Re: [RFC PATCH 1/2] vfio-mdev: Wire in a request handler for mdev parent

[Eric Farman <farman@linux.ibm.com>]

On 11/19/20 11:27 AM, Alex Williamson wrote:
> On Thu, 19 Nov 2020 12:30:26 +0100
> Cornelia Huck <cohuck@redhat.com> wrote:
> 
>> On Tue, 17 Nov 2020 04:21:38 +0100
>> Eric Farman <farman@linux.ibm.com> wrote:
>>
>>> While performing some destructive tests with vfio-ccw, where the
>>> paths to a device are forcible removed and thus the device itself
>>> is unreachable, it is rather easy to end up in an endless loop in
>>> vfio_del_group_dev() due to the lack of a request callback for the
>>> associated device.
>>>
>>> In this example, one MDEV (77c) is used by a guest, while another
>>> (77b) is not. The symptom is that the iommu is detached from the
>>> mdev for 77b, but not 77c, until that guest is shutdown:
>>>
>>>      [  238.794867] vfio_ccw 0.0.077b: MDEV: Unregistering
>>>      [  238.794996] vfio_mdev 11f2d2bc-4083-431d-a023-eff72715c4f0: Removing from iommu group 2
>>>      [  238.795001] vfio_mdev 11f2d2bc-4083-431d-a023-eff72715c4f0: MDEV: detaching iommu
>>>      [  238.795036] vfio_ccw 0.0.077c: MDEV: Unregistering
>>>      ...silence...
>>>
>>> Let's wire in the request call back to the mdev device, so that a hot
>>> unplug can be (gracefully?) handled by the parent device at the time
>>> the device is being removed.
>>
>> I think it makes a lot of sense to give the vendor driver a way to
>> handle requests.
>>
>>>
>>> Signed-off-by: Eric Farman <farman@linux.ibm.com>
>>> ---
>>>   drivers/vfio/mdev/vfio_mdev.c | 11 +++++++++++
>>>   include/linux/mdev.h          |  4 ++++
>>>   2 files changed, 15 insertions(+)
>>>
>>> diff --git a/drivers/vfio/mdev/vfio_mdev.c b/drivers/vfio/mdev/vfio_mdev.c
>>> index 30964a4e0a28..2dd243f73945 100644
>>> --- a/drivers/vfio/mdev/vfio_mdev.c
>>> +++ b/drivers/vfio/mdev/vfio_mdev.c
>>> @@ -98,6 +98,16 @@ static int vfio_mdev_mmap(void *device_data, struct vm_area_struct *vma)
>>>   	return parent->ops->mmap(mdev, vma);
>>>   }
>>>   
>>> +static void vfio_mdev_request(void *device_data, unsigned int count)
>>> +{
>>> +	struct mdev_device *mdev = device_data;
>>> +	struct mdev_parent *parent = mdev->parent;
>>> +
>>> +	if (unlikely(!parent->ops->request))
>>
>> Hm. Do you think that all drivers should implement a ->request()
>> callback?
> 
> It's considered optional for bus drivers in vfio-core, obviously
> mdev-core could enforce presence of this callback, but then we'd break
> existing out of tree drivers.  We don't make guarantees to out of tree
> drivers, but it feels a little petty.  We could instead encourage such
> support by printing a warning for drivers that register without a
> request callback.

Coincidentally, I'd considered adding a dev_warn_once() message in
drivers/vfio/vfio.c:vfio_del_group_dev() when vfio_device->ops->request
is NULL, and thus we're looping endlessly (and silently). But adding 
this patch and not patch 2 made things silent again, so I left it out. 
Putting a warning when the driver registers seems cool.

> 
> Minor nit, I tend to prefer:
> 
> 	if (callback for thing)
> 		call thing
> 
> Rather than
> 
> 	if (!callback for thing)
> 		return;
> 	call thing

I like it too.  I'll set it up that way in v2.

> 
> Thanks,
> Alex
> 
>>
>>> +		return;
>>> +	parent->ops->request(mdev, count);
>>> +}
>>> +
>>>   static const struct vfio_device_ops vfio_mdev_dev_ops = {
>>>   	.name		= "vfio-mdev",
>>>   	.open		= vfio_mdev_open,
>