Re: [Qemu-devel] [RFC PATCH v4 1/3] Mediated device Core driver

[Kirti Wankhede]

Thanks Alex.

I'll consider all the nits and fix those in next version of patch.

More below:

On 5/26/2016 4:09 AM, Alex Williamson wrote:
> On Wed, 25 May 2016 01:28:15 +0530
> Kirti Wankhede <address@hidden> wrote:
>

...

>> +
>> +config MDEV
>> +    tristate "Mediated device driver framework"
>> +    depends on VFIO
>> +    default n
>> +    help
>> +        MDEV provides a framework to virtualize device without
SR-IOV cap
>> +        See Documentation/mdev.txt for more details.
>
> I don't see that file anywhere in this series.

Yes, missed this file in this patch. I'll add it in next version of patch.
Since mdev module is moved in vfio directory, should I place this file
in vfio directory, Documentation/vfio/mdev.txt? or keep documentation of
mdev module within vfio.txt itself?


>> +    if (phy_dev) {
>> +            mutex_lock(&phy_devices.list_lock);
>> +
>> +            /*
>> +            * If vendor driver doesn't return success that means vendor
>> +            * driver doesn't support hot-unplug
>> +            */
>> +            if (phy_dev->ops->destroy) {
>> +                    if (phy_dev->ops->destroy(phy_dev->dev, mdevice->uuid,
>> +                                              mdevice->instance)) {
>> +                            mutex_unlock(&phy_devices.list_lock);
>> +                            return;
>> +                    }
>> +            }
>> +
>> +            mdev_remove_attribute_group(&mdevice->dev,
>> +                                        phy_dev->ops->mdev_attr_groups);
>> +            mdevice->phy_dev = NULL;
>> +            mutex_unlock(&phy_devices.list_lock);
>
> Locking here appears arbitrary, how does the above code interact with
> phy_devices.dev_list?
>

Sorry for not being clear about phy_devices.list_lock, probably I
shouldn't have named it 'list_lock'. This lock is also to synchronize
register_device & unregister_device and physical device specific
callbacks: supported_config, create, destroy, start and shutdown.
Although supported_config, create and destroy are per phy_device
specific callbacks while start and shutdown could refer to multiple
phy_devices indirectly when there are multiple mdev devices of same type
on different physical devices. There could be race condition in start
callback and destroy & unregister_device. I'm revisiting this lock again
and will see to use per phy device lock for phy_device specific callbacks.


>> +struct mdev_device {
>> +    struct kref             kref;
>> +    struct device           dev;
>> +    struct phy_device       *phy_dev;
>> +    struct iommu_group      *group;
>> +    void                    *iommu_data;
>> +    uuid_le                 uuid;
>> +    uint32_t                instance;
>> +    void                    *driver_data;
>> +    struct mutex            ops_lock;
>> +    struct list_head        next;
>> +};
>
> Could this be in the private header?  Seems like this should be opaque
> outside of mdev core.
>

No, this structure is used in mediated device call back functions to
vendor driver so that vendor driver could identify mdev device, similar
to pci_dev structure in pci bus subsystem. (I'll remove kref which is
not being used at all.)


>> + * @read:           Read emulation callback
>> + *                  @mdev: mediated device structure
>> + *                  @buf: read buffer
>> + *                  @count: number bytes to read
>> + *                  @address_space: specifies for which address
>> + *                  space the request is: pci_config_space, IO
>> + *                  register space or MMIO space.
>
> Seems like I asked before and it's no more clear in the code, how do we
> handle multiple spaces for various types?  ie. a device might have
> multiple MMIO spaces.
>
>> + *                  @pos: offset from base address.

Sorry, updated the code but missed to update comment here.
pos = base_address + offset
(its not 'pos' anymore, will rename it to addr)

so vendor driver is aware about base addresses of multiple MMIO spaces
and its size, they can identify MMIO space based on addr.

>> +/*
>> + * Physical Device
>> + */
>> +struct phy_device {
>> +    struct device                   *dev;
>> +    const struct phy_device_ops     *ops;
>> +    struct list_head                next;
>> +};
>
> I would really like to be able to use the mediated device interface to
> create a purely virtual device, is the expectation that my physical
> device interface would create a virtual struct device which would
> become the parent and control point in sysfs for creating all the mdev
> devices? Should we be calling this a host_device or mdev_parent_dev in
> that case since there's really no requirement that it be a physical
> device?

Makes sense. I'll rename it to parent_device.

Thanks,
Kirti.