powerpc/pseries: Add papr-platform-dump character driver for dump retrieval

ibm,platform-dump RTAS call in combination with writable mapping
/dev/mem is issued to collect platform dump from the hypervisor
and may need multiple calls to get the complete dump. The current
implementation uses rtas_platform_dump() API provided by librtas
library to issue these RTAS calls. But /dev/mem access by the
user space is prohibited under system lockdown.

The solution should be to restrict access to RTAS function in user
space and provide kernel interfaces to collect dump. This patch
adds papr-platform-dump character driver and expose standard
interfaces such as open / ioctl/ read to user space in ways that
are compatible with lockdown.

PAPR (7.3.3.4.1 ibm,platform-dump) provides a method to obtain
the complete dump:
- Each dump will be identified by ID called dump tag.
- A sequence of RTAS calls have to be issued until retrieve the
  complete dump. The hypervisor expects the first RTAS call with
  the sequence 0 and the subsequent calls with the sequence
  number returned from the previous calls.
- The hypervisor returns "dump complete" status once the complete
  dump is retrieved. But expects one more RTAS call from the
  partition with the NULL buffer to invalidate dump which means
  the dump will be removed in the hypervisor.
- Sequence of calls are allowed with different dump IDs at the
  same time but not with the same dump ID.

Expose these interfaces to user space with a /dev/papr-platform-dump
character device using the following programming model:

   int devfd = open("/dev/papr-platform-dump", O_RDONLY);
   int fd = ioctl(devfd,PAPR_PLATFORM_DUMP_IOC_CREATE_HANDLE, &dump_id)
	- Restrict user space to access with the same dump ID.
          Typically we do not expect user space requests the dump
          again for the same dump ID.
   char *buf = malloc(size);
   length = read(fd, buf, size);
        - size should be minimum 1K based on PAPR and  <= 4K based
          on RTAS work area size. It will be restrict to RTAS work
          area size. Using 4K work area based on the current
          implementation in librtas library
        - Each read call issue RTAS call to get the data based on
          the size requirement and returns bytes returned from the
          hypervisor
        - If the previous call returns dump complete status, the
          next read returns 0 like EOF.
   ret = ioctl(PAPR_PLATFORM_DUMP_IOC_INVALIDATE, &dump_id)
	- RTAS call with NULL buffer to invalidates the dump.

The read API should use the file descriptor obtained from ioctl
based on dump ID so that gets dump contents for the corresponding
dump ID. Implemented support in librtas (rtas_platform_dump()) for
this new ABI to support system lockdown.

Signed-off-by: Haren Myneni <haren@linux.ibm.com>
Tested-by: Sathvika Vasireddy <sv@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250416225743.596462-7-haren@linux.ibm.com

0 files changed, 0 insertions, 0 deletions