Re: [PATCH v7] hw/i386/cpu: remove default_cpu_version and simplify

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Mar 07, 2025 at 03:53:28PM +0530, Ani Sinha 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. Use of CPU_VERSION_LATEST was removed in [2].
> 
> This change cleans up the complicated logic around default_cpu_version
> including getting rid of default_cpu_version property itself. One new
> flag is introduced for the legacy model for machines 4.0.1 and older.
> 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.
> 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")
> 2) See commit XXX ("microvm: do not use the lastest cpu version")
> 
> CC: imammedo@redhat.com
> Signed-off-by: Ani Sinha <anisinha@redhat.com>
> Reviewed-by: Zhao Liu <zhao1.liu@intel.com>





Anyone merging this? If so:
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>

> ---
>  hw/i386/microvm.c     |  2 +-
>  hw/i386/pc.c          |  1 +
>  hw/i386/pc_piix.c     | 10 +++++---
>  hw/i386/pc_q35.c      | 10 +++++---
>  hw/i386/x86-common.c  |  4 +---
>  include/hw/i386/pc.h  |  6 ++---
>  include/hw/i386/x86.h |  2 +-
>  target/i386/cpu.c     | 56 ++++++++++++-------------------------------
>  target/i386/cpu.h     | 16 +------------
>  9 files changed, 37 insertions(+), 70 deletions(-)
> 
> changelog:
> v2: explain in commit log why use of CPU_VERSION_LATEST for machines
> is problematic.
> v3: fix a bug that broke the pipeline
> https://gitlab.com/mstredhat/qemu/-/pipelines/1626171267
> when cpu versions are explicitly specified in the command line,
> respect that and do not enforce legacy (unversioned) cpu logic.
> The pipeline is green now with the fix:
> https://gitlab.com/anisinha/qemu/-/pipelines/1626783632
> v4: made changes as per Zhao's suggestions.
> Pipeline passes https://gitlab.com/anisinha/qemu/-/pipelines/1635829877
> v5: adjustment of pc_init_cpus() declaration as per Zhao's suggestion. This
> simplifies things and also passes compilation.
> CI still passes https://gitlab.com/anisinha/qemu/-/pipelines/1637657451
> v6: cosmetic commit log correction as suggested by Igor. rebase, added tags.
> v7: This patch is based off the top of
> https://mail.gnu.org/archive/html/qemu-devel/2025-03/msg00934.html.
> Igor's inputs taken into consideration.
> CI passed.
> Note: The above patch has not merged to QEMU upstream yet. Hence, its
> commit hash is marked as XXX in the commit log for this patch.
> 
> diff --git a/hw/i386/microvm.c b/hw/i386/microvm.c
> index a340a5fd39..0eca629958 100644
> --- a/hw/i386/microvm.c
> +++ b/hw/i386/microvm.c
> @@ -491,7 +491,7 @@ static void microvm_machine_state_init(MachineState *machine)
>  
>      microvm_memory_init(mms);
>  
> -    x86_cpus_init(x86ms, 1);
> +    x86_cpus_init(x86ms);
>  
>      microvm_devices_init(mms);
>  }
> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> index 63a96cd23f..6a2ccfd42a 100644
> --- a/hw/i386/pc.c
> +++ b/hw/i386/pc.c
> @@ -30,6 +30,7 @@
>  #include "hw/hyperv/hv-balloon.h"
>  #include "hw/i386/fw_cfg.h"
>  #include "hw/i386/vmport.h"
> +#include "target/i386/cpu.h"
>  #include "system/cpus.h"
>  #include "hw/ide/ide-bus.h"
>  #include "hw/timer/hpet.h"
> diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
> index 04d2957adc..b793a4c765 100644
> --- a/hw/i386/pc_piix.c
> +++ b/hw/i386/pc_piix.c
> @@ -181,7 +181,12 @@ static void pc_init1(MachineState *machine, const char *pci_type)
>      }
>  
>      pc_machine_init_sgx_epc(pcms);
> -    x86_cpus_init(x86ms, pcmc->default_cpu_version);
> +
> +    if (pcmc->no_versioned_cpu_model) {
> +        /* use legacy cpu as it does not support versions */
> +        x86_cpu_set_legacy_version();
> +    }
> +    x86_cpus_init(x86ms);
>  
>      if (kvm_enabled()) {
>          kvmclock_create(pcmc->kvmclock_create_always);
> @@ -457,7 +462,6 @@ 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)";
> @@ -669,7 +673,7 @@ 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 77536dd697..be7775e9de 100644
> --- a/hw/i386/pc_q35.c
> +++ b/hw/i386/pc_q35.c
> @@ -187,7 +187,12 @@ static void pc_q35_init(MachineState *machine)
>      }
>  
>      pc_machine_init_sgx_epc(pcms);
> -    x86_cpus_init(x86ms, pcmc->default_cpu_version);
> +
> +    if (pcmc->no_versioned_cpu_model) {
> +        /* use legacy cpu as it does not support versions */
> +        x86_cpu_set_legacy_version();
> +    }
> +    x86_cpus_init(x86ms);
>  
>      if (kvm_enabled()) {
>          kvmclock_create(pcmc->kvmclock_create_always);
> @@ -339,7 +344,6 @@ 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)";
> @@ -547,7 +551,7 @@ 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 1b0671c523..e8c827c1da 100644
> --- a/hw/i386/x86-common.c
> +++ b/hw/i386/x86-common.c
> @@ -66,15 +66,13 @@ out:
>      object_unref(cpu);
>  }
>  
> -void x86_cpus_init(X86MachineState *x86ms, int default_cpu_version)
> +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 103b54301f..477960275e 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;
> @@ -125,6 +122,9 @@ 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"
> diff --git a/include/hw/i386/x86.h b/include/hw/i386/x86.h
> index d43cb3908e..2d2b987fa1 100644
> --- a/include/hw/i386/x86.h
> +++ b/include/hw/i386/x86.h
> @@ -114,7 +114,7 @@ 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_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 2da2cf36fd..e5c8dd98d0 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -5624,30 +5624,16 @@ 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;
> -
> -void x86_cpu_set_default_version(X86CPUVersion version)
> +static bool no_cpu_alias;
> +void x86_cpu_set_legacy_version(void)
>  {
> -    /* Translating CPU_VERSION_AUTO to CPU_VERSION_AUTO doesn't make sense */
> -    assert(version != CPU_VERSION_AUTO);
> -    default_cpu_version = version;
> +    no_cpu_alias = true;
>  }
>  
>  /* 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;
> -    }
> -    return v;
> +    return model->version;
>  }
>  
>  static const Property max_x86_cpu_properties[] = {
> @@ -6251,10 +6237,15 @@ static char *x86_cpu_class_get_alias_of(X86CPUClass *cc)
>      if (!cc->model || !cc->model->is_alias) {
>          return NULL;
>      }
> -    version = x86_cpu_model_resolve_version(cc->model);
> -    if (version <= 0) {
> +
> +    if (no_cpu_alias) {
> +        /* legacy cpu models do not support cpu aliases */
>          return NULL;
>      }
> +
> +    version = x86_cpu_model_resolve_version(cc->model);
> +    assert(version);
> +
>      return x86_cpu_versioned_model_name(cc->model->cpudef, version);
>  }
>  
> @@ -6268,11 +6259,7 @@ 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);
> -        }
> +        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);
> @@ -6375,13 +6362,8 @@ static void x86_cpu_definition_entry(gpointer data, gpointer user_data)
>      } else {
>          info->deprecated = false;
>      }
> -    /*
> -     * 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) {
> -        info->alias_of = x86_cpu_class_get_alias_of(cc);
> -    }
> +
> +    info->alias_of = x86_cpu_class_get_alias_of(cc);
>  
>      QAPI_LIST_PREPEND(*cpu_list, info);
>  }
> @@ -6553,10 +6535,6 @@ 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) {
> -        return;
> -    }
> -
>      for (vdef = x86_cpu_def_get_versions(model->cpudef); vdef->version; vdef++) {
>          PropValue *p;
>  
> @@ -6583,10 +6561,6 @@ 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) {
> -        return cache_info;
> -    }
> -
>      for (vdef = x86_cpu_def_get_versions(model->cpudef); vdef->version; vdef++) {
>          if (vdef->cache_info) {
>              cache_info = vdef->cache_info;
> @@ -6718,7 +6692,7 @@ 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 = 1;
>      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 113cf10aea..ee1edaa987 100644
> --- a/target/i386/cpu.h
> +++ b/target/i386/cpu.h
> @@ -2739,23 +2739,9 @@ 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 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.
> - */
> -void x86_cpu_set_default_version(X86CPUVersion version);
> +void x86_cpu_set_legacy_version(void);
>  
>  #ifndef CONFIG_USER_ONLY
>  
> -- 
> 2.45.2