i386/cpu: Migrate MSR_IA32_PL0_SSP for FRED and CET-SHSTK

Both FRED and CET-SHSTK need MSR_IA32_PL0_SSP, so add the vmstate for
this MSR.

When CET-SHSTK is not supported, MSR_IA32_PL0_SSP keeps accessible, but
its value doesn't take effect. Therefore, treat this vmstate as a
subsection rather than a fix for the previous FRED vmstate.

Tested-by: Farrah Chen <farrah.chen@intel.com>
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Co-developed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20251211060801.3600039-17-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/i386/machine.c

@@ -1668,6 +1668,28 @@ static const VMStateDescription vmstate_triple_fault = {
     }
 };
 
+static bool pl0_ssp_needed(void *opaque)
+{
+    X86CPU *cpu = opaque;
+
+    /*
+     * CPUID_7_1_EAX_FRED and CPUID_7_0_ECX_CET_SHSTK are checked because
+     * if all of these bits are zero and the MSR will not be settable.
+     */
+    return !!(cpu->env.pl0_ssp);
+}
+
+static const VMStateDescription vmstate_pl0_ssp = {
+    .name = "cpu/msr_pl0_ssp",
+    .version_id = 1,
+    .minimum_version_id = 1,
+    .needed = pl0_ssp_needed,
+    .fields = (const VMStateField[]) {
+        VMSTATE_UINT64(env.pl0_ssp, X86CPU),
+        VMSTATE_END_OF_LIST()
+    }
+};
+
 const VMStateDescription vmstate_x86_cpu = {
     .name = "cpu",
     .version_id = 12,
@@ -1817,6 +1839,7 @@ const VMStateDescription vmstate_x86_cpu = {
 #endif
         &vmstate_arch_lbr,
         &vmstate_triple_fault,
+        &vmstate_pl0_ssp,
         NULL
     }
 };