its own module using the
VISIBLE_IF_KUNIT/EXPORT_SYMBOL_IF_KUNIT machinery. This keeps it out of
vmlinus on Fedora and makes the kconfig work in the normal way. There is
no cost if kunit is disabled.

Fixes: 56e1a4cc2588 ("iommu/arm-smmu-v3: Add unit tests for arm_smmu_write_entry")
Reported-by: Thorsten Leemhuis <linux@leemhuis.info>
Link: https://lore.kernel.org/all/aeea8546-5bce-4c51-b506-5d2008e52fef@leemhuis.info
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Tested-by: Thorsten Leemhuis <linux@leemhuis.info>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/0-v1-24cba6c0f404+2ae-smmu_kunit_module_jgg@nvidia.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>

iommu/arm-smmu-v3: Add unit tests for arm_smmu_write_entry

Add tests for some of the more common STE update operations that we expect
to see, as well as some artificial STE updates to test the edges of
arm_smmu_write_entry. These also serve as a record of which common
operation is expected to be hitless, and how many syncs they require.

arm_smmu_write_entry implements a generic algorithm that updates an STE/CD
to any other abritrary STE/CD configuration. The update requires a
sequence of write+sync operations with some invariants that must be held
true after each sync. arm_smmu_write_entry lends itself well to
unit-testing since the function's interaction with the STE/CD is already
abstracted by input callbacks that we can hook to introspect into the
sequence of operations. We can use these hooks to guarantee that
invariants are held throughout the entire update operation.

Link: https://lore.kernel.org/r/20240106083617.1173871-3-mshavit@google.com
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Michael Shavit <mshavit@google.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/9-v9-5040dc602008+177d7-smmuv3_newapi_p2_jgg@nvidia.com
Signed-off-by: Will Deacon <will@kernel.org>