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

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Date: Fri, 12 Apr 2024 11:41:48 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Wujie Duan <wjduan@...x-info.com>
Cc: zhaotianrui@...ngson.cn, maobibo@...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 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.