Re: [PATCH RFCv1 08/14] iommufd: Add IOMMU_VIOMMU_SET_DEV_ID ioctl

[Nicolin Chen <nicolinc@nvidia.com>]

On Sun, May 12, 2024 at 11:58:27AM -0300, Jason Gunthorpe wrote:
> On Fri, Apr 12, 2024 at 08:47:05PM -0700, Nicolin Chen wrote:
> > Introduce a new ioctl to set a per-viommu device virtual id that should be
> > linked to the physical device id (or just a struct device pointer).
> > 
> > Since a viommu (user space IOMMU instance) can have multiple devices while
> > it's not ideal to confine a device to one single user space IOMMU instance
> > either, these two shouldn't just do a 1:1 mapping. Add two xarrays in their
> > structures to bind them bidirectionally.
> 
> Since I would like to retain the dev_id, I think this is probably
> better done with an allocated struct per dev-id:
> 
> struct dev_id {
>     struct iommufd_device *idev;
>     struct iommufd_viommu *viommu;
>     u64 vdev_id;
>     u64 driver_private; // Ie the driver can store the pSID here
>     struct list_head idev_entry;
> };

I implemented it with a small tweak, to align with viommu_alloc
and vqueue_alloc:

	// core
	struct iommufd_vdev_id {
		struct iommufd_viommu *viommu;
		struct device *dev;
		u64 vdev_id;
		struct list_head idev_item;
	};

	// driver
	struct my_driver_vdev_id {
	    struct iommufd_vdev_id core;
	    unsigned int private_attrs;
	};

	static struct iommufd_vdev_id *
	my_driver_set_vdev_id(struct iommufd_viommu *viommu,
			      struct device *dev, u64 id)
	{
	    struct my_driver_vdev_id *my_vdev_id;

	    my_vdev_id = kzalloc(sizeof(*my_vdev_id), GFP_KERNEL);
	    .... /* set private_attrs */
	    return &my_driver_vdev_id->core;
	}

	static void my_driver_unset_vdev_id(struct iommufd_vdev_id *vdev_id)
	{
	    struct my_driver_vdev_id *my_vdev_id =
		    container_of(vdev_id, struct my_driver_vdev_id, core);
	    .... /* unset private_attrs */
	}

Please let me know if you like it inverted as you wrote above.

> > @@ -135,7 +135,16 @@ void iommufd_device_destroy(struct iommufd_object *obj)
> >  {
> >  	struct iommufd_device *idev =
> >  		container_of(obj, struct iommufd_device, obj);
> > +	struct iommufd_viommu *viommu;
> > +	unsigned long index;
> >  
> > +	xa_for_each(&idev->viommus, index, viommu) {
> > +		if (viommu->ops->unset_dev_id)
> > +			viommu->ops->unset_dev_id(viommu, idev->dev);
> > +		xa_erase(&viommu->idevs, idev->obj.id);
> > +		xa_erase(&idev->viommus, index);
> > +	}
> 
> Then this turns into list_for_each(idev->viommu_vdevid_list)

Done.

> > +int iommufd_viommu_set_device_id(struct iommufd_ucmd *ucmd)
> > +{
...
> > +	rc = xa_alloc(&idev->viommus, &viommu_id, viommu,
> > +		      XA_LIMIT(viommu->obj.id, viommu->obj.id),
> > +		      GFP_KERNEL_ACCOUNT);
> > +	if (rc)
> > +		goto out_put_viommu;
> > +
> > +	rc = xa_alloc(&viommu->idevs, &dev_id, idev,
> > +		      XA_LIMIT(dev_id, dev_id), GFP_KERNEL_ACCOUNT);
> > +	if (rc)
> > +		goto out_xa_erase_viommu;
> 
> Both of these are API mis-uses, you don't want an allocating xarray
> you just want to use xa_cmpxchg
> 
> Put an xarray in the viommu object and fill it with pointers to the
> struct dev_id thing above
> 
> The driver can piggyback on this xarray too if it wants, so we only
> need one.

I also moved xa_cmpxchg to the driver, as I feel that the vdev_id
here is very driver/iommu specific. There can be some complication
if iommufd core handles this u64 vdev_id: most likely we will use
this u64 vdev_id to index the xarray that takes an unsigned-long
xa_index for a fast vdev_id-to-pdev_id lookup, while only a driver
knows whether u64 vdev_id is compatible with unsigned long or not.

And, we have a list_head in the structure idev, so a device unbind
will for-each the list and unset all the vdev_ids in it, meanwhile
the viommu stays. I wonder if we need to add another list_head in
the structure viommu, so a viommu tear down will for-each its list
and unset all the vdev_ids on its side while a device (idev) stays.
I don't see a use case of that though..any thought?

Thanks
Nicolin