Power11 has the same PCR (Processor Compatibility Register) value, as
Power10.

Due to this, QEMU considers Power11 as a valid compat-mode for Power10,
ie. earlier it was possible to run QEMU with '-M pseries,max-compat-mode=power11 --cpu power10'

Same PCR also introduced a regression where `-M pseries --cpu power10`
boots as Power11 (ie. logical PVR is of Power11, even though PVR is Power10).
The regression was due to 'do_client_architecture_support' checking for
valid compat modes and finding Power11 to be a valid compat mode for
Power10 (it happens even without passing 'max-compat-mode' explicitly).

Fix compat-mode issue and regression, by ensuring a future Power
processor (with a higher logical_pvr value, eg. P11) cannot be valid
compat-mode for an older Power processor (eg. P10)

Cc: Cédric Le Goater <clg@kaod.org>
Cc: Daniel Henrique Barboza <danielhb413@gmail.com>
Cc: Mahesh J Salgaonkar <mahesh@linux.ibm.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Aditya Gupta <adityag@linux.ibm.com>

---
Multiple alternative approaches were tried to fix this:

1. New PCR for Power11: No
2. 'Hacky fix': chose (n-1) entry in compat table in case of Power10.
  Commit: https://github.com/adi-g15-ibm/qemu/commit/c79bd667791883d4fb5e643979ae9c86a97bdf66
  Pros: Doesn't require adding a new field
  Cons: Hacky fix, will need change if Power12 comes with same PCR as
  Power10 (unlikely)
3. Setting current Processor's .logical_pvr as the max_compat to be checked
during CAS (Client Architecture Support).
  Commit: https://github.com/adi-g15-ibm/qemu/commit/30e430be71c6f3296017177298e54532570c2c87
  This causes CAS to not consider Power11 if cpu is Power10
---

---
 target/ppc/compat.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/target/ppc/compat.c b/target/ppc/compat.c
index 12dd8ae290ca..168a3c06316f 100644
--- a/target/ppc/compat.c
+++ b/target/ppc/compat.c
@@ -139,6 +139,10 @@ static bool pcc_compat(PowerPCCPUClass *pcc, uint32_t compat_pvr,
         /* Outside specified range */
         return false;
     }
+    if (compat->pvr > pcc->logical_pvr) {
+        /* Older CPU cannot support a newer processor's compat mode */
+        return false;
+    }
     if (!(pcc->pcr_supported & compat->pcr_level)) {
         /* Not supported by this CPU */
         return false;
-- 
2.45.1