Hi Ani,
Sorry for late reply.
On Thu, Jan 16, 2025 at 09:04:18AM +0530, Ani Sinha wrote:
> Date: Thu, 16 Jan 2025 09:04:18 +0530
> From: Ani Sinha <anisi...@redhat.com>
> Subject: [PATCH v3] hw/i386/cpu: remove default_cpu_version and simplify
> X-Mailer: git-send-email 2.45.2
>
> 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.
I like the idea to remove CPU_VERSION_AUTO. Though this patch introduces
2 more new static variables ("use_legacy_cpu" and "use_last_cpu_version"),
as you said, once 4.0.1 and older machines are removed, it's easy to
clean up "use_legacy_cpu".
> 1) See commit dcafd1ef0af227 ("i386: Register versioned CPU models")
>
> CC: imamm...@redhat.com
> Signed-off-by: Ani Sinha <anisi...@redhat.com>
> ---
[snip]
> -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);
> +}
Could we simplify it even further, i.e., omit these two new helpers and
just add x86_cpu_uses_lastest_version() and machine_uses_legacy_cpu() to
the initialization of the PC & microvm, e.g.,
--- a/hw/i386/microvm.c
+++ b/hw/i386/microvm.c
@@ -458,7 +458,8 @@ static void microvm_machine_state_init(MachineState
*machine)
microvm_memory_init(mms);
- x86_cpus_init_with_latest_cpu_version(x86ms);
+ x86_cpu_uses_lastest_version();
+ x86_cpus_init(x86ms);
microvm_devices_init(mms);
}
and
--- a/include/hw/i386/pc.h
+++ b/include/hw/i386/pc.h
@@ -138,11 +138,10 @@ static inline void pc_init_cpus(MachineState *ms)
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);
+ machine_uses_legacy_cpu();
}
+
+ x86_cpus_init(x86ms);
}
/* ioapic.c */
[snip]
> diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
> index a558705cb9..ad43a233d8 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)
I think there's no need to declare as `inline`.
> +{
> + 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 */
As my comment above, we can just call machine_uses_legacy_cpu()
if pcmc->no_versioned_cpu_model is true.
[snip]
> -/*
> - * 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)
What about this name, "x86_cpu_set_legacy_version"?
> +{
> + use_legacy_cpu = true;
> +}
>
> -void x86_cpu_set_default_version(X86CPUVersion version)
> +static bool use_last_cpu_version;
Maybe "use_lastest_cpu"? Keep it in the same style as "use_legacy_cpu".
> +void x86_cpu_uses_lastest_version(void)
Similarly, What about "x86_cpu_set_latest_version"?
> {
> - /* 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)
> @@ -5376,14 +5375,11 @@ 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 const Property max_x86_cpu_properties[] = {
> @@ -5987,6 +5983,12 @@ 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) {
I understand this non-NULL check is origianl for legacy CPU version.
So it's also necessary to remove it, or convert it to "assert(version)"?
> return NULL;
> @@ -6004,11 +6006,7 @@ 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);
> @@ -6115,7 +6113,7 @@ 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);
> }
Do we need the check of "!use_legacy_cpu"?
x86_cpu_class_get_alias_of() returns NULL if use_legacy_cpu is true.
Thanks,
Zhao
