Re: [PATCH v2 3/3] target/loongarch: add no_pv_feature compat flag

[Tao Cui <cui.tao@linux.dev>]

在 2026/7/10 11:09, Bibo Mao 写道:
> 
> 
> On 2026/7/10 上午8:50, Tao Cui wrote:
>> From: Tao Cui <cuitao@kylinos.cn>
>>
>> Advertising pv features to the guest affects the migration stream. Add a
>> no_pv_feature compat flag so the behavior can be gated per machine version:
>> LoongArchCPU gets a bool field, and the virt machine gains a
>> LoongArchVirtMachineClass field that defaults the CPU flag per version.
>>
>> kvm_cpu_check_pv_features() skips populating pv features when the flag is
>> set. The 11.1 machine type keeps pv advertisement off (no_pv_feature is
>> true, preserving the previous behavior); from the new 11.2 machine type it
>> is on by default.
>>
>> Signed-off-by: Tao Cui <cuitao@kylinos.cn>
>> ---
>>   hw/loongarch/virt.c         | 14 +++++++++++++-
>>   include/hw/loongarch/virt.h |  9 ++++++++-
>>   target/loongarch/cpu.h      |  1 +
>>   target/loongarch/kvm/kvm.c  |  5 +++++
>>   4 files changed, 27 insertions(+), 2 deletions(-)
>>
>> diff --git a/hw/loongarch/virt.c b/hw/loongarch/virt.c
>> index 6693dea647..53c84c8def 100644
>> --- a/hw/loongarch/virt.c
>> +++ b/hw/loongarch/virt.c
>> @@ -955,6 +955,8 @@ static void virt_init(MachineState *machine)
>>                            machine->cpu_type);
>>               exit(EXIT_FAILURE);
>>           }
>> +        LOONGARCH_CPU(cpuobj)->no_pv_feature =
>> +            LOONGARCH_VIRT_MACHINE_GET_CLASS(lvms)->no_pv_feature;
> I think it will better to set in function virt_cpu_plug().
On virt_cpu_plug(): 
  agreed, that's cleaner — I'll move the propagation there.
>>           qdev_realize_and_unref(DEVICE(cpuobj), NULL, &error_fatal);
>>       }
>>       virt_check_dmsi(machine);
>> @@ -1584,6 +1586,7 @@ static const TypeInfo virt_machine_info = {
>>       .parent         = TYPE_MACHINE,
>>       .abstract       = true,
>>       .instance_size  = sizeof(LoongArchVirtMachineState),
>> +    .class_size     = sizeof(LoongArchVirtMachineClass),
>>       .class_init     = virt_class_init,
>>       .instance_init  = virt_initfn,
>>       .instance_finalize = virt_instance_finalize,
>> @@ -1601,6 +1604,15 @@ static void machvirt_machine_init(void)
>>   type_init(machvirt_machine_init);
>>     static void virt_machine_11_1_options(MachineClass *mc)
>> +{
>> +    LoongArchVirtMachineClass *vmc = LOONGARCH_VIRT_MACHINE_CLASS(mc);
>> +
>> +    /* pv feature advertisement is enabled from 11.2; keep 11.1 unchanged */
>> +    vmc->no_pv_feature = true;
>> +}
>> +DEFINE_VIRT_MACHINE(11, 1)
>> +
>> +static void virt_machine_11_2_options(MachineClass *mc)
>>   {
>>   }
>> -DEFINE_VIRT_MACHINE_AS_LATEST(11, 1)
>> +DEFINE_VIRT_MACHINE_AS_LATEST(11, 2)
>> diff --git a/include/hw/loongarch/virt.h b/include/hw/loongarch/virt.h
>> index d39a9bbf5d..cf8d9e1fd2 100644
>> --- a/include/hw/loongarch/virt.h
>> +++ b/include/hw/loongarch/virt.h
>> @@ -130,8 +130,15 @@ struct LoongArchVirtMachineState {
>>       bool highmem_mmio;
>>   };
>>   +struct LoongArchVirtMachineClass {
>> +    MachineClass parent_obj;
>> +
>> +    bool no_pv_feature;   /* compat: 11.1 and older do not advertise pv features */
>> +};
>> +
>>   #define TYPE_LOONGARCH_VIRT_MACHINE  MACHINE_TYPE_NAME("virt")
>> -OBJECT_DECLARE_SIMPLE_TYPE(LoongArchVirtMachineState, LOONGARCH_VIRT_MACHINE)
>> +OBJECT_DECLARE_TYPE(LoongArchVirtMachineState, LoongArchVirtMachineClass,
>> +                    LOONGARCH_VIRT_MACHINE)
>>   void virt_acpi_setup(LoongArchVirtMachineState *lvms);
>>   void virt_fdt_setup(LoongArchVirtMachineState *lvms);
>>   diff --git a/target/loongarch/cpu.h b/target/loongarch/cpu.h
>> index ad30c73167..11bbb5f36e 100644
>> --- a/target/loongarch/cpu.h
>> +++ b/target/loongarch/cpu.h
>> @@ -447,6 +447,7 @@ struct ArchCPU {
>>       OnOffAuto msgint;
>>       OnOffAuto kvm_pv_ipi;
>>       OnOffAuto kvm_steal_time;
>> +    bool no_pv_feature;   /* compat: do not advertise KVM pv features */
>>       int32_t socket_id;  /* socket-id of this CPU */
>>       int32_t core_id;    /* core-id of this CPU */
>>       int32_t thread_id;  /* thread-id of this CPU */
>> diff --git a/target/loongarch/kvm/kvm.c b/target/loongarch/kvm/kvm.c
>> index c557ee3c3d..878914928b 100644
>> --- a/target/loongarch/kvm/kvm.c
>> +++ b/target/loongarch/kvm/kvm.c
>> @@ -1119,6 +1119,11 @@ static int kvm_cpu_check_pv_features(CPUState *cs, Error **errp)
>>       CPULoongArchState *env = cpu_env(cs);
>>       bool kvm_supported;
>>   +    /* compat: older machine types do not advertise pv features to the guest */
>> +    if (cpu->no_pv_feature) {
>> +        return 0;
>> +    }
>> +
> This piece of code is unnecessary, with QEMU-11.1 machine type, kvm_cpu_check_pv_features checking is skipped when CPU is created, why?

On the check_pv_features guard: 
you're right, it's unnecessary. As you
pointed out in v1, check_pv_features() is host detection and should run
regardless of machine version. I'll remove the guard and check
no_pv_feature in kvm_set_pv_features() instead, so the push is skipped for
11.1 while detection still happens normally.

One timing concern: 
virt_cpu_plug() runs after qdev_realize(), but the
first FULL_STATE sync (post_init) fires during realize — so the push could
happen before no_pv_feature is set in plug. Should I keep setting the flag
before qdev_realize() (in virt_init) for now, or is there a better
mechanism?

> 
> Regards
> Bibo Mao
>>       kvm_supported = kvm_feature_supported(cs, LOONGARCH_FEATURE_PV_IPI);
>>       if (cpu->kvm_pv_ipi == ON_OFF_AUTO_ON) {
>>           if (!kvm_supported) {
>>
>