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Subject: Re: [PATCH v6 3/7] s390: ap: setup relation betwen KVM and mediated
 device
To:     pmorel@linux.ibm.com, borntraeger@de.ibm.com
Cc:     alex.williamson@redhat.com, cohuck@redhat.com,
        linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org,
        kvm@vger.kernel.org, frankja@linux.ibm.com, pasic@linux.ibm.com,
        david@redhat.com, schwidefsky@de.ibm.com,
        heiko.carstens@de.ibm.com, freude@linux.ibm.com, mimu@linux.ibm.com
References: <1553265828-27823-1-git-send-email-pmorel@linux.ibm.com>
 <1553265828-27823-4-git-send-email-pmorel@linux.ibm.com>
 <1ea236d1-ca0b-03c0-3699-0c0deb435785@linux.ibm.com>
 <3cd496d0-3eec-78e8-9ea5-4d62fe0cff1c@linux.ibm.com>
 <eaac369f-132b-b83f-fa14-8749819388ba@linux.ibm.com>
 <cce25d53-9076-d54f-10d7-46526b5f7e54@linux.ibm.com>
From:   Tony Krowiak <akrowiak@linux.ibm.com>
Date:   Fri, 29 Mar 2019 09:06:00 -0400
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On 3/29/19 4:58 AM, Pierre Morel wrote:
> On 28/03/2019 18:25, Tony Krowiak wrote:
>> On 3/28/19 12:27 PM, Pierre Morel wrote:
>>> On 28/03/2019 17:12, Tony Krowiak wrote:
>>>> On 3/22/19 10:43 AM, Pierre Morel wrote:
>>>>> When the mediated device is open we setup the relation with KVM 
>>>>> unset it
>>>>> when the mediated device is released.
>>>>
>>>> s/open we setup/open, we set up/
>>>> s/with KVM unset/with KVM and unset/
>>>>
>>>>>
>>>>> We lock the matrix mediated device to avoid any change until the
>>>>> open is done.
>>>>> We make sure that KVM is present when opening the mediated device
>>>>> otherwise we return an error.
>>>>
>>>> s/mediated device/mediated device,/
>>>>
>>>>>
>>>>> Increase kvm's refcount to ensure the KVM structures are still 
>>>>> available
>>>>> during the use of the mediated device by the guest.
>>>>>
>>>>> Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
>>>>> ---
>>>>>   drivers/s390/crypto/vfio_ap_ops.c | 143 
>>>>> +++++++++++++++++++++-----------------
>>>>>   1 file changed, 79 insertions(+), 64 deletions(-)
>>>>>
>>>>> diff --git a/drivers/s390/crypto/vfio_ap_ops.c 
>>>>> b/drivers/s390/crypto/vfio_ap_ops.c
>>>>> index 77f7bac..bdb36e0 100644
>>>>> --- a/drivers/s390/crypto/vfio_ap_ops.c
>>>>> +++ b/drivers/s390/crypto/vfio_ap_ops.c
>>>>> @@ -787,74 +787,24 @@ static const struct attribute_group 
>>>>> *vfio_ap_mdev_attr_groups[] = {
>>>>>       NULL
>>>>>   };
>>>>> -/**
>>>>> - * vfio_ap_mdev_set_kvm
>>>>> - *
>>>>> - * @matrix_mdev: a mediated matrix device
>>>>> - * @kvm: reference to KVM instance
>>>>> - *
>>>>> - * Verifies no other mediated matrix device has @kvm and sets a 
>>>>> reference to
>>>>> - * it in @matrix_mdev->kvm.
>>>>> - *
>>>>> - * Return 0 if no other mediated matrix device has a reference to 
>>>>> @kvm;
>>>>> - * otherwise, returns an -EPERM.
>>>>> - */
>>>>> -static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
>>>>> -                struct kvm *kvm)
>>>>> -{
>>>>> -    struct ap_matrix_mdev *m;
>>>>> -
>>>>> -    mutex_lock(&matrix_dev->lock);
>>>>> -
>>>>> -    list_for_each_entry(m, &matrix_dev->mdev_list, node) {
>>>>> -        if ((m != matrix_mdev) && (m->kvm == kvm)) {
>>>>> -            mutex_unlock(&matrix_dev->lock);
>>>>> -            return -EPERM;
>>>>> -        }
>>>>> -    }
>>>>> -
>>>>> -    matrix_mdev->kvm = kvm;
>>>>> -    mutex_unlock(&matrix_dev->lock);
>>>>> -
>>>>> -    return 0;
>>>>> -}
>>>>> -
>>>>>   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;
>>>>>   }
>>>>> -static int vfio_ap_mdev_reset_queues(struct mdev_device *mdev)
>>>>> +static int vfio_ap_mdev_reset_queues(struct ap_matrix_mdev 
>>>>> *matrix_mdev)
>>>>>   {
>>>>>       int ret;
>>>>>       int rc = 0;
>>>>> -    struct ap_matrix_mdev *matrix_mdev = mdev_get_drvdata(mdev);
>>>>>       struct vfio_ap_queue *q;
>>>>>       list_for_each_entry(q, &matrix_mdev->qlist, list) {
>>>>> @@ -871,41 +821,106 @@ static int vfio_ap_mdev_reset_queues(struct 
>>>>> mdev_device *mdev)
>>>>>       return rc;
>>>>>   }
>>>>> +/**
>>>>> + * vfio_ap_mdev_set_kvm
>>>>> + *
>>>>> + * @matrix_mdev: a mediated matrix device
>>>>> + *
>>>>> + * - Verifies that the hook is free and install the PQAP hook
>>>>> + * - Copy the matrix masks inside the CRYCB
>>>>> + * - Increment the KVM rerference count
>>>>> + *
>>>>> + * Return 0 if no other mediated matrix device has a reference to 
>>>>> @kvm;
>>>>> + * otherwise, returns an -EPERM.
>>>>> + */
>>>>> +static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev)
>>>>> +{
>>>>> +    if (matrix_mdev->kvm->arch.crypto.pqap_hook)
>>>>> +        return -EPERM;
>>>>
>>>> How would this happen; in other words, why are we checking this?
>>>
>>> I check this to verify that no other AP mediated device is already in 
>>> use by this VM.
>>
>> Maybe you should insert a comment to that effect.
> 
> Please notice that there is already a comment on this in the description 
> of the function.

True, but that comment merely states that the function verifies the
hook is free, not the reason why that particular check is done. When
I reviewed the code and saw this check, I wondered why it was necessary.
The comment you have would not have helped in this regard, so maybe
you need to update your comment.

> 
> Regards,
> Pierre
> 
> 
>