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Subject: Re: [PATCH v5 05/13] s390: vfio-ap: register matrix device with VFIO
 mdev framework
To:     Tony Krowiak <akrowiak@linux.vnet.ibm.com>, pmorel@linux.ibm.com,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     freude@de.ibm.com, schwidefsky@de.ibm.com,
        heiko.carstens@de.ibm.com, borntraeger@de.ibm.com,
        cohuck@redhat.com, kwankhede@nvidia.com,
        bjsdjshi@linux.vnet.ibm.com, pbonzini@redhat.com,
        alex.williamson@redhat.com, pmorel@linux.vnet.ibm.com,
        alifm@linux.vnet.ibm.com, mjrosato@linux.vnet.ibm.com,
        jjherne@linux.vnet.ibm.com, thuth@redhat.com,
        pasic@linux.vnet.ibm.com, berrange@redhat.com,
        fiuczy@linux.vnet.ibm.com, buendgen@de.ibm.com
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 <afd54b28-e61c-a81e-41a3-6142d7277400@linux.vnet.ibm.com>
From:   Halil Pasic <pasic@linux.ibm.com>
Date:   Tue, 15 May 2018 17:48:37 +0200
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On 05/15/2018 05:16 PM, Tony Krowiak wrote:
> On 05/15/2018 10:17 AM, Pierre Morel wrote:
>> On 14/05/2018 21:42, Tony Krowiak wrote:
>>> On 05/11/2018 01:18 PM, Halil Pasic wrote:
>>>>
>>>>
>>>> On 05/07/2018 05:11 PM, Tony Krowiak wrote:
>>>>> Registers the matrix device created by the VFIO AP device
>>>>> driver with the VFIO mediated device framework.
>>>>> Registering the matrix device will create the sysfs
>>>>> structures needed to create mediated matrix devices
>>>>> each of which will be used to configure the AP matrix
>>>>> for a guest and connect it to the VFIO AP device driver.
>>>>>
>>>> [..]
>>>>> diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
>>>>> new file mode 100644
>>>>> index 0000000..d7d36fb
>>>>> --- /dev/null
>>>>> +++ b/drivers/s390/crypto/vfio_ap_ops.c
>>>>> @@ -0,0 +1,106 @@
>>>>> +// SPDX-License-Identifier: GPL-2.0+
>>>>> +/*
>>>>> + * Adjunct processor matrix VFIO device driver callbacks.
>>>>> + *
>>>>> + * Copyright IBM Corp. 2017
>>>>> + * Author(s): Tony Krowiak <akrowiak@linux.vnet.ibm.com>
>>>>> + *
>>>>> + */
>>>>> +#include <linux/string.h>
>>>>> +#include <linux/vfio.h>
>>>>> +#include <linux/device.h>
>>>>> +#include <linux/list.h>
>>>>> +#include <linux/ctype.h>
>>>>> +
>>>>> +#include "vfio_ap_private.h"
>>>>> +
>>>>> +#define VFOP_AP_MDEV_TYPE_HWVIRT "passthrough"
>>>>> +#define VFIO_AP_MDEV_NAME_HWVIRT "VFIO AP Passthrough Device"
>>>>> +
>>>>> +static int vfio_ap_mdev_create(struct kobject *kobj, struct mdev_device *mdev)
>>>>> +{
>>>>> +    struct ap_matrix *ap_matrix = to_ap_matrix(mdev_parent_dev(mdev));
>>>>> +
>>>>> +    ap_matrix->available_instances--;
>>>>> +
>>>>> +    return 0;
>>>>> +}
>>>>> +
>>>>> +static int vfio_ap_mdev_remove(struct mdev_device *mdev)
>>>>> +{
>>>>> +    struct ap_matrix *ap_matrix = to_ap_matrix(mdev_parent_dev(mdev));
>>>>> +
>>>>> +    ap_matrix->available_instances++;
>>>>> +
>>>>> +    return 0;
>>>>> +}
>>>>> +
>>>>
>>>> The above functions seem to be called with the lock of this auto-generated
>>>> mdev parent device held. That's why we don't have to care about synchronization
>>>> ourselves, right?
>>>
>>> I would assume as much. The comments for the 'struct mdev_parent_ops' in
>>> include/linux/mdev.h do not mention anything about synchronization, nor did I
>>> see any locking or synchronization in the vfio_ccw implementation after which
>>> I modeled my code, so frankly it is something I did not consider.
>>>
>>>>
>>>>
>>>> A small comment in the code could be helpful for mdev non-experts. Hell, I would
>>>> even consider documenting it for all mdev -- took me some time to figure out.
>>>
>>> You may want to bring this up with the VFIO mdev maintainers, but I'd be happy to
>>> include a comment in the functions in question if you think it important.
>>>
>>>>
>>>>
>>>> [..]
>>>>
>>>>
>>>>> +int vfio_ap_mdev_register(struct ap_matrix *ap_matrix)
>>>>> +{
>>>>> +    int ret;
>>>>> +
>>>>> +    ret = mdev_register_device(&ap_matrix->device, &vfio_ap_matrix_ops);
>>>>> +    if (ret)
>>>>> +        return ret;
>>>>> +
>>>>> +    ap_matrix->available_instances = AP_MATRIX_MAX_AVAILABLE_INSTANCES;
>>>>> +
>>>>> +    return 0;
>>>>> +}
>>>>> +
>>>>> +void vfio_ap_mdev_unregister(struct ap_matrix *ap_matrix)
>>>>> +{
>>>>> +    ap_matrix->available_instances--;
>>>>
>>>> What is this for? I don't understand.
>>>
>>> To control the number of mediated devices one can create for the matrix device.
>>> Once the max is reached, the mdev framework will not allow creation of another
>>> mediated device until one is removed. This counter keeps track of the number
>>> of instances that can be created. This is documented with the mediated
>>> framework. You may want to take a look at:
>>>
>>> Documentation/vfio-mediated-device.txt
>>> Documentation/vfio.txt
>>> Documentation/virtual/kvm/devices/vfio.txt
>>
>> This is what you do in create/remove.
>> But here in unregister I agree with Halil, it does not seem to be usefull.
> 
> If that is in fact what Halil was asking, then I misinterpreted his question; I
> thought he was asking what the available_instances was used for. You are
> correct, this does not belong here although it makes little difference given
> this is called only when the driver, which creates the matrix device, is unloaded.
> It is necessary in the register function to initialize its value, but I'll
> remove it from here.
> 

I questioned the dubious usage of ap_matrix->available_instances rather than
asking what is the variable for.

If I've had this deemed damaging I would have asked if it's damaging in a way
I think it is. For example take my comment on 'KVM: s390: interfaces to manage
guest's AP matrix'.

Regards,
Halil

>>
>>
>>>
>>>>
>>>>
>>>> Regards,
>>>> Halil
>>>>
>>>>> + mdev_unregister_device(&ap_matrix->device);
>>>>> +}
>>>
>>>
>>
>