Add support for OCP protocol

[n-ramacciotti]

RFC PR for observability and configuration protocol

This is the linux-side handler.

It is still a work in progress. For now it is only used to test SVSM_OCP_LIST request.
It contains the platform device and driver registration + all the ocp calls implementation.

This contains a [test] commit with a module that can be loaded and will do a single call to SVSM_OCP_LIST during initialization. The parameter of this call can be changed based on a configuration parameter that can be specified when inserting the module. This can be inserted and removed multiple times.

see PR coconut-svsm/svsm#1138

[n-ramacciotti]

v2:

• Introduced support for read and write
• Introduced testing for write operation

[n-ramacciotti]

v3:

• updated svsm calls to account for the two list functionalities
• add platform driver with basic registration and memory allocation (similarly to vtpm)
• add platform device (similarly to vtpm)

TODO:

• add ioctl interface

[n-ramacciotti]

v4:

• added ioctl interface
• formatted based on scripts/checkpatch.pl feedback:
  • used kzalloc_obj instead of kzalloc
  • other formatting issues

Tested locally (without SNP, but with fake replies and a very basic userspace application)

TODO:

• test with snp
• Introduce userspace application (will be in a separate repository)
• for the moment one ioctl call is one svsm call, but it is possible to query svsm multiple times if user requires more bytes than max allowed in a single svsm call
• return bytes_read/written, number of sources/object as ioctl return value instead of 0?
• write call in ioctl is a copy-paste of read and should be fixed
• Does it make sense to keep "Source" and "Details" in uapi? As these are not used by kernel but only by svsm and guest

[n-ramacciotti]

v5:

• added WITH Linux-syscall-note in ocp_svsm.h
• fixed typo in list_object_sources where list_objects was used instead of list_sources
• Add support for missing struct in uapi (object_details)
• Add size assertion in uapi
• Fixed write source call

Other todo in #20 (comment) are open