RTAS runs in real mode (MSR[DR] and MSR[IR] unset) and in 32-bit big
endian mode (MSR[SF,LE] unset).

The change in MSR is done in enter_rtas() in a relatively complex way,
since the MSR value could be hardcoded.

Furthermore, a panic has been reported when hitting the watchdog interrupt
while running in RTAS, this leads to the following stack trace:

  watchdog: CPU 24 Hard LOCKUP
  watchdog: CPU 24 TB:997512652051031, last heartbeat TB:997504470175378 (15980ms ago)
  ...
  Supported: No, Unreleased kernel
  CPU: 24 PID: 87504 Comm: drmgr Kdump: loaded Tainted: G            E  X    5.14.21-150400.71.1.bz196362_2-default #1 SLE15-SP4 (unreleased) 0d821077ef4faa8dfaf370efb5fdca1fa35f4e2c
  NIP:  000000001fb41050 LR: 000000001fb4104c CTR: 0000000000000000
  REGS: c00000000fc33d60 TRAP: 0100   Tainted: G            E  X     (5.14.21-150400.71.1.bz196362_2-default)
  MSR:  8000000002981000 <SF,VEC,VSX,ME>  CR: 48800002  XER: 20040020
  CFAR: 000000000000011c IRQMASK: 1
  GPR00: 0000000000000003 ffffffffffffffff 0000000000000001 00000000000050dc
  GPR04: 000000001ffb6100 0000000000000020 0000000000000001 000000001fb09010
  GPR08: 0000000020000000 0000000000000000 0000000000000000 0000000000000000
  GPR12: 80040000072a40a8 c00000000ff8b680 0000000000000007 0000000000000034
  GPR16: 000000001fbf6e94 000000001fbf6d84 000000001fbd1db0 000000001fb3f008
  GPR20: 000000001fb41018 ffffffffffffffff 000000000000017f fffffffffffff68f
  GPR24: 000000001fb18fe8 000000001fb3e000 000000001fb1adc0 000000001fb1cf40
  GPR28: 000000001fb26000 000000001fb460f0 000000001fb17f18 000000001fb17000
  NIP [000000001fb41050] 0x1fb41050
  LR [000000001fb4104c] 0x1fb4104c
  Call Trace:
  Instruction dump:
  XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
  XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
  Oops: Unrecoverable System Reset, sig: 6 [#1]
  LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA pSeries
  ...
  Supported: No, Unreleased kernel
  CPU: 24 PID: 87504 Comm: drmgr Kdump: loaded Tainted: G            E  X    5.14.21-150400.71.1.bz196362_2-default #1 SLE15-SP4 (unreleased) 0d821077ef4faa8dfaf370efb5fdca1fa35f4e2c
  NIP:  000000001fb41050 LR: 000000001fb4104c CTR: 0000000000000000
  REGS: c00000000fc33d60 TRAP: 0100   Tainted: G            E  X     (5.14.21-150400.71.1.bz196362_2-default)
  MSR:  8000000002981000 <SF,VEC,VSX,ME>  CR: 48800002  XER: 20040020
  CFAR: 000000000000011c IRQMASK: 1
  GPR00: 0000000000000003 ffffffffffffffff 0000000000000001 00000000000050dc
  GPR04: 000000001ffb6100 0000000000000020 0000000000000001 000000001fb09010
  GPR08: 0000000020000000 0000000000000000 0000000000000000 0000000000000000
  GPR12: 80040000072a40a8 c00000000ff8b680 0000000000000007 0000000000000034
  GPR16: 000000001fbf6e94 000000001fbf6d84 000000001fbd1db0 000000001fb3f008
  GPR20: 000000001fb41018 ffffffffffffffff 000000000000017f fffffffffffff68f
  GPR24: 000000001fb18fe8 000000001fb3e000 000000001fb1adc0 000000001fb1cf40
  GPR28: 000000001fb26000 000000001fb460f0 000000001fb17f18 000000001fb17000
  NIP [000000001fb41050] 0x1fb41050
  LR [000000001fb4104c] 0x1fb4104c
  Call Trace:
  Instruction dump:
  XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
  XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
  ---[ end trace 3ddec07f638c34a2 ]---

This happens because MSR[RI] is unset when entering RTAS but there is no
valid reason to not set it here.

RTAS is expected to be called with MSR[RI] as specified in PAPR+ section
"7.2.1 Machine State":

  R1–7.2.1–9. If called with MSR[RI] equal to 1, then RTAS must protect
  its own critical regions from recursion by setting the MSR[RI] bit to
  0 when in the critical regions.

Fixing this by reviewing the way MSR is compute before calling RTAS. Now a
hardcoded value meaning real mode, 32 bits big endian mode and Recoverable
Interrupt is loaded. In the case MSR[S] is set, it will remain set while
entering RTAS as only urfid can unset it (thanks Fabiano).

In addition a check is added in do_enter_rtas() to detect calls made with
MSR[RI] unset, as we are forcing it on later.

This patch has been tested on the following machines:
Power KVM Guest
  P8 S822L (host Ubuntu kernel 5.11.0-49-generic)
PowerVM LPAR
  P8 9119-MME (FW860.A1)
  p9 9008-22L (FW950.00)
  P10 9080-HEX (FW1010.00)
Suggested-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220504101244.12107-1-ldufour@linux.ibm.com