Re: [PATCH] KVM: nVMX: do not pin the VMCS12

[Christian Borntraeger]

On 07/28/2017 08:57 AM, Paolo Bonzini wrote:
> On 27/07/2017 19:20, David Matlack wrote:
>>> +       kvm_vcpu_write_guest_page(&vmx->vcpu,
>>> +                                 vmx->nested.current_vmptr >> PAGE_SHIFT,
>>> +                                 vmx->nested.cached_vmcs12, 0, VMCS12_SIZE);
>> Have you hit any "suspicious RCU usage" error messages during VM
>> teardown with this patch? We did when we replaced memcpy with
>> kvm_write_guest a while back. IIRC it was due to kvm->srcu not being
>> held in one of the teardown paths. kvm_write_guest() expects it to be
>> held in order to access memslots.
>>
>> We fixed this by skipping the VMCS12 flush during VMXOFF. I'll send
>> that patch along with a few other nVMX dirty tracking related patches
>> I've been meaning to get upstreamed.
> 
> Oh, right.  I had this other (untested) patch in the queue after
> Christian recently annotated everything with RCU checks:
> 

So you make the checks not trigger for users_count == 0 to cope with
the teardown pathes?
Since for users_count==0 all file descriptors are gone, no
memslot/bus can be changed by userspace so this makes sense.


> Paolo
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 890b706d1943..07e3b02a1be3 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -477,7 +477,8 @@ struct kvm {
>  static inline struct kvm_io_bus *kvm_get_bus(struct kvm *kvm, enum kvm_bus idx)
>  {
>  	return srcu_dereference_check(kvm->buses[idx], &kvm->srcu,
> -				      lockdep_is_held(&kvm->slots_lock));
> +				      lockdep_is_held(&kvm->slots_lock) ||
> +				      !refcount_read(&kvm->users_count));
>  }
> 
>  static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i)
> @@ -570,7 +571,8 @@ void kvm_put_kvm(struct kvm *kvm);
>  static inline struct kvm_memslots *__kvm_memslots(struct kvm *kvm, int as_id)
>  {
>  	return srcu_dereference_check(kvm->memslots[as_id], &kvm->srcu,
> -			lockdep_is_held(&kvm->slots_lock));
> +			lockdep_is_held(&kvm->slots_lock) ||
> +			!refcount_read(&kvm->users_count));
>  }
> 
>  static inline struct kvm_memslots *kvm_memslots(struct kvm *kvm)
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index f3f74271f1a9..6a21c98b22bf 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -655,7 +655,6 @@ static struct kvm *kvm_create_vm(unsigned long type)
>  	mutex_init(&kvm->lock);
>  	mutex_init(&kvm->irq_lock);
>  	mutex_init(&kvm->slots_lock);
> -	refcount_set(&kvm->users_count, 1);
>  	INIT_LIST_HEAD(&kvm->devices);
> 
>  	r = kvm_arch_init_vm(kvm, type);
> @@ -701,6 +700,7 @@ static struct kvm *kvm_create_vm(unsigned long type)
>  	if (r)
>  		goto out_err;
> 
> +	refcount_set(&kvm->users_count, 1);
>  	spin_lock(&kvm_lock);
>  	list_add(&kvm->vm_list, &vm_list);
>  	spin_unlock(&kvm_lock);
> @@ -717,10 +717,9 @@ static struct kvm *kvm_create_vm(unsigned long type)
>  	hardware_disable_all();
>  out_err_no_disable:
>  	for (i = 0; i < KVM_NR_BUSES; i++)
> -		kfree(rcu_access_pointer(kvm->buses[i]));
> +		kfree(kvm_get_bus(kvm, i));
>  	for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++)
> -		kvm_free_memslots(kvm,
> -			rcu_dereference_protected(kvm->memslots[i], 1));
> +		kvm_free_memslots(kvm, __kvm_memslots(kvm, i));
>  	kvm_arch_free_vm(kvm);
>  	mmdrop(current->mm);
>  	return ERR_PTR(r);
> @@ -754,9 +754,8 @@ static void kvm_destroy_vm(struct kvm *kvm)
>  	spin_unlock(&kvm_lock);
>  	kvm_free_irq_routing(kvm);
>  	for (i = 0; i < KVM_NR_BUSES; i++) {
> -		struct kvm_io_bus *bus;
> +		struct kvm_io_bus *bus = kvm_get_bus(kvm, i);
> 
> -		bus = rcu_dereference_protected(kvm->buses[i], 1);
>  		if (bus)
>  			kvm_io_bus_destroy(bus);
>  		kvm->buses[i] = NULL;
> @@ -770,8 +769,7 @@ static void kvm_destroy_vm(struct kvm *kvm)
>  	kvm_arch_destroy_vm(kvm);
>  	kvm_destroy_devices(kvm);
>  	for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++)
> -		kvm_free_memslots(kvm,
> -			rcu_dereference_protected(kvm->memslots[i], 1));
> +		kvm_free_memslots(kvm, __kvm_memslots(kvm, i));
>  	cleanup_srcu_struct(&kvm->irq_srcu);
>  	cleanup_srcu_struct(&kvm->srcu);
>  	kvm_arch_free_vm(kvm);
>