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

[maobibo]

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@xxxxxxxxxxxxx>
> > ---
> >   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