Re: [PATCH v9 8/9] KVM: x86: Allow userspace set maximum VCPU id for VM

[Zeng Guang <guang.zeng@intel.com>]

On 5/3/2022 12:07 AM, Paolo Bonzini wrote:
> On 4/19/22 17:44, Zeng Guang wrote:
>> +Userspace is able to calculate the limit to APIC ID values from designated CPU
>> +topology. This capability allows userspace to specify maximum possible APIC ID
>> +assigned for current VM session prior to the creation of vCPUs. By design, it
>> +can set only once and doesn't accept change any more. KVM will manage memory
>> +allocation of VM-scope structures which depends on the value of APIC ID.
>> +
>> +Calling KVM_CHECK_EXTENSION for this capability returns the value of maximum APIC
>> +ID that KVM supports at runtime. It sets as KVM_MAX_VCPU_IDS by default.
> Better:
>
> This capability allows userspace to specify maximum possible APIC ID
> assigned for current VM session prior to the creation of vCPUs, saving
> memory for data structures indexed by the APIC ID.  Userspace is able
> to calculate the limit to APIC ID values from designated
> CPU topology.
>
> The value can be changed only until KVM_ENABLE_CAP is set to a nonzero
> value or until a vCPU is created.  Upon creation of the first vCPU,
> if the value was set to zero or KVM_ENABLE_CAP was not invoked, KVM
> uses the return value of KVM_CHECK_EXTENSION(KVM_CAP_MAX_VCPU_ID) as
> the maximum APIC ID.
>
>>    	case KVM_CAP_MAX_VCPU_ID:
>> -		r = KVM_MAX_VCPU_IDS;
>> +		if (!kvm->arch.max_vcpu_ids)
>> +			r = KVM_MAX_VCPU_IDS;
>> +		else
>> +			r = kvm->arch.max_vcpu_ids;
> I think returning the constant KVM_CAP_MAX_VCPU_IDS is better.
>
> Paolo
Thanks. I will change as you suggested.