linux-kernel - Re: [PATCH v5 4/7] s390: ap: setup relation betwen KVM and mediated device

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Date:   Tue, 19 Mar 2019 15:23:13 +0100
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     Halil Pasic <pasic@...ux.ibm.com>
Cc:     borntraeger@...ibm.com, alex.williamson@...hat.com,
        cohuck@...hat.com, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, kvm@...r.kernel.org,
        frankja@...ux.ibm.com, akrowiak@...ux.ibm.com, david@...hat.com,
        schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
        freude@...ux.ibm.com, mimu@...ux.ibm.com
Subject: Re: [PATCH v5 4/7] s390: ap: setup relation betwen KVM and mediated
 device

On 19/03/2019 12:54, Halil Pasic wrote:
> On Tue, 19 Mar 2019 10:38:42 +0100
> Pierre Morel <pmorel@...ux.ibm.com> wrote:
> 
>> On 15/03/2019 19:15, Halil Pasic wrote:
>>> On Wed, 13 Mar 2019 17:05:01 +0100
>>> Pierre Morel <pmorel@...ux.ibm.com> wrote:
>>>
>>>> When the mediated device is open we setup the relation with KVM unset it
>>>> when the mediated device is released.
>>>>
>>>> We ensure KVM is present on opening of the mediated device.
>>>>
>>>> We ensure that KVM survives the mediated device, and establish a direct
>>>
>>> survives?
>>
>> what alternative do you prefer?
>>
> 
> Increase kvm's refcount to ensure the guest is alive when the
> ap_matrix_mdev is active. An ap mp_matrix becomes active with
> a successful open() and ceases to be active with a release().

Right, it is mdev usage not mdev.

> 
> Your sentence was materially wrong as the mdev is allowed to outlive
> the KVM. BTW survive tends to have an 'in spite of' note to it, which
> outlive does not. vfio-ap is, I hope, not a calamity that threatens
> the life of KVM ;). https://en.oxforddictionaries.com/definition/survive

Thanks, your description is much better.

> 
>>>
>>>> link from KVM to the mediated device to simplify the relationship.
>>>>
>>>> Signed-off-by: Pierre Morel <pmorel@...ux.ibm.com>
>>>> ---
>>
>> ...snip...
>>
>>>>    static int vfio_ap_mdev_group_notifier(struct notifier_block *nb,
>>>>    				       unsigned long action, void *data)
>>>>    {
>>>> -	int ret;
>>>>    	struct ap_matrix_mdev *matrix_mdev;
>>>>    
>>>>    	if (action != VFIO_GROUP_NOTIFY_SET_KVM)
>>>>    		return NOTIFY_OK;
>>>>    
>>>>    	matrix_mdev = container_of(nb, struct ap_matrix_mdev, group_notifier);
>>>> -
>>>> -	if (!data) {
>>>> -		matrix_mdev->kvm = NULL;
>>>> -		return NOTIFY_OK;
>>>> -	}
>>>> -
>>>> -	ret = vfio_ap_mdev_set_kvm(matrix_mdev, data);
>>>> -	if (ret)
>>>> -		return NOTIFY_DONE;
>>>> -
>>>> -	/* If there is no CRYCB pointer, then we can't copy the masks */
>>>> -	if (!matrix_mdev->kvm->arch.crypto.crycbd)
>>>> -		return NOTIFY_DONE;
>>>> -
>>>> -	kvm_arch_crypto_set_masks(matrix_mdev->kvm, matrix_mdev->matrix.apm,
>>>> -				  matrix_mdev->matrix.aqm,
>>>> -				  matrix_mdev->matrix.adm);
>>>> +	matrix_mdev->kvm = data;
>>>>    
>>>>    	return NOTIFY_OK;
>>>>    }
>>>> @@ -888,6 +873,12 @@ static int vfio_ap_mdev_open(struct mdev_device *mdev)
>>>>    	if (ret)
>>>>    		goto err_group;
>>>>    
>>>> +	/* We do not support opening the mediated device without KVM */
>>>> +	if (!matrix_mdev->kvm) {
>>>> +		ret = -ENODEV;
>>>> +		goto err_group;
>>>> +	}
>>>> +
>>>>    	matrix_mdev->iommu_notifier.notifier_call = vfio_ap_mdev_iommu_notifier;
>>>>    	events = VFIO_IOMMU_NOTIFY_DMA_UNMAP;
>>>>    
>>>> @@ -896,8 +887,15 @@ static int vfio_ap_mdev_open(struct mdev_device *mdev)
>>>>    	if (ret)
>>>>    		goto err_iommu;
>>>>    
>>>> +	ret = vfio_ap_mdev_set_kvm(matrix_mdev);
>>>
>>> At this point the matrix_mdev->kvm ain't guaranteed to be valid IMHO. Or
>>> am I wrong? If I'm right kvm_get_kvm(matrix_mdev->kvm) could be too late.
>>
>> What about the if (!matrix_mdev->kvm) 10 lines above ?
>>
> 
> That check is not sufficient.
> 
> You should do the kvm_get_kvm() in vfio_ap_mdev_group_notifier(). VFIO
> must ensure that the kvm pointer you get is valid, in a sense that it
> points to a valid struct kvm and the kvm object is alive, while you are
> in the callback. But not beyond.
> 
> If another thread were to decrement the refcount of the kvm object you
> would end up with matrix_mdev->kvm pointing to an object that has already
> died.
> 
> Does my analysis make sense to you?

Yes thanks the explication is good, it would have been worth to get it 
the first time.

> 
>>>
>>>> +	if (ret)
>>>> +		goto err_kvm;
>>>> +
>>>>    	return 0;
>>>>    
>>>> +err_kvm:
>>>> +	vfio_unregister_notifier(mdev_dev(mdev), VFIO_IOMMU_NOTIFY,
>>>> +				 &matrix_mdev->iommu_notifier);
>>>>    err_iommu:
>>>>    	vfio_unregister_notifier(mdev_dev(mdev), VFIO_GROUP_NOTIFY,
>>>>    				 &matrix_mdev->group_notifier);
>>>> @@ -906,19 +904,33 @@ static int vfio_ap_mdev_open(struct mdev_device *mdev)
>>>>    	return ret;
>>>>    }
>>>>    
>>>> -static void vfio_ap_mdev_release(struct mdev_device *mdev)
>>>> +static int vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev)
>>>>    {
>>>> -	struct ap_matrix_mdev *matrix_mdev = mdev_get_drvdata(mdev);
>>>> +	struct kvm *kvm = matrix_mdev->kvm;
>>>>    
>>>>    	if (matrix_mdev->kvm)
>>>>    		kvm_arch_crypto_clear_masks(matrix_mdev->kvm);
>>>
>>> This still conditional?
>>
>> Yes, nothing to clear if there is no KVM.
>>
> 
> Since we have ensured the open only works if there is a KVM at that
> point in time, and we have taken a reference to KVM, I would expect
> KVM can not go away before we give up our reference.

Right.


Thanks,
Pierre

-- 
Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany