[PATCH RFC 0/2] powerpc/pseries: new character devices for RTAS functions

[Nathan Lynch via B4 Relay <devnull+nathanl.linux.ibm.com@kernel.org>]

This is a proposal for adding chardev-based access to a select subset
of RTAS functions on the pseries platform.

The problem: important platform features are enabled on Linux VMs
through the powerpc-specific rtas() syscall in combination with
writeable mappings of /dev/mem. In typical usage, this is encapsulated
behind APIs provided by the librtas library. This paradigm is
incompatible with lockdown, which prohibits /dev/mem access.

The solution I'm working on is to add a small pseries-specific
"driver" for each functional area, exposing the relevant features to
user space in ways that are compatible with lockdown. In most of these
areas, I believe it's possible to change librtas to prefer the new
chardev interfaces without disrupting existing users.

I've broken down the affected functions into the following areas and
priorities:

High priority:
* VPD retrieval.
* System parameters: retrieval and update.

Medium priority:
* Platform dump retrieval.
* Light path diagnostics (get/set-dynamic-indicator,
  get-dynamic-sensor-state, get-indices).

Low priority (may never happen):
* Error injection: would have to be carefully restricted.
* Physical attestation: no known users.
* LPAR perftools: no known users.

Out of scope:
* DLPAR (configure-connector et al): involves device tree updates
  which must be handled entirely in-kernel for lockdown. This is the
  object of a separate effort.

See https://github.com/ibm-power-utilities/librtas/issues/29 for more
details.

In this RFC, I've included a single driver for VPD retrieval. Clients
use ioctl() to obtain a file descriptor-based handle for the VPD they
want. I think this could be a good model for the other areas too, but
I'd like to get opinions on it.

In the next iteration I expect to add a separate driver for system
parameters.

For reference, I floated a different approach for system parameters
here:

https://lore.kernel.org/linuxppc-dev/20220730000458.130938-1-nathanl@linux.ibm.com/

---
Nathan Lynch (2):
      powerpc/pseries: papr-vpd char driver for VPD retrieval
      powerpc/selftests: add test for papr-vpd

 Documentation/userspace-api/ioctl/ioctl-number.rst |   2 +
 arch/powerpc/include/uapi/asm/papr-vpd.h           |  29 ++
 arch/powerpc/platforms/pseries/Makefile            |   1 +
 arch/powerpc/platforms/pseries/papr-vpd.c          | 353 +++++++++++++++++++++
 tools/testing/selftests/powerpc/Makefile           |   1 +
 .../testing/selftests/powerpc/papr_vpd/.gitignore  |   1 +
 tools/testing/selftests/powerpc/papr_vpd/Makefile  |  12 +
 .../testing/selftests/powerpc/papr_vpd/papr_vpd.c  | 351 ++++++++++++++++++++
 8 files changed, 750 insertions(+)
---
base-commit: d77497508a229529830850ba07e1e52596463d21
change-id: 20230817-papr-sys_rtas-vs-lockdown-5c54505db792

Best regards,
-- 
Nathan Lynch <nathanl@linux.ibm.com>