[Qemu-devel] [PATCH v2] target/s390x/cpu_models: Allow some additional feature bits for the "qemu" CPU

[Qemu-devel] [PATCH v2] target/s390x/cpu_models: Allow some additional feature bits for the "qemu" CPU

[Thomas Huth]

Currently we only present the plain z900 feature bits to the guest,
but QEMU already emulates some additional features (but not all of
the next CPU generation, so we can not use the next CPU level as
default yet). Since newer Linux kernels are checking the feature bits
and refuse to work if a required feature is missing, it would be nice
to have a way to present more of the supported features when we are
running with the "qemu" CPU.
This patch now adds the supported features to the "full_feat" bitmap,
so that additional features can be enabled on the command line now,
for example with:

 qemu-system-s390x -cpu qemu,stfle=true,ldisp=true,eimm=true,stckf=true

Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 v2: Mark feats array with "static const"

 target/s390x/cpu_models.c | 34 +++++++++++++++++++++++++++++++---
 1 file changed, 31 insertions(+), 3 deletions(-)

diff --git a/target/s390x/cpu_models.c b/target/s390x/cpu_models.c
index 8d27363..e5e005a 100644
--- a/target/s390x/cpu_models.c
+++ b/target/s390x/cpu_models.c
@@ -658,6 +658,30 @@ static void check_compatibility(const S390CPUModel *max_model,
                   "available in the configuration: ");
 }
 
+/**
+ * The base TCG CPU model "qemu" is based on the z900. However, we already
+ * can also emulate some additional features of later CPU generations, so
+ * we add these additional feature bits here.
+ */
+static void add_qemu_cpu_model_features(S390FeatBitmap fbm)
+{
+    static const int feats[] = {
+        S390_FEAT_STFLE,
+        S390_FEAT_EXTENDED_IMMEDIATE,
+        S390_FEAT_LONG_DISPLACEMENT,
+        S390_FEAT_LONG_DISPLACEMENT_FAST,
+        S390_FEAT_STORE_CLOCK_FAST,
+        S390_FEAT_GENERAL_INSTRUCTIONS_EXT,
+        S390_FEAT_EXECUTE_EXT,
+        S390_FEAT_STFLE_45,
+    };
+    int i;
+
+    for (i = 0; i < ARRAY_SIZE(feats); i++) {
+        set_bit(feats[i], fbm);
+    }
+}
+
 static S390CPUModel *get_max_cpu_model(Error **errp)
 {
     static S390CPUModel max_model;
@@ -670,10 +694,11 @@ static S390CPUModel *get_max_cpu_model(Error **errp)
     if (kvm_enabled()) {
         kvm_s390_get_host_cpu_model(&max_model, errp);
     } else {
-        /* TCG emulates a z900 */
+        /* TCG emulates a z900 (with some optional additional features) */
         max_model.def = &s390_cpu_defs[0];
         bitmap_copy(max_model.features, max_model.def->default_feat,
                     S390_FEAT_MAX);
+        add_qemu_cpu_model_features(max_model.features);
     }
     if (!*errp) {
         cached = true;
@@ -925,11 +950,14 @@ static void s390_host_cpu_model_initfn(Object *obj)
 
 static void s390_qemu_cpu_model_initfn(Object *obj)
 {
+    static S390CPUDef s390_qemu_cpu_defs;
     S390CPU *cpu = S390_CPU(obj);
 
     cpu->model = g_malloc0(sizeof(*cpu->model));
-    /* TCG emulates a z900 */
-    cpu->model->def = &s390_cpu_defs[0];
+    /* TCG emulates a z900 (with some optional additional features) */
+    memcpy(&s390_qemu_cpu_defs, &s390_cpu_defs[0], sizeof(s390_qemu_cpu_defs));
+    add_qemu_cpu_model_features(s390_qemu_cpu_defs.full_feat);
+    cpu->model->def = &s390_qemu_cpu_defs;
     bitmap_copy(cpu->model->features, cpu->model->def->default_feat,
                 S390_FEAT_MAX);
 }
-- 
1.8.3.1

Re: [Qemu-devel] [PATCH v2] target/s390x/cpu_models: Allow some additional feature bits for the "qemu" CPU

[Aurelien Jarno]

On 2017-05-25 11:22, Thomas Huth wrote:
> Currently we only present the plain z900 feature bits to the guest,
> but QEMU already emulates some additional features (but not all of
> the next CPU generation, so we can not use the next CPU level as
> default yet). Since newer Linux kernels are checking the feature bits
> and refuse to work if a required feature is missing, it would be nice
> to have a way to present more of the supported features when we are
> running with the "qemu" CPU.
> This patch now adds the supported features to the "full_feat" bitmap,
> so that additional features can be enabled on the command line now,
> for example with:
> 
>  qemu-system-s390x -cpu qemu,stfle=true,ldisp=true,eimm=true,stckf=true
> 
> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
>  v2: Mark feats array with "static const"
> 
>  target/s390x/cpu_models.c | 34 +++++++++++++++++++++++++++++++---
>  1 file changed, 31 insertions(+), 3 deletions(-)

Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien@aurel32.net                 http://www.aurel32.net

Re: [Qemu-devel] [PATCH v2] target/s390x/cpu_models: Allow some additional feature bits for the "qemu" CPU

[David Hildenbrand]

On 25.05.2017 11:22, Thomas Huth wrote:
> Currently we only present the plain z900 feature bits to the guest,
> but QEMU already emulates some additional features (but not all of
> the next CPU generation, so we can not use the next CPU level as
> default yet). Since newer Linux kernels are checking the feature bits
> and refuse to work if a required feature is missing, it would be nice
> to have a way to present more of the supported features when we are
> running with the "qemu" CPU.
> This patch now adds the supported features to the "full_feat" bitmap,
> so that additional features can be enabled on the command line now,
> for example with:
> 
>  qemu-system-s390x -cpu qemu,stfle=true,ldisp=true,eimm=true,stckf=true>
> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
>  v2: Mark feats array with "static const"
> 
>  target/s390x/cpu_models.c | 34 +++++++++++++++++++++++++++++++---
>  1 file changed, 31 insertions(+), 3 deletions(-)
> 
> diff --git a/target/s390x/cpu_models.c b/target/s390x/cpu_models.c
> index 8d27363..e5e005a 100644
> --- a/target/s390x/cpu_models.c
> +++ b/target/s390x/cpu_models.c
> @@ -658,6 +658,30 @@ static void check_compatibility(const S390CPUModel *max_model,
>                    "available in the configuration: ");
>  }
>  
> +/**
> + * The base TCG CPU model "qemu" is based on the z900. However, we already
> + * can also emulate some additional features of later CPU generations, so
> + * we add these additional feature bits here.
> + */
> +static void add_qemu_cpu_model_features(S390FeatBitmap fbm)
> +{
> +    static const int feats[] = {
> +        S390_FEAT_STFLE,
> +        S390_FEAT_EXTENDED_IMMEDIATE,
> +        S390_FEAT_LONG_DISPLACEMENT,
> +        S390_FEAT_LONG_DISPLACEMENT_FAST,
> +        S390_FEAT_STORE_CLOCK_FAST,
> +        S390_FEAT_GENERAL_INSTRUCTIONS_EXT,
> +        S390_FEAT_EXECUTE_EXT,
> +        S390_FEAT_STFLE_45,
> +    };
> +    int i;
> +
> +    for (i = 0; i < ARRAY_SIZE(feats); i++) {
> +        set_bit(feats[i], fbm);
> +    }
> +}
> +
>  static S390CPUModel *get_max_cpu_model(Error **errp)
>  {
>      static S390CPUModel max_model;
> @@ -670,10 +694,11 @@ static S390CPUModel *get_max_cpu_model(Error **errp)
>      if (kvm_enabled()) {
>          kvm_s390_get_host_cpu_model(&max_model, errp);
>      } else {
> -        /* TCG emulates a z900 */
> +        /* TCG emulates a z900 (with some optional additional features) */
>          max_model.def = &s390_cpu_defs[0];
>          bitmap_copy(max_model.features, max_model.def->default_feat,
>                      S390_FEAT_MAX);
> +        add_qemu_cpu_model_features(max_model.features);
>      }
>      if (!*errp) {
>          cached = true;
> @@ -925,11 +950,14 @@ static void s390_host_cpu_model_initfn(Object *obj)
>  
>  static void s390_qemu_cpu_model_initfn(Object *obj)
>  {
> +    static S390CPUDef s390_qemu_cpu_defs;
>      S390CPU *cpu = S390_CPU(obj);
>  
>      cpu->model = g_malloc0(sizeof(*cpu->model));
> -    /* TCG emulates a z900 */
> -    cpu->model->def = &s390_cpu_defs[0];
> +    /* TCG emulates a z900 (with some optional additional features) */
> +    memcpy(&s390_qemu_cpu_defs, &s390_cpu_defs[0], sizeof(s390_qemu_cpu_defs));
> +    add_qemu_cpu_model_features(s390_qemu_cpu_defs.full_feat);
> +    cpu->model->def = &s390_qemu_cpu_defs;
>      bitmap_copy(cpu->model->features, cpu->model->def->default_feat,
>                  S390_FEAT_MAX);

That should work for the general case, at least for now.

arch_query_cpu_model_baseline() will still drop the additional featues.

Another option would be to directly bump up the CPU model to a z9, with
missing base features (for now). Which looks cleaner in my eyes:


>From 59c944c6fef87808e4456ba9565ace7772d236d7 Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@redhat.com>
Date: Fri, 26 May 2017 13:54:27 +0200
Subject: [PATCH] s390x/cpu_models: Allow some additional feature bits
for the
 "qemu" CPU

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 hw/s390x/s390-virtio-ccw.c | 13 +++++++++++
 target/s390x/cpu.h         |  2 ++
 target/s390x/cpu_models.c  | 55
+++++++++++++++++++++++++++++++++++++++-------
 3 files changed, 62 insertions(+), 8 deletions(-)

diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
index fdd4384..3963a84 100644
--- a/hw/s390x/s390-virtio-ccw.c
+++ b/hw/s390x/s390-virtio-ccw.c
@@ -454,6 +454,19 @@ DEFINE_CCW_MACHINE(2_10, "2.10", true);

 static void ccw_machine_2_9_instance_options(MachineState *machine)
 {
+    static const int feats[] = {
+        S390_FEAT_GROUP_PLO,
+        S390_FEAT_ESAN3,
+        S390_FEAT_ZARCH,
+    };
+    S390FeatBitmap qemu_features = {};
+    int i;
+
+    for (i = 0; i < ARRAY_SIZE(feats); i++) {
+        set_bit(feats[i], qemu_features);
+    }
+    s390_set_qemu_cpu_model_compat(0x2064, 7, 1, qemu_features);
+
     ccw_machine_2_10_instance_options(machine);
 }

diff --git a/target/s390x/cpu.h b/target/s390x/cpu.h
index 240b8a5..8a11104 100644
--- a/target/s390x/cpu.h
+++ b/target/s390x/cpu.h
@@ -637,6 +637,8 @@ void s390_cpu_model_register_props(Object *obj);
 void s390_cpu_model_class_register_props(ObjectClass *oc);
 void s390_realize_cpu_model(CPUState *cs, Error **errp);
 ObjectClass *s390_cpu_class_by_name(const char *name);
+void s390_set_qemu_cpu_model_compat(uint16_t type, uint8_t gen, uint8_t
ec_ga,
+                                    S390FeatBitmap features);

 #define EXCP_EXT 1 /* external interrupt */
 #define EXCP_SVC 2 /* supervisor call (syscall) */
diff --git a/target/s390x/cpu_models.c b/target/s390x/cpu_models.c
index 8d27363..04643d7 100644
--- a/target/s390x/cpu_models.c
+++ b/target/s390x/cpu_models.c
@@ -658,6 +658,28 @@ static void check_compatibility(const S390CPUModel
*max_model,
                   "available in the configuration: ");
 }

+static void get_max_qemu_cpu_model_features(S390FeatBitmap features)
+{
+    static const int feats[] = {
+        S390_FEAT_GROUP_PLO,
+        S390_FEAT_ESAN3,
+        S390_FEAT_ZARCH,
+        S390_FEAT_STFLE,
+        S390_FEAT_EXTENDED_IMMEDIATE,
+        S390_FEAT_LONG_DISPLACEMENT,
+        S390_FEAT_LONG_DISPLACEMENT_FAST,
+        S390_FEAT_STORE_CLOCK_FAST,
+        S390_FEAT_GENERAL_INSTRUCTIONS_EXT,
+        S390_FEAT_EXECUTE_EXT,
+        S390_FEAT_STFLE_45,
+    };
+    int i;
+
+    for (i = 0; i < ARRAY_SIZE(feats); i++) {
+        set_bit(feats[i], features);
+    }
+}
+
 static S390CPUModel *get_max_cpu_model(Error **errp)
 {
     static S390CPUModel max_model;
@@ -670,10 +692,9 @@ static S390CPUModel *get_max_cpu_model(Error **errp)
     if (kvm_enabled()) {
         kvm_s390_get_host_cpu_model(&max_model, errp);
     } else {
-        /* TCG emulates a z900 */
-        max_model.def = &s390_cpu_defs[0];
-        bitmap_copy(max_model.features, max_model.def->default_feat,
-                    S390_FEAT_MAX);
+        /* QEMU emulates a z9 but with some missing base features */
+        max_model.def = s390_find_cpu_def(0x2094, 9, 1, NULL);
+        get_max_qemu_cpu_model_features(max_model.features);
     }
     if (!*errp) {
         cached = true;
@@ -923,15 +944,27 @@ static void s390_host_cpu_model_initfn(Object *obj)
 }
 #endif

+static uint16_t qemu_type;
+static uint8_t qemu_gen;
+static uint8_t qemu_ec_ga;
+static S390FeatBitmap qemu_features;
+
+void s390_set_qemu_cpu_model_compat(uint16_t type, uint8_t gen, uint8_t
ec_ga,
+                                    S390FeatBitmap features)
+{
+    qemu_type = type;
+    qemu_gen = gen;
+    qemu_ec_ga = ec_ga;
+    bitmap_copy(qemu_features, features, S390_FEAT_MAX);
+}
+
 static void s390_qemu_cpu_model_initfn(Object *obj)
 {
     S390CPU *cpu = S390_CPU(obj);

     cpu->model = g_malloc0(sizeof(*cpu->model));
-    /* TCG emulates a z900 */
-    cpu->model->def = &s390_cpu_defs[0];
-    bitmap_copy(cpu->model->features, cpu->model->def->default_feat,
-                S390_FEAT_MAX);
+    cpu->model->def = s390_find_cpu_def(qemu_type, qemu_gen,
qemu_ec_ga, NULL);
+    bitmap_copy(cpu->model->features, qemu_features, S390_FEAT_MAX);
 }

 static void s390_cpu_model_finalize(Object *obj)
@@ -1064,6 +1097,12 @@ static void register_types(void)
                               s390_cpu_defs[i].full_feat);
     }

+    /* QEMU emulates a z9 but with some missing base features */
+    qemu_type = 0x2094;
+    qemu_gen = 9;
+    qemu_ec_ga = 1;
+    get_max_qemu_cpu_model_features(qemu_features);
+
     for (i = 0; i < ARRAY_SIZE(s390_cpu_defs); i++) {
         char *base_name = s390_base_cpu_type_name(s390_cpu_defs[i].name);
         TypeInfo ti_base = {
-- 
2.9.3




-- 

Thanks,

David

Re: [Qemu-devel] [PATCH v2] target/s390x/cpu_models: Allow some additional feature bits for the "qemu" CPU

[Thomas Huth]

On 26.05.2017 14:14, David Hildenbrand wrote:
> On 25.05.2017 11:22, Thomas Huth wrote:
>> Currently we only present the plain z900 feature bits to the guest,
>> but QEMU already emulates some additional features (but not all of
>> the next CPU generation, so we can not use the next CPU level as
>> default yet). Since newer Linux kernels are checking the feature bits
>> and refuse to work if a required feature is missing, it would be nice
>> to have a way to present more of the supported features when we are
>> running with the "qemu" CPU.
>> This patch now adds the supported features to the "full_feat" bitmap,
>> so that additional features can be enabled on the command line now,
>> for example with:
>>
>>  qemu-system-s390x -cpu qemu,stfle=true,ldisp=true,eimm=true,stckf=true>
>> Signed-off-by: Thomas Huth <thuth@redhat.com>
>> ---
>>  v2: Mark feats array with "static const"
>>
>>  target/s390x/cpu_models.c | 34 +++++++++++++++++++++++++++++++---
>>  1 file changed, 31 insertions(+), 3 deletions(-)
>>
>> diff --git a/target/s390x/cpu_models.c b/target/s390x/cpu_models.c
>> index 8d27363..e5e005a 100644
>> --- a/target/s390x/cpu_models.c
>> +++ b/target/s390x/cpu_models.c
>> @@ -658,6 +658,30 @@ static void check_compatibility(const S390CPUModel *max_model,
>>                    "available in the configuration: ");
>>  }
>>  
>> +/**
>> + * The base TCG CPU model "qemu" is based on the z900. However, we already
>> + * can also emulate some additional features of later CPU generations, so
>> + * we add these additional feature bits here.
>> + */
>> +static void add_qemu_cpu_model_features(S390FeatBitmap fbm)
>> +{
>> +    static const int feats[] = {
>> +        S390_FEAT_STFLE,
>> +        S390_FEAT_EXTENDED_IMMEDIATE,
>> +        S390_FEAT_LONG_DISPLACEMENT,
>> +        S390_FEAT_LONG_DISPLACEMENT_FAST,
>> +        S390_FEAT_STORE_CLOCK_FAST,
>> +        S390_FEAT_GENERAL_INSTRUCTIONS_EXT,
>> +        S390_FEAT_EXECUTE_EXT,
>> +        S390_FEAT_STFLE_45,
>> +    };
>> +    int i;
>> +
>> +    for (i = 0; i < ARRAY_SIZE(feats); i++) {
>> +        set_bit(feats[i], fbm);
>> +    }
>> +}
>> +
>>  static S390CPUModel *get_max_cpu_model(Error **errp)
>>  {
>>      static S390CPUModel max_model;
>> @@ -670,10 +694,11 @@ static S390CPUModel *get_max_cpu_model(Error **errp)
>>      if (kvm_enabled()) {
>>          kvm_s390_get_host_cpu_model(&max_model, errp);
>>      } else {
>> -        /* TCG emulates a z900 */
>> +        /* TCG emulates a z900 (with some optional additional features) */
>>          max_model.def = &s390_cpu_defs[0];
>>          bitmap_copy(max_model.features, max_model.def->default_feat,
>>                      S390_FEAT_MAX);
>> +        add_qemu_cpu_model_features(max_model.features);
>>      }
>>      if (!*errp) {
>>          cached = true;
>> @@ -925,11 +950,14 @@ static void s390_host_cpu_model_initfn(Object *obj)
>>  
>>  static void s390_qemu_cpu_model_initfn(Object *obj)
>>  {
>> +    static S390CPUDef s390_qemu_cpu_defs;
>>      S390CPU *cpu = S390_CPU(obj);
>>  
>>      cpu->model = g_malloc0(sizeof(*cpu->model));
>> -    /* TCG emulates a z900 */
>> -    cpu->model->def = &s390_cpu_defs[0];
>> +    /* TCG emulates a z900 (with some optional additional features) */
>> +    memcpy(&s390_qemu_cpu_defs, &s390_cpu_defs[0], sizeof(s390_qemu_cpu_defs));
>> +    add_qemu_cpu_model_features(s390_qemu_cpu_defs.full_feat);
>> +    cpu->model->def = &s390_qemu_cpu_defs;
>>      bitmap_copy(cpu->model->features, cpu->model->def->default_feat,
>>                  S390_FEAT_MAX);
> 
> That should work for the general case, at least for now.
> 
> arch_query_cpu_model_baseline() will still drop the additional featues.

It's only for experimenting with the additional feature right now, not
for serious usage yet (since some features are still missing to run
Linux kernels that are compiled for something > z900) - so I don't think
we need this for QMP already, i.e. IMHO we do not need care too much
about arch_query_cpu_model_baseline() yet.

> Another option would be to directly bump up the CPU model to a z9, with
> missing base features (for now). Which looks cleaner in my eyes:

But couldn't this cause issues, too? What if a kernel just assumes that
certain features are available with a z9 machine, without having a
closer look at the feature bits? Also, what about the features that we
already provide, but that were not available in the z9 yet, like
S390_FEAT_GENERAL_INSTRUCTIONS_EXT (which was introduced with the z10 as
far as I can see)? Don't you have the same issue with
arch_query_cpu_model_baseline() there?

 Thomas

Re: [Qemu-devel] [PATCH v2] target/s390x/cpu_models: Allow some additional feature bits for the "qemu" CPU

[David Hildenbrand]

On 26.05.2017 22:14, Thomas Huth wrote:
> On 26.05.2017 14:14, David Hildenbrand wrote:
>> On 25.05.2017 11:22, Thomas Huth wrote:
>>> Currently we only present the plain z900 feature bits to the guest,
>>> but QEMU already emulates some additional features (but not all of
>>> the next CPU generation, so we can not use the next CPU level as
>>> default yet). Since newer Linux kernels are checking the feature bits
>>> and refuse to work if a required feature is missing, it would be nice
>>> to have a way to present more of the supported features when we are
>>> running with the "qemu" CPU.
>>> This patch now adds the supported features to the "full_feat" bitmap,
>>> so that additional features can be enabled on the command line now,
>>> for example with:
>>>
>>>  qemu-system-s390x -cpu qemu,stfle=true,ldisp=true,eimm=true,stckf=true>
>>> Signed-off-by: Thomas Huth <thuth@redhat.com>
>>> ---
>>>  v2: Mark feats array with "static const"
>>>
>>>  target/s390x/cpu_models.c | 34 +++++++++++++++++++++++++++++++---
>>>  1 file changed, 31 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/target/s390x/cpu_models.c b/target/s390x/cpu_models.c
>>> index 8d27363..e5e005a 100644
>>> --- a/target/s390x/cpu_models.c
>>> +++ b/target/s390x/cpu_models.c
>>> @@ -658,6 +658,30 @@ static void check_compatibility(const S390CPUModel *max_model,
>>>                    "available in the configuration: ");
>>>  }
>>>  
>>> +/**
>>> + * The base TCG CPU model "qemu" is based on the z900. However, we already
>>> + * can also emulate some additional features of later CPU generations, so
>>> + * we add these additional feature bits here.
>>> + */
>>> +static void add_qemu_cpu_model_features(S390FeatBitmap fbm)
>>> +{
>>> +    static const int feats[] = {
>>> +        S390_FEAT_STFLE,
>>> +        S390_FEAT_EXTENDED_IMMEDIATE,
>>> +        S390_FEAT_LONG_DISPLACEMENT,
>>> +        S390_FEAT_LONG_DISPLACEMENT_FAST,
>>> +        S390_FEAT_STORE_CLOCK_FAST,
>>> +        S390_FEAT_GENERAL_INSTRUCTIONS_EXT,
>>> +        S390_FEAT_EXECUTE_EXT,
>>> +        S390_FEAT_STFLE_45,
>>> +    };
>>> +    int i;
>>> +
>>> +    for (i = 0; i < ARRAY_SIZE(feats); i++) {
>>> +        set_bit(feats[i], fbm);
>>> +    }
>>> +}
>>> +
>>>  static S390CPUModel *get_max_cpu_model(Error **errp)
>>>  {
>>>      static S390CPUModel max_model;
>>> @@ -670,10 +694,11 @@ static S390CPUModel *get_max_cpu_model(Error **errp)
>>>      if (kvm_enabled()) {
>>>          kvm_s390_get_host_cpu_model(&max_model, errp);
>>>      } else {
>>> -        /* TCG emulates a z900 */
>>> +        /* TCG emulates a z900 (with some optional additional features) */
>>>          max_model.def = &s390_cpu_defs[0];
>>>          bitmap_copy(max_model.features, max_model.def->default_feat,
>>>                      S390_FEAT_MAX);
>>> +        add_qemu_cpu_model_features(max_model.features);
>>>      }
>>>      if (!*errp) {
>>>          cached = true;
>>> @@ -925,11 +950,14 @@ static void s390_host_cpu_model_initfn(Object *obj)
>>>  
>>>  static void s390_qemu_cpu_model_initfn(Object *obj)
>>>  {
>>> +    static S390CPUDef s390_qemu_cpu_defs;
>>>      S390CPU *cpu = S390_CPU(obj);
>>>  
>>>      cpu->model = g_malloc0(sizeof(*cpu->model));
>>> -    /* TCG emulates a z900 */
>>> -    cpu->model->def = &s390_cpu_defs[0];
>>> +    /* TCG emulates a z900 (with some optional additional features) */
>>> +    memcpy(&s390_qemu_cpu_defs, &s390_cpu_defs[0], sizeof(s390_qemu_cpu_defs));
>>> +    add_qemu_cpu_model_features(s390_qemu_cpu_defs.full_feat);
>>> +    cpu->model->def = &s390_qemu_cpu_defs;
>>>      bitmap_copy(cpu->model->features, cpu->model->def->default_feat,
>>>                  S390_FEAT_MAX);
>>
>> That should work for the general case, at least for now.
>>
>> arch_query_cpu_model_baseline() will still drop the additional featues.
> 
> It's only for experimenting with the additional feature right now, not
> for serious usage yet (since some features are still missing to run
> Linux kernels that are compiled for something > z900) - so I don't think
> we need this for QMP already, i.e. IMHO we do not need care too much
> about arch_query_cpu_model_baseline() yet.
> 

Yes, for a short term solution this should be just fine.

>> Another option would be to directly bump up the CPU model to a z9, with
>> missing base features (for now). Which looks cleaner in my eyes:
> 
> But couldn't this cause issues, too? What if a kernel just assumes that
> certain features are available with a z9 machine, without having a
> closer look at the feature bits? Also, what about the features that we

A tcg guest currently only sees the feature part of CPU models. So e.g.
CPUID is not correctly set. So it most likely makes sense to connect
that properly to the CPU model, too, before we change the CPU
generation. (otherwise we will have guest visible changes once we
connect it - not the features will change but the CPU type/generation).

> already provide, but that were not available in the z9 yet, like
> S390_FEAT_GENERAL_INSTRUCTIONS_EXT (which was introduced with the z10 as
> far as I can see)? Don't you have the same issue with
> arch_query_cpu_model_baseline() there?

Both point are true. In your approach, I don't like creation of fake,
duplicate CPU definitions. But as this will be a short term "hack" to
allow enabling these features, this should be fine for now. We should
just keep in mind that once we want to enable these features
permanently, that we should get rid of the hacked up CPU definition and
implement proper migration compatibility support.

So
Acked-by: David Hildenbrand <david@redhat.com>

Thanks!


> 
>  Thomas
> 


-- 

Thanks,

David