Re: [PATCH 4/5] virtio: introduce a vDPA based transport

Jason Wang
2020-01-17T09:33:40Z

On 2020/1/16 下午11:38, Jason Gunthorpe wrote:
> On Thu, Jan 16, 2020 at 08:42:30PM +0800, Jason Wang wrote:
>> diff --git a/drivers/virtio/virtio_vdpa.c b/drivers/virtio/virtio_vdpa.c
>> new file mode 100644
>> index 000000000000..86936e5e7ec3
>> +++ b/drivers/virtio/virtio_vdpa.c
>> @@ -0,0 +1,400 @@
>> +// SPDX-License-Identifier: GPL-2.0-only
>> +/*
>> + * VIRTIO based driver for vDPA device
>> + *
>> + * Copyright (c) 2020, Red Hat. All rights reserved.
>> + * Author: Jason Wang <jasowang@redhat.com>
>> + *
>> + */
>> +
>> +#include <linux/init.h>
>> +#include <linux/module.h>
>> +#include <linux/device.h>
>> +#include <linux/kernel.h>
>> +#include <linux/slab.h>
>> +#include <linux/uuid.h>
>> +#include <linux/virtio.h>
>> +#include <linux/vdpa.h>
>> +#include <linux/virtio_config.h>
>> +#include <linux/virtio_ring.h>
>> +
>> +#define MOD_VERSION "0.1"
>> +#define MOD_AUTHOR "Jason Wang <jasowang@redhat.com>"
>> +#define MOD_DESC "vDPA bus driver for virtio devices"
>> +#define MOD_LICENSE "GPL v2"
>> +
>> +#define to_virtio_vdpa_device(dev) \
>> + container_of(dev, struct virtio_vdpa_device, vdev)
> Should be a static function

Ok.

>
>> +struct virtio_vdpa_device {
>> + struct virtio_device vdev;
>> + struct vdpa_device *vdpa;
>> + u64 features;
>> +
>> + /* The lock to protect virtqueue list */
>> + spinlock_t lock;
>> + /* List of virtio_vdpa_vq_info */
>> + struct list_head virtqueues;
>> +};
>> +
>> +struct virtio_vdpa_vq_info {
>> + /* the actual virtqueue */
>> + struct virtqueue *vq;
>> +
>> + /* the list node for the virtqueues list */
>> + struct list_head node;
>> +};
>> +
>> +static struct vdpa_device *vd_get_vdpa(struct virtio_device *vdev)
>> +{
>> + struct virtio_vdpa_device *vd_dev = to_virtio_vdpa_device(vdev);
>> + struct vdpa_device *vdpa = vd_dev->vdpa;
>> +
>> + return vdpa;
> Bit of a long way to say
>
> return to_virtio_vdpa_device(vdev)->vdpa
>
> ?

Right.

>
>> +err_vq:
>> + vring_del_virtqueue(vq);
>> +error_new_virtqueue:
>> + ops->set_vq_ready(vdpa, index, 0);
>> + WARN_ON(ops->get_vq_ready(vdpa, index));
> A warn_on during error unwind? Sketchy, deserves a comment I think

Yes, it's a hint of bug in the vDPA driver. Will add a comment.

>
>> +static void virtio_vdpa_release_dev(struct device *_d)
>> +{
>> + struct virtio_device *vdev =
>> + container_of(_d, struct virtio_device, dev);
>> + struct virtio_vdpa_device *vd_dev =
>> + container_of(vdev, struct virtio_vdpa_device, vdev);
>> + struct vdpa_device *vdpa = vd_dev->vdpa;
>> +
>> + devm_kfree(&vdpa->dev, vd_dev);
>> +}
> It is unusual for the release function to not be owned by the
> subsystem, through the class.

This is how virtio_pci and virtio_mmio work now. Virtio devices may have
different transports which require different release functions. I think
this is the reason why virtio

> I'm not sure there are enough module ref
> counts to ensure that this function is not unloaded?

Let me double check this.

>
> Usually to make this all work sanely the subsytem provides some
> allocation function
>
> vdpa_dev = vdpa_alloc_dev(parent, ops, sizeof(struct virtio_vdpa_device))
> struct virtio_vdpa_device *priv = vdpa_priv(vdpa_dev)
>
> Then the subsystem naturally owns all the memory.
>
> Otherwise it gets tricky to ensure that the module doesn't unload
> before all the krefs are put.

I see.

>
>> +
>> +static int virtio_vdpa_probe(struct device *dev)
>> +{
>> + struct vdpa_device *vdpa = dev_to_vdpa(dev);
> The probe function for a class should accept the classes type already,
> no casting.

Right.

>
>> + const struct vdpa_config_ops *ops = vdpa->config;
>> + struct virtio_vdpa_device *vd_dev;
>> + int rc;
>> +
>> + vd_dev = devm_kzalloc(dev, sizeof(*vd_dev), GFP_KERNEL);
>> + if (!vd_dev)
>> + return -ENOMEM;
> This is not right, the struct device lifetime is controled by a kref,
> not via devm. If you want to use a devm unwind then the unwind is
> put_device, not devm_kfree.

I'm not sure I get the point here. The lifetime is bound to underlying
vDPA device and devres allow to be freed before the vpda device is
released. But I agree using devres of underlying vdpa device looks wired.

>
> In this simple situation I don't see a reason to use devm.
>
>> + vd_dev->vdev.dev.parent = &vdpa->dev;
>> + vd_dev->vdev.dev.release = virtio_vdpa_release_dev;
>> + vd_dev->vdev.config = &virtio_vdpa_config_ops;
>> + vd_dev->vdpa = vdpa;
>> + INIT_LIST_HEAD(&vd_dev->virtqueues);
>> + spin_lock_init(&vd_dev->lock);
>> +
>> + vd_dev->vdev.id.device = ops->get_device_id(vdpa);
>> + if (vd_dev->vdev.id.device == 0)
>> + return -ENODEV;
>> +
>> + vd_dev->vdev.id.vendor = ops->get_vendor_id(vdpa);
>> + rc = register_virtio_device(&vd_dev->vdev);
>> + if (rc)
>> + put_device(dev);
> And a ugly unwind like this is why you want to have device_initialize()
> exposed to the driver,

In this context, which "driver" did you mean here? (Note, virtio-vdpa is
the driver for vDPA bus here).

> so there is a clear pairing that calling
> device_initialize() must be followed by put_device. This should also
> use the goto unwind style
>
>> + else
>> + dev_set_drvdata(dev, vd_dev);
>> +
>> + return rc;
>> +}
>> +
>> +static void virtio_vdpa_remove(struct device *dev)
>> +{
> Remove should also already accept the right type

Yes.

>
>> + struct virtio_vdpa_device *vd_dev = dev_get_drvdata(dev);
>> +
>> + unregister_virtio_device(&vd_dev->vdev);
>> +}
>> +
>> +static struct vdpa_driver virtio_vdpa_driver = {
>> + .drv = {
>> + .name = "virtio_vdpa",
>> + },
>> + .probe = virtio_vdpa_probe,
>> + .remove = virtio_vdpa_remove,
>> +};
> Still a little unclear on binding, is this supposed to bind to all
> vdpa devices?

Yes, it expected to drive all vDPA devices.

>
> Where is the various THIS_MODULE's I expect to see in a scheme like
> this?
>
> All function pointers must be protected by a held module reference
> count, ie the above probe/remove and all the pointers in ops.

Will double check, since I don't see this in other virtio transport
drivers (PCI or MMIO).

>
>> +static int __init virtio_vdpa_init(void)
>> +{
>> + return register_vdpa_driver(&virtio_vdpa_driver);
>> +}
>> +
>> +static void __exit virtio_vdpa_exit(void)
>> +{
>> + unregister_vdpa_driver(&virtio_vdpa_driver);
>> +}
>> +
>> +module_init(virtio_vdpa_init)
>> +module_exit(virtio_vdpa_exit)
> Best to provide the usual 'module_pci_driver' like scheme for this
> boiler plate.

Ok.

>
>> +MODULE_VERSION(MOD_VERSION);
>> +MODULE_LICENSE(MOD_LICENSE);
>> +MODULE_AUTHOR(MOD_AUTHOR);
>> +MODULE_DESCRIPTION(MOD_DESC);
> Why the indirection with 2nd defines?

Will fix.

Thanks

>
> Jason
>