Re: [PATCH v2] hw/i386/cpu: remove default_cpu_version and simplify

[Ani Sinha]

On Thu, Dec 12, 2024 at 8:19 PM Ani Sinha <anisinha@redhat.com> wrote:
>
> commit 0788a56bd1ae3 ("i386: Make unversioned CPU models be aliases")
> introduced 'default_cpu_version' for PCMachineClass. This created three
> categories of CPU models:
>  - Most unversioned CPU models would use version 1 by default.
>  - For machines 4.0.1 and older that do not support cpu model aliases, a
>    special default_cpu_version value of CPU_VERSION_LEGACY is used.
>  - It was thought that future machines would use the latest value of cpu
>    versions corresponding to default_cpu_version value of
>    CPU_VERSION_LATEST [1].
>
> All pc machines still use the default cpu version of 1 for
> unversioned cpu models. CPU_VERSION_LATEST is a moving target and
> changes with time. Therefore, if machines use CPU_VERSION_LATEST, it would
> mean that over a period of time, for the same machine type, the cpu version
> would be different depending on what is latest at that time. This would
> break guests even when they use a constant machine type. Therefore, for
> pc machines, use of CPU_VERSION_LATEST is not possible. Currently, only
> microvms use CPU_VERSION_LATEST.
>
> This change cleans up the complicated logic around default_cpu_version
> including getting rid of default_cpu_version property itself. A couple of new
> flags are introduced, one for the legacy model for machines 4.0.1 and older
> and other for microvms. For older machines, a new pc machine property is
> introduced that separates pc machine versions 4.0.1 and older from the newer
> machines. 4.0.1 and older machines are scheduled to be deleted towards
> end of 2025 since they would be 6 years old by then. At that time, we can
> remove all logic around legacy cpus. Microvms are the only machines that
> continue to use the latest cpu version. If this changes later, we can
> remove all logic around x86_cpu_model_last_version(). Default cpu version
> for unversioned cpu models is hardcoded to the value 1 and applies
> unconditionally for all pc machine types of version 4.1 and above.
>
> This change also removes all complications around CPU_VERSION_AUTO
> including removal of the value itself.
>
> 1) See commit dcafd1ef0af227 ("i386: Register versioned CPU models")
>
> CC: imammedo@redhat.com
> Signed-off-by: Ani Sinha <anisinha@redhat.com>

Has this been picked up by anyone?

> ---
>  hw/i386/microvm.c     |  2 +-
>  hw/i386/pc_piix.c     |  6 +++---
>  hw/i386/pc_q35.c      |  6 +++---
>  hw/i386/x86-common.c  | 15 ++++++++++---
>  include/hw/i386/pc.h  | 21 +++++++++++++++---
>  include/hw/i386/x86.h |  5 ++++-
>  target/i386/cpu.c     | 50 +++++++++++++++++++++----------------------
>  target/i386/cpu.h     | 21 ++++--------------
>  8 files changed, 69 insertions(+), 57 deletions(-)
>
> changelog:
> v2: explain in commit log why use of CPU_VERSION_LATEST for machines
> is problematic.
>
> diff --git a/hw/i386/microvm.c b/hw/i386/microvm.c
> index 86637afa0f..2cbe08caea 100644
> --- a/hw/i386/microvm.c
> +++ b/hw/i386/microvm.c
> @@ -458,7 +458,7 @@ static void microvm_machine_state_init(MachineState 
> *machine)
>
>      microvm_memory_init(mms);
>
> -    x86_cpus_init(x86ms, CPU_VERSION_LATEST);
> +    x86_cpus_init_with_latest_cpu_version(x86ms);
>
>      microvm_devices_init(mms);
>  }
> diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
> index 2bf6865d40..8e0ab0387f 100644
> --- a/hw/i386/pc_piix.c
> +++ b/hw/i386/pc_piix.c
> @@ -181,7 +181,8 @@ static void pc_init1(MachineState *machine, const char 
> *pci_type)
>      }
>
>      pc_machine_init_sgx_epc(pcms);
> -    x86_cpus_init(x86ms, pcmc->default_cpu_version);
> +
> +    pc_init_cpus(machine);
>
>      if (kvm_enabled()) {
>          kvmclock_create(pcmc->kvmclock_create_always);
> @@ -454,7 +455,6 @@ static void pc_i440fx_machine_options(MachineClass *m)
>      ObjectClass *oc = OBJECT_CLASS(m);
>      pcmc->default_south_bridge = TYPE_PIIX3_DEVICE;
>      pcmc->pci_root_uid = 0;
> -    pcmc->default_cpu_version = 1;
>
>      m->family = "pc_piix";
>      m->desc = "Standard PC (i440FX + PIIX, 1996)";
> @@ -661,7 +661,7 @@ static void pc_i440fx_machine_4_0_options(MachineClass *m)
>  {
>      PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
>      pc_i440fx_machine_4_1_options(m);
> -    pcmc->default_cpu_version = CPU_VERSION_LEGACY;
> +    pcmc->no_versioned_cpu_model = true;
>      compat_props_add(m->compat_props, hw_compat_4_0, hw_compat_4_0_len);
>      compat_props_add(m->compat_props, pc_compat_4_0, pc_compat_4_0_len);
>  }
> diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
> index 8319b6d45e..9a4f14b336 100644
> --- a/hw/i386/pc_q35.c
> +++ b/hw/i386/pc_q35.c
> @@ -187,7 +187,8 @@ static void pc_q35_init(MachineState *machine)
>      }
>
>      pc_machine_init_sgx_epc(pcms);
> -    x86_cpus_init(x86ms, pcmc->default_cpu_version);
> +
> +    pc_init_cpus(machine);
>
>      if (kvm_enabled()) {
>          kvmclock_create(pcmc->kvmclock_create_always);
> @@ -336,7 +337,6 @@ static void pc_q35_machine_options(MachineClass *m)
>  {
>      PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
>      pcmc->pci_root_uid = 0;
> -    pcmc->default_cpu_version = 1;
>
>      m->family = "pc_q35";
>      m->desc = "Standard PC (Q35 + ICH9, 2009)";
> @@ -537,7 +537,7 @@ static void pc_q35_machine_4_0_1_options(MachineClass *m)
>  {
>      PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
>      pc_q35_machine_4_1_options(m);
> -    pcmc->default_cpu_version = CPU_VERSION_LEGACY;
> +    pcmc->no_versioned_cpu_model = true;
>      /*
>       * This is the default machine for the 4.0-stable branch. It is basically
>       * a 4.0 that doesn't use split irqchip by default. It MUST hence apply 
> the
> diff --git a/hw/i386/x86-common.c b/hw/i386/x86-common.c
> index dc031af662..37dfdb7300 100644
> --- a/hw/i386/x86-common.c
> +++ b/hw/i386/x86-common.c
> @@ -66,15 +66,24 @@ out:
>      object_unref(cpu);
>  }
>
> -void x86_cpus_init(X86MachineState *x86ms, int default_cpu_version)
> +void x86_legacy_cpus_init(X86MachineState *x86ms)
> +{
> +    machine_uses_legacy_cpu();
> +    x86_cpus_init(x86ms);
> +}
> +
> +void x86_cpus_init_with_latest_cpu_version(X86MachineState *x86ms)
> +{
> +    x86_cpu_uses_lastest_version();
> +    x86_cpus_init(x86ms);
> +}
> +void x86_cpus_init(X86MachineState *x86ms)
>  {
>      int i;
>      const CPUArchIdList *possible_cpus;
>      MachineState *ms = MACHINE(x86ms);
>      MachineClass *mc = MACHINE_GET_CLASS(x86ms);
>
> -    x86_cpu_set_default_version(default_cpu_version);
> -
>      /*
>       * Calculates the limit to CPU APIC ID values
>       *
> diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
> index 14ee06287d..b514b957d9 100644
> --- a/include/hw/i386/pc.h
> +++ b/include/hw/i386/pc.h
> @@ -92,9 +92,6 @@ struct PCMachineClass {
>
>      /* Compat options: */
>
> -    /* Default CPU model version.  See x86_cpu_set_default_version(). */
> -    int default_cpu_version;
> -
>      /* ACPI compat: */
>      bool has_acpi_build;
>      int pci_root_uid;
> @@ -125,11 +122,29 @@ struct PCMachineClass {
>       * check for memory.
>       */
>      bool broken_32bit_mem_addr_check;
> +
> +    /* whether the machine supports versioned cpu models */
> +    bool no_versioned_cpu_model;
>  };
>
>  #define TYPE_PC_MACHINE "generic-pc-machine"
>  OBJECT_DECLARE_TYPE(PCMachineState, PCMachineClass, PC_MACHINE)
>
> +static inline void pc_init_cpus(MachineState *ms)
> +{
> +    X86MachineState *x86ms = X86_MACHINE(ms);
> +    PCMachineState *pcms = PC_MACHINE(ms);
> +    PCMachineClass *pcmc = PC_MACHINE_GET_CLASS(pcms);
> +
> +    if (pcmc->no_versioned_cpu_model) {
> +        /* use legacy cpu as it does not support versions */
> +        x86_legacy_cpus_init(x86ms);
> +    } else {
> +        /* use non-legacy cpus */
> +        x86_cpus_init(x86ms);
> +    }
> +}
> +
>  /* ioapic.c */
>
>  GSIState *pc_gsi_create(qemu_irq **irqs, bool pci_enabled);
> diff --git a/include/hw/i386/x86.h b/include/hw/i386/x86.h
> index d43cb3908e..a90464f74b 100644
> --- a/include/hw/i386/x86.h
> +++ b/include/hw/i386/x86.h
> @@ -114,7 +114,10 @@ void init_topo_info(X86CPUTopoInfo *topo_info, const 
> X86MachineState *x86ms);
>  uint32_t x86_cpu_apic_id_from_index(X86MachineState *x86ms,
>                                      unsigned int cpu_index);
>
> -void x86_cpus_init(X86MachineState *pcms, int default_cpu_version);
> +void x86_cpus_init(X86MachineState *pcms);
> +void x86_legacy_cpus_init(X86MachineState *x86ms);
> +void x86_cpus_init_with_latest_cpu_version(X86MachineState *x86ms);
> +
>  void x86_rtc_set_cpus_count(ISADevice *rtc, uint16_t cpus_count);
>  void x86_cpu_pre_plug(HotplugHandler *hotplug_dev,
>                        DeviceState *dev, Error **errp);
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index 3725dbbc4b..14f5aa359b 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -191,6 +191,9 @@ struct CPUID2CacheDescriptorInfo 
> cpuid2_cache_descriptors[] = {
>   */
>  #define CACHE_DESCRIPTOR_UNAVAILABLE 0xFF
>
> +/* default cpu version to use */
> +#define DEFAULT_CPU_VERSION 1
> +
>  /*
>   * Return a CPUID 2 cache descriptor for a given cache.
>   * If no known descriptor is found, return CACHE_DESCRIPTOR_UNAVAILABLE
> @@ -5343,20 +5346,16 @@ static const X86CPUDefinition builtin_x86_defs[] = {
>      },
>  };
>
> -/*
> - * We resolve CPU model aliases using -v1 when using "-machine
> - * none", but this is just for compatibility while libvirt isn't
> - * adapted to resolve CPU model versions before creating VMs.
> - * See "Runnability guarantee of CPU models" at
> - * docs/about/deprecated.rst.
> - */
> -X86CPUVersion default_cpu_version = 1;
> +static bool use_legacy_cpu;
> +void machine_uses_legacy_cpu(void)
> +{
> +    use_legacy_cpu = true;
> +}
>
> -void x86_cpu_set_default_version(X86CPUVersion version)
> +static bool use_last_cpu_version;
> +void x86_cpu_uses_lastest_version(void)
>  {
> -    /* Translating CPU_VERSION_AUTO to CPU_VERSION_AUTO doesn't make sense */
> -    assert(version != CPU_VERSION_AUTO);
> -    default_cpu_version = version;
> +    use_last_cpu_version = true;
>  }
>
>  static X86CPUVersion x86_cpu_model_last_version(const X86CPUModel *model)
> @@ -5374,14 +5373,11 @@ static X86CPUVersion x86_cpu_model_last_version(const 
> X86CPUModel *model)
>  /* Return the actual version being used for a specific CPU model */
>  static X86CPUVersion x86_cpu_model_resolve_version(const X86CPUModel *model)
>  {
> -    X86CPUVersion v = model->version;
> -    if (v == CPU_VERSION_AUTO) {
> -        v = default_cpu_version;
> -    }
> -    if (v == CPU_VERSION_LATEST) {
> +    if (use_last_cpu_version) {
>          return x86_cpu_model_last_version(model);
>      }
> -    return v;
> +
> +    return model->version;
>  }
>
>  static Property max_x86_cpu_properties[] = {
> @@ -5986,6 +5982,12 @@ static char *x86_cpu_class_get_alias_of(X86CPUClass 
> *cc)
>      if (!cc->model || !cc->model->is_alias) {
>          return NULL;
>      }
> +
> +    if (use_legacy_cpu) {
> +        /* legacy cpu models do not support cpu aliases */
> +        return NULL;
> +    }
> +
>      version = x86_cpu_model_resolve_version(cc->model);
>      if (version <= 0) {
>          return NULL;
> @@ -6003,11 +6005,7 @@ static void x86_cpu_list_entry(gpointer data, gpointer 
> user_data)
>      g_autofree char *model_id = x86_cpu_class_get_model_id(cc);
>
>      if (!desc && alias_of) {
> -        if (cc->model && cc->model->version == CPU_VERSION_AUTO) {
> -            desc = g_strdup("(alias configured by machine type)");
> -        } else {
>              desc = g_strdup_printf("(alias of %s)", alias_of);
> -        }
>      }
>      if (!desc && cc->model && cc->model->note) {
>          desc = g_strdup_printf("%s [%s]", model_id, cc->model->note);
> @@ -6114,7 +6112,7 @@ static void x86_cpu_definition_entry(gpointer data, 
> gpointer user_data)
>       * Old machine types won't report aliases, so that alias translation
>       * doesn't break compatibility with previous QEMU versions.
>       */
> -    if (default_cpu_version != CPU_VERSION_LEGACY) {
> +    if (!use_legacy_cpu) {
>          info->alias_of = x86_cpu_class_get_alias_of(cc);
>      }
>
> @@ -6288,7 +6286,7 @@ static void x86_cpu_apply_version_props(X86CPU *cpu, 
> X86CPUModel *model)
>      const X86CPUVersionDefinition *vdef;
>      X86CPUVersion version = x86_cpu_model_resolve_version(model);
>
> -    if (version == CPU_VERSION_LEGACY) {
> +    if (use_legacy_cpu) {
>          return;
>      }
>
> @@ -6318,7 +6316,7 @@ static const CPUCaches 
> *x86_cpu_get_versioned_cache_info(X86CPU *cpu,
>      X86CPUVersion version = x86_cpu_model_resolve_version(model);
>      const CPUCaches *cache_info = model->cpudef->cache_info;
>
> -    if (version == CPU_VERSION_LEGACY) {
> +    if (use_legacy_cpu) {
>          return cache_info;
>      }
>
> @@ -6453,7 +6451,7 @@ static void x86_register_cpudef_types(const 
> X86CPUDefinition *def)
>      /* Unversioned model: */
>      m = g_new0(X86CPUModel, 1);
>      m->cpudef = def;
> -    m->version = CPU_VERSION_AUTO;
> +    m->version = DEFAULT_CPU_VERSION;
>      m->is_alias = true;
>      x86_register_cpu_model_type(def->name, m);
>
> diff --git a/target/i386/cpu.h b/target/i386/cpu.h
> index 4c239a6970..5ad80c8a97 100644
> --- a/target/i386/cpu.h
> +++ b/target/i386/cpu.h
> @@ -2658,28 +2658,15 @@ void cpu_report_tpr_access(CPUX86State *env, 
> TPRAccess access);
>  void apic_handle_tpr_access_report(DeviceState *d, target_ulong ip,
>                                     TPRAccess access);
>
> -/* Special values for X86CPUVersion: */
> -
> -/* Resolve to latest CPU version */
> -#define CPU_VERSION_LATEST -1
> -
> -/*
> - * Resolve to version defined by current machine type.
> - * See x86_cpu_set_default_version()
> - */
> -#define CPU_VERSION_AUTO   -2
> -
> -/* Don't resolve to any versioned CPU models, like old QEMU versions */
> -#define CPU_VERSION_LEGACY  0
> -
>  typedef int X86CPUVersion;
>
>  /*
> - * Set default CPU model version for CPU models having
> - * version == CPU_VERSION_AUTO.
> + * Set CPU model version to the lastest version.
> + * Currently, this is only used by microvm.
>   */
> -void x86_cpu_set_default_version(X86CPUVersion version);
> +void x86_cpu_uses_lastest_version(void);
>
> +void machine_uses_legacy_cpu(void);
>  #ifndef CONFIG_USER_ONLY
>
>  void do_cpu_sipi(X86CPU *cpu);
> --
> 2.45.2
>