--- a/qemu/target-i386/cpu.h
+++ b/qemu/target-i386/cpu.h
@@ -405,6 +405,8 @@
 #define CPUID_EXT3_IBS     (1 << 10)
 #define CPUID_EXT3_SKINIT  (1 << 12)
 
+#define PRIVATE_FEATURE_A20MASK (1 << 0)
+
 #define CPUID_VENDOR_INTEL_1 0x756e6547 /* "Genu" */
 #define CPUID_VENDOR_INTEL_2 0x49656e69 /* "ineI" */
 #define CPUID_VENDOR_INTEL_3 0x6c65746e /* "ntel" */
@@ -685,6 +687,9 @@ typedef struct CPUX86State {
     uint32_t cpuid_apic_id;
     int cpuid_vendor_override;
 
+    /* private features not defined in CPUID */
+    uint32_t private_features;
+
     /* MTRRs */
     uint64_t mtrr_fixed[11];
     uint64_t mtrr_deftype;
diff --git a/qemu/target-i386/helper.c b/qemu/target-i386/helper.c
index c961544..3580740 100644
--- a/qemu/target-i386/helper.c
+++ b/qemu/target-i386/helper.c
@@ -57,11 +57,18 @@ static const char *ext3_feature_name[] = {
     NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL,
     NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL,
 };
+static const char *private_feature_name[] = {
+    "a20mask", NULL, NULL, NULL, NULL, NULL, NULL, NULL,
+    NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL,
+    NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL,
+    NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL,
+};
 
 static void add_flagname_to_bitmaps(const char *flagname, uint32_t *features,
                                     uint32_t *ext_features,
                                     uint32_t *ext2_features,
-                                    uint32_t *ext3_features)
+                                    uint32_t *ext3_features,
+                                    uint32_t *private_features)
 {
     int i;
     int found = 0;
@@ -86,6 +93,11 @@ static void add_flagname_to_bitmaps(const char *flagname, uint32_t *features,
             *ext3_features |= 1 << i;
             found = 1;
         }
+    for ( i = 0 ; i < 32 ; i++ )
+        if (private_feature_name[i] && !strcmp (flagname, private_feature_name[i])) {
+            *private_features |= 1 << i;
+            found = 1;
+        }
     if (!found) {
         fprintf(stderr, "CPU feature %s not found\n", flagname);
     }
@@ -99,6 +111,7 @@ typedef struct x86_def_t {
     int model;
     int stepping;
     uint32_t features, ext_features, ext2_features, ext3_features;
+    uint32_t private_features;
     uint32_t xlevel;
     char model_id[48];
     int vendor_override;
@@ -375,6 +388,7 @@ static int cpu_x86_find_by_name(x86_def_t *x86_cpu_def, const char *cpu_model)
     char *featurestr, *name = strtok(s, ",");
     uint32_t plus_features = 0, plus_ext_features = 0, plus_ext2_features = 0, plus_ext3_features = 0;
     uint32_t minus_features = 0, minus_ext_features = 0, minus_ext2_features = 0, minus_ext3_features = 0;
+    uint32_t plus_private_features = 0, minus_private_features = 0;
     uint32_t numvalue;
 
     def = NULL;
@@ -393,16 +407,21 @@ static int cpu_x86_find_by_name(x86_def_t *x86_cpu_def, const char *cpu_model)
     }
 
     add_flagname_to_bitmaps("hypervisor", &plus_features,
-        &plus_ext_features, &plus_ext2_features, &plus_ext3_features);
+        &plus_ext_features, &plus_ext2_features, &plus_ext3_features,
+        &plus_private_features);
 
     featurestr = strtok(NULL, ",");
 
     while (featurestr) {
         char *val;
         if (featurestr[0] == '+') {
-            add_flagname_to_bitmaps(featurestr + 1, &plus_features, &plus_ext_features, &plus_ext2_features, &plus_ext3_features);
+            add_flagname_to_bitmaps(featurestr + 1, &plus_features,
+                &plus_ext_features, &plus_ext2_features, &plus_ext3_features,
+                &plus_private_features);
         } else if (featurestr[0] == '-') {
-            add_flagname_to_bitmaps(featurestr + 1, &minus_features, &minus_ext_features, &minus_ext2_features, &minus_ext3_features);
+            add_flagname_to_bitmaps(featurestr + 1, &minus_features,
+                &minus_ext_features, &minus_ext2_features, &minus_ext3_features,
+                &minus_private_features);
         } else if ((val = strchr(featurestr, '='))) {
             *val = 0; val++;
             if (!strcmp(featurestr, "family")) {
@@ -479,10 +498,12 @@ static int cpu_x86_find_by_name(x86_def_t *x86_cpu_def, const char *cpu_model)
     x86_cpu_def->ext_features |= plus_ext_features;
     x86_cpu_def->ext2_features |= plus_ext2_features;
     x86_cpu_def->ext3_features |= plus_ext3_features;
+    x86_cpu_def->private_features |= plus_private_features;
     x86_cpu_def->features &= ~minus_features;
     x86_cpu_def->ext_features &= ~minus_ext_features;
     x86_cpu_def->ext2_features &= ~minus_ext2_features;
     x86_cpu_def->ext3_features &= ~minus_ext3_features;
+    x86_cpu_def->private_features &= ~minus_private_features;
     free(s);
     return 0;
 
@@ -528,6 +549,7 @@ static int cpu_x86_register (CPUX86State *env, const char *cpu_model)
     env->cpuid_ext2_features = def->ext2_features;
     env->cpuid_xlevel = def->xlevel;
     env->cpuid_ext3_features = def->ext3_features;
+    env->private_features = def->private_features;
     {
         const char *model_id = def->model_id;
         int c, len, i;
@@ -569,7 +591,11 @@ void cpu_reset(CPUX86State *env)
     env->hflags2 |= HF2_GIF_MASK;
 
     cpu_x86_update_cr0(env, 0x60000010);
-    env->a20_mask = ~0x0;
+    if (env->private_features & PRIVATE_FEATURE_A20MASK) {
+        env->a20_mask = 0xfffff;
+    } else {
+        env->a20_mask = ~0x0;
+    }
     env->smbase = 0x30000;
 
     env->idt.limit = 0xffff;
@@ -938,7 +964,15 @@ void cpu_x86_set_a20(CPUX86State *env, int a20_state)
         /* when a20 is changed, all the MMU mappings are invalid, so
            we must flush everything */
         tlb_flush(env, 1);
-        env->a20_mask = ~(1 << 20) | (a20_state << 20);
+        if (env->private_features & PRIVATE_FEATURE_A20MASK) {
+            if (a20_state) {
+                env->a20_mask = ~0x0;
+            } else {
+                env->a20_mask = 0xfffff;
+            }
+        } else {
+            env->a20_mask = ~(1 << 20) | (a20_state << 20);
+        }
     }
 }
 
