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Message-ID: <da1fc56f-bcfe-cc6e-aff7-0f95a3bc6d20@intel.com>
Date:   Tue, 3 May 2022 10:33:44 +0800
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Subject: Re: [PATCH v9 8/9] KVM: x86: Allow userspace set maximum VCPU id for
 VM
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To:     Paolo Bonzini <pbonzini@redhat.com>,
        "Christopherson,, Sean" <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "Luck, Tony" <tony.luck@intel.com>,
        Kan Liang <kan.liang@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Kim Phillips <kim.phillips@amd.com>,
        Jarkko Sakkinen <jarkko@kernel.org>,
        Jethro Beekman <jethro@fortanix.com>,
        "Huang, Kai" <kai.huang@intel.com>
Cc:     "x86@kernel.org" <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Hu, Robert" <robert.hu@intel.com>,
        "Gao, Chao" <chao.gao@intel.com>
References: <20220419154444.11888-1-guang.zeng@intel.com>
 <a06997fe-8dd7-e91a-2017-912827f554e7@redhat.com>
From:   Zeng Guang <guang.zeng@intel.com>
In-Reply-To: <a06997fe-8dd7-e91a-2017-912827f554e7@redhat.com>
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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.