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Date:   Tue, 30 Aug 2022 09:42:42 -0400
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Subject: Re: [PATCH 01/15] vfio: Add helpers for unifying vfio_device life
 cycle
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To:     Kevin Tian <kevin.tian@intel.com>,
        Zhenyu Wang <zhenyuw@linux.intel.com>,
        Zhi Wang <zhi.a.wang@intel.com>,
        Jani Nikula <jani.nikula@linux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Eric Farman <farman@linux.ibm.com>,
        Matthew Rosato <mjrosato@linux.ibm.com>,
        Halil Pasic <pasic@linux.ibm.com>,
        Vineeth Vijayan <vneethv@linux.ibm.com>,
        Peter Oberparleiter <oberpar@linux.ibm.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        Sven Schnelle <svens@linux.ibm.com>,
        Jason Herne <jjherne@linux.ibm.com>,
        Harald Freudenberger <freude@linux.ibm.com>,
        Diana Craciun <diana.craciun@oss.nxp.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Longfang Liu <liulongfang@huawei.com>,
        Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Yishai Hadas <yishaih@nvidia.com>,
        Eric Auger <eric.auger@redhat.com>,
        Kirti Wankhede <kwankhede@nvidia.com>,
        Leon Romanovsky <leon@kernel.org>,
        Abhishek Sahu <abhsahu@nvidia.com>,
        intel-gvt-dev@lists.freedesktop.org,
        intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
        linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     Yi Liu <yi.l.liu@intel.com>
References: <20220827171037.30297-1-kevin.tian@intel.com>
 <20220827171037.30297-2-kevin.tian@intel.com>
From:   Anthony Krowiak <akrowiak@linux.ibm.com>
In-Reply-To: <20220827171037.30297-2-kevin.tian@intel.com>
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Reviewed-by: Tony Krowiak <akrowiak@linux.ibm.com>

I have a couple of review comments, but nothing critical that would 
prevent my r-b.

On 8/27/22 1:10 PM, Kevin Tian wrote:
> The idea is to let vfio core manage the vfio_device life cycle instead
> of duplicating the logic cross drivers. This is also a preparatory
> step for adding struct device into vfio_device.
>
> New pair of helpers together with a kref in vfio_device:
>
>   - vfio_alloc_device()
>   - vfio_put_device()
>
> Drivers can register @init/@release callbacks to manage any private
> state wrapping the vfio_device.
>
> However vfio-ccw doesn't fit this model due to a life cycle mess
> that its private structure mixes both parent and mdev info hence must
> be allocated/free'ed outside of the life cycle of vfio device.
>
> Per prior discussions this won't be fixed in short term by IBM folks.
>
> Instead of waiting introduce another helper vfio_init_device() so ccw
> can call it to initialize a pre-allocated vfio_device.
>
> Further implication of the ccw trick is that vfio_device cannot be
> free'ed uniformly in vfio core. Instead, require *EVERY* driver to
> implement @release and free vfio_device inside. Then ccw can choose
> to delay the free at its own discretion.
>
> Another trick down the road is that kvzalloc() is used to accommodate
> the need of gvt which uses vzalloc() while all others use kzalloc().
> So drivers should call a helper vfio_free_device() to free the
> vfio_device instead of assuming that kfree() or vfree() is appliable.
>
> Later once the ccw mess is fixed we can remove those tricks and
> fully handle structure alloc/free in vfio core.
>
> Existing vfio_{un}init_group_dev() will be deprecated after all
> existing usages are converted to the new model.
>
> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> Co-developed-by: Yi Liu <yi.l.liu@intel.com>
> Signed-off-by: Yi Liu <yi.l.liu@intel.com>
> Signed-off-by: Kevin Tian <kevin.tian@intel.com>
> ---
>   drivers/vfio/vfio_main.c | 92 ++++++++++++++++++++++++++++++++++++++++
>   include/linux/vfio.h     | 25 ++++++++++-
>   2 files changed, 116 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> index 7cb56c382c97..af8aad116f2b 100644
> --- a/drivers/vfio/vfio_main.c
> +++ b/drivers/vfio/vfio_main.c
> @@ -496,6 +496,98 @@ void vfio_uninit_group_dev(struct vfio_device *device)
>   }
>   EXPORT_SYMBOL_GPL(vfio_uninit_group_dev);
>   
> +/*
> + * Alloc and initialize vfio_device so it can be registered to vfio
> + * core.
> + *
> + * Drivers should use the wrapper vfio_alloc_device() for allocation.
> + * @size is the size of the structure to be allocated, including any
> + * private data used by the driver.


It seems the purpose of the wrapper is to ensure that the object being 
allocated has as its first field a struct vfio_device object and to 
return its container. Why not just make that a requirement for this 
function - which I would rename vfio_alloc_device - and document it in 
the prologue? The caller can then cast the return pointer or use 
container_of.


> + *
> + * Driver may provide an @init callback to cover device private data.
> + *
> + * Use vfio_put_device() to release the structure after success return.
> + */
> +struct vfio_device *_vfio_alloc_device(size_t size, struct device *dev,
> +		const struct vfio_device_ops *ops)
> +{
> +	struct vfio_device *device;
> +	int ret;
> +
> +	if (WARN_ON(size < sizeof(struct vfio_device)))
> +		return ERR_PTR(-EINVAL);
> +
> +	device = kvzalloc(size, GFP_KERNEL);
> +	if (!device)
> +		return ERR_PTR(-ENOMEM);
> +
> +	ret = vfio_init_device(device, dev, ops);
> +	if (ret)
> +		goto out_free;
> +	return device;
> +
> +out_free:
> +	kvfree(device);
> +	return ERR_PTR(ret);
> +}
> +EXPORT_SYMBOL_GPL(_vfio_alloc_device);
> +
> +/*
> + * Initialize a vfio_device so it can be registered to vfio core.
> + *
> + * Only vfio-ccw driver should call this interface.
> + */
> +int vfio_init_device(struct vfio_device *device, struct device *dev,
> +		     const struct vfio_device_ops *ops)
> +{
> +	int ret;
> +
> +	vfio_init_group_dev(device, dev, ops);
> +
> +	if (ops->init) {
> +		ret = ops->init(device);
> +		if (ret)
> +			goto out_uninit;
> +	}
> +
> +	kref_init(&device->kref);
> +	return 0;
> +
> +out_uninit:
> +	vfio_uninit_group_dev(device);
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(vfio_init_device);
> +
> +/*
> + * The helper called by driver @release callback to free the device
> + * structure. Drivers which don't have private data to clean can
> + * simply use this helper as its @release.
> + */
> +void vfio_free_device(struct vfio_device *device)
> +{
> +	kvfree(device);
> +}
> +EXPORT_SYMBOL_GPL(vfio_free_device);
> +
> +/* Release helper called by vfio_put_device() */
> +void vfio_device_release(struct kref *kref)
> +{
> +	struct vfio_device *device =
> +			container_of(kref, struct vfio_device, kref);
> +
> +	vfio_uninit_group_dev(device);
> +
> +	/*
> +	 * kvfree() cannot be done here due to a life cycle mess in
> +	 * vfio-ccw. Before the ccw part is fixed all drivers are
> +	 * required to support @release and call vfio_free_device()
> +	 * from there.
> +	 */
> +	device->ops->release(device);
> +}
> +EXPORT_SYMBOL_GPL(vfio_device_release);
> +
>   static struct vfio_group *vfio_noiommu_group_alloc(struct device *dev,
>   		enum vfio_group_type type)
>   {
> diff --git a/include/linux/vfio.h b/include/linux/vfio.h
> index e05ddc6fe6a5..e1e9e8352903 100644
> --- a/include/linux/vfio.h
> +++ b/include/linux/vfio.h
> @@ -45,7 +45,8 @@ struct vfio_device {
>   	struct kvm *kvm;
>   
>   	/* Members below here are private, not for driver use */
> -	refcount_t refcount;
> +	struct kref kref;	/* object life cycle */
> +	refcount_t refcount;	/* user count on registered device*/
>   	unsigned int open_count;
>   	struct completion comp;
>   	struct list_head group_next;
> @@ -55,6 +56,8 @@ struct vfio_device {
>   /**
>    * struct vfio_device_ops - VFIO bus driver device callbacks
>    *
> + * @init: initialize private fields in device structure
> + * @release: Reclaim private fields in device structure
>    * @open_device: Called when the first file descriptor is opened for this device
>    * @close_device: Opposite of open_device
>    * @read: Perform read(2) on device file descriptor
> @@ -72,6 +75,8 @@ struct vfio_device {
>    */
>   struct vfio_device_ops {
>   	char	*name;
> +	int	(*init)(struct vfio_device *vdev);
> +	void	(*release)(struct vfio_device *vdev);
>   	int	(*open_device)(struct vfio_device *vdev);
>   	void	(*close_device)(struct vfio_device *vdev);
>   	ssize_t	(*read)(struct vfio_device *vdev, char __user *buf,
> @@ -137,6 +142,24 @@ static inline int vfio_check_feature(u32 flags, size_t argsz, u32 supported_ops,
>   	return 1;
>   }
>   
> +struct vfio_device *_vfio_alloc_device(size_t size, struct device *dev,
> +				       const struct vfio_device_ops *ops);
> +#define vfio_alloc_device(dev_struct, member, dev, ops)				\
> +	container_of(_vfio_alloc_device(sizeof(struct dev_struct) +		\
> +					BUILD_BUG_ON_ZERO(offsetof(		\
> +						struct dev_struct, member)),	\
> +					dev, ops),				\
> +		     struct dev_struct, member)


I found the use of this macro confusing and unnecessary. I'd prefer 
vfio_alloc_device be a function (see my comments above).


> +
> +int vfio_init_device(struct vfio_device *device, struct device *dev,
> +		     const struct vfio_device_ops *ops);
> +void vfio_free_device(struct vfio_device *device);
> +void vfio_device_release(struct kref *kref);
> +static inline void vfio_put_device(struct vfio_device *device)
> +{
> +	kref_put(&device->kref, vfio_device_release);
> +}
> +
>   void vfio_init_group_dev(struct vfio_device *device, struct device *dev,
>   			 const struct vfio_device_ops *ops);
>   void vfio_uninit_group_dev(struct vfio_device *device);