Fix real mode guest segments dpl value in savevm

Older KVM version put invalid value in the segments registers dpl field for
real mode guests (0x3).
This breaks migration from those hosts to hosts with unrestricted guest support.
We detect it by checking CS dpl value for real mode guest and fix the dpl values
of all the segment registers.
Signed-off-by: NOrit Wasserman <owasserm@redhat.com>
Reviewed-by: NJuan Quintela <quintela@redhat.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>

target-i386/machine.c

@@ -252,6 +252,24 @@ static void cpu_pre_save(void *opaque)
     }
 
     env->fpregs_format_vmstate = 0;
+
+    /*
+     * Real mode guest segments register DPL should be zero.
+     * Older KVM version were setting it wrongly.
+     * Fixing it will allow live migration to host with unrestricted guest
+     * support (otherwise the migration will fail with invalid guest state
+     * error).
+     */
+    if (!(env->cr[0] & CR0_PE_MASK) &&
+        (env->segs[R_CS].flags >> DESC_DPL_SHIFT & 3) != 0) {
+        env->segs[R_CS].flags &= ~(env->segs[R_CS].flags & DESC_DPL_MASK);
+        env->segs[R_DS].flags &= ~(env->segs[R_DS].flags & DESC_DPL_MASK);
+        env->segs[R_ES].flags &= ~(env->segs[R_ES].flags & DESC_DPL_MASK);
+        env->segs[R_FS].flags &= ~(env->segs[R_FS].flags & DESC_DPL_MASK);
+        env->segs[R_GS].flags &= ~(env->segs[R_GS].flags & DESC_DPL_MASK);
+        env->segs[R_SS].flags &= ~(env->segs[R_SS].flags & DESC_DPL_MASK);
+    }
+
 }
 
 static int cpu_post_load(void *opaque, int version_id)