linux-kernel - Re: [PATCH v5 05/13] s390: vfio-ap: register matrix device with VFIO mdev framework

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Message-Id: <1f9117bd-ed14-bde4-fdbd-cb3733c8c633@linux.ibm.com>
Date:   Tue, 15 May 2018 16:17:01 +0200
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     Tony Krowiak <akrowiak@...ux.vnet.ibm.com>,
        Halil Pasic <pasic@...ux.ibm.com>, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     freude@...ibm.com, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, borntraeger@...ibm.com,
        cohuck@...hat.com, kwankhede@...dia.com,
        bjsdjshi@...ux.vnet.ibm.com, pbonzini@...hat.com,
        alex.williamson@...hat.com, pmorel@...ux.vnet.ibm.com,
        alifm@...ux.vnet.ibm.com, mjrosato@...ux.vnet.ibm.com,
        jjherne@...ux.vnet.ibm.com, thuth@...hat.com,
        pasic@...ux.vnet.ibm.com, berrange@...hat.com,
        fiuczy@...ux.vnet.ibm.com, buendgen@...ibm.com
Subject: Re: [PATCH v5 05/13] s390: vfio-ap: register matrix device with VFIO
 mdev framework

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@...ux.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.

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

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