From: Xin Wang <wangxinxin.wang@huawei.com>

For now, qemu save/load CPU exception info(such as exception_nr and
has_error_code), while the exception error_code is ignored. This will
cause the dest hypervisor reinject a vCPU exception with error_code(0),
potentially causing a guest kernel panic.

For instance, if src VM stopped with an user-mode write #PF (error_code 6),
the dest hypervisor will reinject an #PF with error_code(0) when vCPU resume,
then guest kernel panic as:
  BUG: unable to handle page fault for address: 00007f80319cb010
  #PF: supervisor read access in user mode
  #PF: error_code(0x0000) - not-present page
  RIP: 0033:0x40115d

To fix it, support save/load exception error_code.

Signed-off-by: Xin Wang <wangxinxin.wang@huawei.com>
Link: https://lore.kernel.org/r/20250819145834.3998-1-wangxinxin.wang@huawei.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 target/i386/machine.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/target/i386/machine.c b/target/i386/machine.c
index dd2dac1d443..45b7cea80aa 100644
--- a/target/i386/machine.c
+++ b/target/i386/machine.c
@@ -462,6 +462,24 @@ static const VMStateDescription vmstate_exception_info = {
     }
 };
 
+static bool cpu_errcode_needed(void *opaque)
+{
+    X86CPU *cpu = opaque;
+
+    return cpu->env.has_error_code != 0;
+}
+
+static const VMStateDescription vmstate_error_code = {
+    .name = "cpu/error_code",
+    .version_id = 1,
+    .minimum_version_id = 1,
+    .needed = cpu_errcode_needed,
+    .fields = (const VMStateField[]) {
+        VMSTATE_INT32(env.error_code, X86CPU),
+        VMSTATE_END_OF_LIST()
+    }
+};
+
 /* Poll control MSR enabled by default */
 static bool poll_control_msr_needed(void *opaque)
 {
@@ -1746,6 +1764,7 @@ const VMStateDescription vmstate_x86_cpu = {
     },
     .subsections = (const VMStateDescription * const []) {
         &vmstate_exception_info,
+        &vmstate_error_code,
         &vmstate_async_pf_msr,
         &vmstate_async_pf_int_msr,
         &vmstate_pv_eoi_msr,
-- 
2.51.0