From: Vaibhav Jain <redacted>

Currently we see a kernel-oops reported on Power-9 while attaching a
context to an AFU, with radix-mode and sysfs attr 'prefault_mode' set
to anything other than 'none'. The backtrace of the oops is of this
form:

Unable to handle kernel paging request for data at address 0x00000080
Faulting instruction address: 0xc00800000bcf3b20
cpu 0x1: Vector: 300 (Data Access) at [c00000037f003800]
    pc: c00800000bcf3b20: cxl_load_segment+0x178/0x290 [cxl]
    lr: c00800000bcf39f0: cxl_load_segment+0x48/0x290 [cxl]
    sp: c00000037f003a80
   msr: 9000000000009033
   dar: 80
 dsisr: 40000000
  current = 0xc00000037f280000
  paca    = 0xc0000003ffffe600   softe: 3        irq_happened: 0x01
    pid   = 3529, comm = afp_no_int
<snip>
[c00000037f003af0] c00800000bcf4424 cxl_prefault+0xfc/0x248 [cxl]
[c00000037f003b50] c00800000bcf8a40 process_element_entry_psl9+0xd8/0x1a0 [cxl]
[c00000037f003b90] c00800000bcf944c cxl_attach_dedicated_process_psl9+0x44/0x130 [cxl]
[c00000037f003bd0] c00800000bcf5448 native_attach_process+0xc0/0x130 [cxl]
[c00000037f003c50] c00800000bcf16cc afu_ioctl+0x3f4/0x5e0 [cxl]
[c00000037f003d00] c00000000039d98c do_vfs_ioctl+0xdc/0x890
[c00000037f003da0] c00000000039e1a8 ksys_ioctl+0x68/0xf0
[c00000037f003df0] c00000000039e270 sys_ioctl+0x40/0xa0
[c00000037f003e30] c00000000000b320 system_call+0x58/0x6c

--- Exception: c01 (System Call) at 0000000010053bb0

The issue is caused as on Power-8 the AFU attr 'prefault_mode' was
used to improve initial storage fault performance by prefaulting
process segments. However on Power-9 with radix mode we don't have
Storage-Segments that we can prefault. Also prefaulting process Pages
will be too costly and fine-grained.

Hence, since the prefaulting mechanism doesn't makes sense of
radix-mode, this patch updates prefault_mode_store() to not allow any
other value apart from CXL_PREFAULT_NONE when radix mode is enabled.

Cc: <redacted>
Fixes: f24be42aab37 ("cxl: Add psl9 specific code")
Signed-off-by: Vaibhav Jain <redacted>
Acked-by: Frederic Barrat <redacted>
Acked-by: Andrew Donnellan <redacted>