[Qemu-devel] [PATCH 1/4] ppc: Remove broken partial PVR matching

[David Gibson <david@gibson.dropbear.id.au>]

The ppc target contains a ppc_find_by_pvr() function, which looks up a
CPU spec based on a PVR (that is, based on the value in the target cpu's
Processor Version Register).  PVR values contain information on both the
cpu model (upper 16 bits, usually) and on the precise revision (low 16
bits, usually).

ppc_find_by_pvr, as well as making exact PVR matches, attempts to find
"close" PVR matches, when we don't have a CPU spec for the exact revision
specified.  This sounds like a good idea, execpt that the current logic
is completely nonsensical.

It seems to assume CPU families are subdivided bit by bit in the PVR in a
way they just aren't.  Specifically, it requires a match on all bits of the
specified pvr up to the last non-zero bit.  This has the bizarre effect
that when the low bits are simply a sequential revision number (a common
though not universal pattern), then odd specified revisions must be matched
exactly, whereas even specified revisions will also match the next odd
revision, likewise for powers of 4, 8 and so forth.

To correctly do inexact matching we'd need to re-organize the table of CPU
specs to include a mask showing what PVR range the spec is compatible with
(similar to the cputable code in the Linux kernel).

For now, just remove the bogosity by only permitting exact PVR matches.
That at least makes the matching simple and consistent.  If we need inexact
matching we can add the necessary per-subfamily masks later.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 target-ppc/translate_init.c |   38 +++++++-------------------------------
 1 files changed, 7 insertions(+), 31 deletions(-)

diff --git a/target-ppc/translate_init.c b/target-ppc/translate_init.c
index ca0d852..73b49cf 100644
--- a/target-ppc/translate_init.c
+++ b/target-ppc/translate_init.c
@@ -10043,40 +10043,16 @@ int cpu_ppc_register_internal (CPUPPCState *env, const ppc_def_t *def)
 
 static const ppc_def_t *ppc_find_by_pvr (uint32_t pvr)
 {
-    const ppc_def_t *ret;
-    uint32_t pvr_rev;
-    int i, best, match, best_match, max;
+    int i;
 
-    ret = NULL;
-    max = ARRAY_SIZE(ppc_defs);
-    best = -1;
-    pvr_rev = pvr & 0xFFFF;
-    /* We want all specified bits to match */
-    best_match = 32 - ctz32(pvr_rev);
-    for (i = 0; i < max; i++) {
-        /* We check that the 16 higher bits are the same to ensure the CPU
-         * model will be the choosen one.
-         */
-        if (((pvr ^ ppc_defs[i].pvr) >> 16) == 0) {
-            /* We want as much as possible of the low-level 16 bits
-             * to be the same but we allow inexact matches.
-             */
-            match = clz32(pvr_rev ^ (ppc_defs[i].pvr & 0xFFFF));
-            /* We check '>=' instead of '>' because the PPC_defs table
-             * is ordered by increasing revision.
-             * Then, we will match the higher revision compatible
-             * with the requested PVR
-             */
-            if (match >= best_match) {
-                best = i;
-                best_match = match;
-            }
+    for (i = 0; i < ARRAY_SIZE(ppc_defs); i++) {
+        /* If we have an exact match, we're done */
+        if (pvr == ppc_defs[i].pvr) {
+            return &ppc_defs[i];
         }
     }
-    if (best != -1)
-        ret = &ppc_defs[best];
 
-    return ret;
+    return NULL;
 }
 
 #include <ctype.h>
-- 
1.7.6.3