From: Eric Auger <eric.auger@redhat.com>

Whenever there is a mismatch between cpreg indexes in the incoming
stream and cpregs exposed by the destination output the name of
the register. We use a print_register_name() wrapper helper. At the
moment we are only able to do a nice decoding of the index for
KVM regs.

Without this patch, the error would be:
qemu-system-aarch64: load of migration failed: Operation not permitted:
error while loading state for instance 0x0 of device 'cpu': post load
hook failed for: cpu, version_id: 22, minimum_version: 22, ret: -1
which is not helpful for the end user to understand the actual
issue.

This patch adds the actual information about the probme:
qemu-system-aarch64: cpu_post_load: system register
op0:3 op1:0 crn:2 crm:0 op2:3 in the incoming stream but
unknown on the destination, fail migration

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20260304101625.1962633-6-eric.auger@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 target/arm/machine.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/target/arm/machine.c b/target/arm/machine.c
index d3d4f2ddc1..d9b65b5eed 100644
--- a/target/arm/machine.c
+++ b/target/arm/machine.c
@@ -1,5 +1,6 @@
 #include "qemu/osdep.h"
 #include "cpu.h"
+#include "cpregs.h"
 #include "trace.h"
 #include "qemu/error-report.h"
 #include "system/kvm.h"
@@ -1047,6 +1048,15 @@ static int cpu_pre_load(void *opaque)
     return 0;
 }
 
+static gchar *print_register_name(uint64_t kvm_regidx)
+{
+    if (kvm_enabled()) {
+        return kvm_print_register_name(kvm_regidx);
+    } else {
+        return g_strdup_printf("system register 0x%x", kvm_to_cpreg_id(kvm_regidx));
+    }
+}
+
 static int cpu_post_load(void *opaque, int version_id)
 {
     ARMCPU *cpu = opaque;
@@ -1085,11 +1095,18 @@ static int cpu_post_load(void *opaque, int version_id)
     for (i = 0, v = 0; i < cpu->cpreg_array_len
              && v < cpu->cpreg_vmstate_array_len; i++) {
         if (cpu->cpreg_vmstate_indexes[v] > cpu->cpreg_indexes[i]) {
-            /* register in our list but not incoming : skip it */
+            g_autofree gchar *name = print_register_name(cpu->cpreg_indexes[i]);
+
+            warn_report("%s: %s "
+                        "expected by the destination but not in the incoming stream: "
+                        "skip it", __func__, name);
             continue;
         }
         if (cpu->cpreg_vmstate_indexes[v] < cpu->cpreg_indexes[i]) {
-            /* register in their list but not ours: fail migration */
+            g_autofree gchar *name = print_register_name(cpu->cpreg_vmstate_indexes[v]);
+
+            error_report("%s: %s in the incoming stream but unknown on the destination: "
+                         "fail migration", __func__, name);
             return -1;
         }
         /* matching register, copy the value over */
-- 
2.43.0