linux-kernel - Re: [PATCH] KVM: loongarch: Add vcpu id check before create vcpu

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Date: Mon, 15 Apr 2024 10:04:17 +0800
From: maobibo <maobibo@...ngson.cn>
To: Sean Christopherson <seanjc@...gle.com>, Wujie Duan <wjduan@...x-info.com>
Cc: zhaotianrui@...ngson.cn, chenhuacai@...nel.org, kernel@...0n.name,
 kvm@...r.kernel.org, loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: loongarch: Add vcpu id check before create vcpu



On 2024/4/13 上午2:41, Sean Christopherson wrote:
> On Fri, Apr 12, 2024, Wujie Duan wrote:
>> Add a pre-allocation arch condition to checks that vcpu id should
>> smaller than max_vcpus
>>
>> Signed-off-by: Wujie Duan <wjduan@...x-info.com>
>> ---
>>   arch/loongarch/kvm/vcpu.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/arch/loongarch/kvm/vcpu.c b/arch/loongarch/kvm/vcpu.c
>> index 3a8779065f73..d41cacf39583 100644
>> --- a/arch/loongarch/kvm/vcpu.c
>> +++ b/arch/loongarch/kvm/vcpu.c
>> @@ -884,6 +884,9 @@ long kvm_arch_vcpu_async_ioctl(struct file *filp,
>>   
>>   int kvm_arch_vcpu_precreate(struct kvm *kvm, unsigned int id)
>>   {
>> +	if (id >= kvm->max_vcpus)
>> +		return -EINVAL;
> 
> Do you have a testcase for this?  If I'm following the LoongArch code correctly,
> I don't think this is actually necessary.
> 
> In arch/loongarch/include/asm/kvm_host.h:
> 
>    #define KVM_MAX_VCPUS			256
> 
> without a #define KVM_MAX_VCPU_IDS in loongarch/, AFAICT.  And so the common
> code in include/linux/kvm_host.h will do:
> 
>    #ifndef KVM_MAX_VCPU_IDS
>    #define KVM_MAX_VCPU_IDS KVM_MAX_VCPUS
>    #endif
> 
> LoongArch's kvm_vm_ioctl_check_extension() reports that to userspace:
> 
> 	case KVM_CAP_MAX_VCPU_ID:
> 		r = KVM_MAX_VCPU_IDS;
> 
> and the common kvm_vm_ioctl_create_vcpu() does:
> 
> 	if (id >= KVM_MAX_VCPU_IDS)
> 		return -EINVAL;
> 
> and the common kvm_create_vm() does:
> 
> 	kvm->max_vcpus = KVM_MAX_VCPUS;
> 
> with again no override of max_vcpus in LoongArch or common KVM.  So unless I'm
> missing something, manually checking max_vcpus in LoongArch's kvm_arch_vcpu_precreate()
> is unnecessary.
> 
yes, you are right. There is already such checking in function 
kvm_vm_ioctl_create_vcpu(), and it is unnecessary to add the same 
checking in function kvm_arch_vcpu_precreate().

And thanks for pointing it out.

Regards
Bibo Mao