Re: [PULL 11/11] target/s390x: Add a CONFIG switch to disable legacy CPUs

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 7/3/25 16:33, Thomas Huth wrote:
> On 07/03/2025 16.27, Philippe Mathieu-Daudé wrote:
>> Hi Thomas,
>>
>> On 24/6/24 11:10, Thomas Huth wrote:
>>> The oldest model that IBM still supports is the z13. Considering
>>> that each generation can "emulate" the previous two generations
>>> in hardware (via the "IBC" feature of the CPUs), this means that
>>> everything that is older than z114/196 is not an officially supported
>>> CPU model anymore. The Linux kernel still support the z10, so if
>>> we also take this into account, everything older than that can
>>> definitely be considered as a legacy CPU model.
>>>
>>> For downstream builds of QEMU, we would like to be able to disable
>>> these legacy CPUs in the build. Thus add a CONFIG switch that can be
>>> used to disable them (and old machine types that use them by default).
>>>
>>> Message-Id: <20240614125019.588928-1-thuth@redhat.com>
>>> Signed-off-by: Thomas Huth <thuth@redhat.com>
>>> ---
>>>   hw/s390x/s390-virtio-ccw.c | 5 +++++
>>>   target/s390x/cpu_models.c  | 9 +++++++++
>>>   target/s390x/Kconfig       | 5 +++++
>>>   3 files changed, 19 insertions(+)
>>>
>>> diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
>>> index 3d0bc3e7f2..cd063f8b64 100644
>>> --- a/hw/s390x/s390-virtio-ccw.c
>>> +++ b/hw/s390x/s390-virtio-ccw.c
>>> @@ -47,6 +47,7 @@
>>>   #include "migration/blocker.h"
>>>   #include "qapi/visitor.h"
>>>   #include "hw/s390x/cpu-topology.h"
>>> +#include CONFIG_DEVICES
>>>   static Error *pv_mig_blocker;
>>> @@ -1126,6 +1127,8 @@ static void 
>>> ccw_machine_2_12_class_options(MachineClass *mc)
>>>   }
>>>   DEFINE_CCW_MACHINE(2_12, "2.12", false);
>>> +#ifdef CONFIG_S390X_LEGACY_CPUS
>>> +
>>>   static void ccw_machine_2_11_instance_options(MachineState *machine)
>>>   {
>>>       static const S390FeatInit qemu_cpu_feat = 
>>> { S390_FEAT_LIST_QEMU_V2_11 };
>>> @@ -1272,6 +1275,8 @@ static void 
>>> ccw_machine_2_4_class_options(MachineClass *mc)
>>>   }
>>>   DEFINE_CCW_MACHINE(2_4, "2.4", false);
>>> +#endif
>>> +
>>>   static void ccw_machine_register_types(void)
>>>   {
>>>       type_register_static(&ccw_machine_info);
>>> diff --git a/target/s390x/cpu_models.c b/target/s390x/cpu_models.c
>>> index efb508cd2e..a27f4b6f79 100644
>>> --- a/target/s390x/cpu_models.c
>>> +++ b/target/s390x/cpu_models.c
>>> @@ -25,6 +25,7 @@
>>>   #ifndef CONFIG_USER_ONLY
>>>   #include "sysemu/sysemu.h"
>>>   #include "target/s390x/kvm/pv.h"
>>> +#include CONFIG_DEVICES
>>>   #endif
>>>   #define CPUDEF_INIT(_type, _gen, _ec_ga, _mha_pow, _hmfai, _name, 
>>> _desc) \
>>> @@ -47,6 +48,13 @@
>>>    * generation 15 one base feature and one optional feature have 
>>> been deprecated.
>>>    */
>>>   static S390CPUDef s390_cpu_defs[] = {
>>> +    /*
>>> +     * Linux requires at least z10 nowadays, and IBM only supports 
>>> recent CPUs
>>> +     * (see https://www.ibm.com/support/pages/ibm-mainframe-life- 
>>> cycle- history),
>>> +     * so we consider older CPUs as legacy that can optionally be 
>>> disabled via
>>> +     * the CONFIG_S390X_LEGACY_CPUS config switch.
>>> +     */
>>> +#if defined(CONFIG_S390X_LEGACY_CPUS) || defined(CONFIG_USER_ONLY)
>>>       CPUDEF_INIT(0x2064, 7, 1, 38, 0x00000000U, "z900", "IBM zSeries 
>>> 900 GA1"),
>>>       CPUDEF_INIT(0x2064, 7, 2, 38, 0x00000000U, "z900.2", "IBM 
>>> zSeries 900 GA2"),
>>>       CPUDEF_INIT(0x2064, 7, 3, 38, 0x00000000U, "z900.3", "IBM 
>>> zSeries 900 GA3"),
>>> @@ -64,6 +72,7 @@ static S390CPUDef s390_cpu_defs[] = {
>>>       CPUDEF_INIT(0x2096, 9, 2, 40, 0x00000000U, "z9BC", "IBM System 
>>> z9 BC GA1"),
>>>       CPUDEF_INIT(0x2094, 9, 3, 40, 0x00000000U, "z9EC.3", "IBM 
>>> System z9 EC GA3"),
>>>       CPUDEF_INIT(0x2096, 9, 3, 40, 0x00000000U, "z9BC.2", "IBM 
>>> System z9 BC GA2"),
>>> +#endif
>>>       CPUDEF_INIT(0x2097, 10, 1, 43, 0x00000000U, "z10EC", "IBM 
>>> System z10 EC GA1"),
>>>       CPUDEF_INIT(0x2097, 10, 2, 43, 0x00000000U, "z10EC.2", "IBM 
>>> System z10 EC GA2"),
>>>       CPUDEF_INIT(0x2098, 10, 2, 43, 0x00000000U, "z10BC", "IBM 
>>> System z10 BC GA1"),
>>> diff --git a/target/s390x/Kconfig b/target/s390x/Kconfig
>>> index d886be48b4..8a95f2bc3f 100644
>>> --- a/target/s390x/Kconfig
>>> +++ b/target/s390x/Kconfig
>>> @@ -2,3 +2,8 @@ config S390X
>>>       bool
>>>       select PCI
>>>       select S390_FLIC
>>> +
>>> +config S390X_LEGACY_CPUS
>>> +    bool
>>> +    default y
>>
>> Do we have plan to deprecate or switch that off?
> 
> No, this is mainly a config knob for downstream builds that do not want 
> to include legacy CPUs, but for upstream it should be fine to keep those 
> around.
> 
> Why do you ask?

WRT single binary, trying to split target/ VS hw/, I was hoping Kconfig
switches would only affect hw/. CPUs are odd devices sitting in between.

Generic registration of QOM types isn't a trivial task neither...

Anyway I can postpone this file analysis, since s390x is a "simple"
target in my setup :)