Re: [PATCH v7 7/8] KVM: x86: Allow userspace set maximum VCPU id for VM

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

On 4/1/2022 10:01 AM, Sean Christopherson wrote:
> On Fri, Mar 04, 2022, Zeng Guang wrote:
>> Introduce new max_vcpu_id in KVM for x86 architecture. Userspace
>> can assign maximum possible vcpu id for current VM session using
>> KVM_CAP_MAX_VCPU_ID of KVM_ENABLE_CAP ioctl().
>>
>> This is done for x86 only because the sole use case is to guide
>> memory allocation for PID-pointer table, a structure needed to
>> enable VMX IPI.
>>
>> By default, max_vcpu_id set as KVM_MAX_VCPU_IDS.
>>
>> Suggested-by: Sean Christopherson <seanjc@google.com>
>> Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
>> Signed-off-by: Zeng Guang <guang.zeng@intel.com>
>> ---
>>   arch/x86/include/asm/kvm_host.h |  6 ++++++
>>   arch/x86/kvm/x86.c              | 11 +++++++++++
> The new behavior needs to be documented in api.rst.

OK. I will prepare document for it.


>>   2 files changed, 17 insertions(+)
>>
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index 6dcccb304775..db16aebd946c 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -1233,6 +1233,12 @@ struct kvm_arch {
>>   	hpa_t	hv_root_tdp;
>>   	spinlock_t hv_root_tdp_lock;
>>   #endif
>> +	/*
>> +	 * VM-scope maximum vCPU ID. Used to determine the size of structures
>> +	 * that increase along with the maximum vCPU ID, in which case, using
>> +	 * the global KVM_MAX_VCPU_IDS may lead to significant memory waste.
>> +	 */
>> +	u32 max_vcpu_id;
> This should be max_vcpu_ids.  I agree the it _should_ be max_vcpu_id, but KVM's API
> for this is awful and we're stuck with the plural name.
>
>>   };
>>   
>>   struct kvm_vm_stat {
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 4f6fe9974cb5..ca17cc452bd3 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -5994,6 +5994,13 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>>   		kvm->arch.exit_on_emulation_error = cap->args[0];
>>   		r = 0;
>>   		break;
>> +	case KVM_CAP_MAX_VCPU_ID:
> I think it makes sense to change kvm_vm_ioctl_check_extension() to return the
> current max, it is a VM-scoped ioctl after all.

kvm_vm_ioctl_check_extension() can return kvm->arch.max_vcpu_ids
as it reflects runtime capability supported on current vm. I will
change it.

> Amusingly, I think we also need a capability to enumerate that KVM_CAP_MAX_VCPU_ID
> is writable.

IIUC, KVM_CAP_*  has intrinsic writable attribute. KVM will return invalid
If not implemented.

>> +		if (cap->args[0] <= KVM_MAX_VCPU_IDS) {
>> +			kvm->arch.max_vcpu_id = cap->args[0];
> This needs to be rejected if kvm->created_vcpus > 0, and that check needs to be
> done under kvm_lock, otherwise userspace can bump the max ID after KVM allocates
> per-VM structures and trigger buffer overflow.

Is it necessary to use kvm_lock ? Seems no use case to call it from multi-threads.

>> +			r = 0;
>> +		} else
> If-elif-else statements need curly braces for all paths if any path needs braces.
> Probably a moot point for this patch due to the above changes.
>
>> +			r = -E2BIG;
> This should be -EINVAL, not -E2BIG.
>
> E.g.
>
> 	case KVM_CAP_MAX_VCPU_ID:
> 		r = -EINVAL;
> 		if (cap->args[0] > KVM_MAX_VCPU_IDS)
> 			break;
>
> 		mutex_lock(&kvm->lock);
> 		if (!kvm->created_vcpus) {
> 			kvm->arch.max_vcpu_id = cap->args[0];
> 			r = 0;
> 		}
> 		mutex_unlock(&kvm->lock);
> 		break;
>
>
>> +		break;
>>   	default:
>>   		r = -EINVAL;
>>   		break;
>> @@ -11067,6 +11074,9 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
>>   	struct page *page;
>>   	int r;
>>   
>> +	if (vcpu->vcpu_id >= vcpu->kvm->arch.max_vcpu_id)
>> +		return -E2BIG;
> Same here, it should be -EINVAL.
>
>> +
>>   	vcpu->arch.last_vmentry_cpu = -1;
>>   	vcpu->arch.regs_avail = ~0;
>>   	vcpu->arch.regs_dirty = ~0;
>> @@ -11589,6 +11599,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>>   	spin_lock_init(&kvm->arch.hv_root_tdp_lock);
>>   	kvm->arch.hv_root_tdp = INVALID_PAGE;
>>   #endif
>> +	kvm->arch.max_vcpu_id = KVM_MAX_VCPU_IDS;
>>   
>>   	INIT_DELAYED_WORK(&kvm->arch.kvmclock_update_work, kvmclock_update_fn);
>>   	INIT_DELAYED_WORK(&kvm->arch.kvmclock_sync_work, kvmclock_sync_fn);
>> -- 
>> 2.27.0
>>