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Date: Wed, 19 Feb 2020 10:52:38 +0800
From: Tiwei Bie <tiwei.bie@...el.com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: mst@...hat.com, jasowang@...hat.com, linux-kernel@...r.kernel.org,
kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
netdev@...r.kernel.org, shahafs@...lanox.com,
rob.miller@...adcom.com, haotian.wang@...ive.com,
eperezma@...hat.com, lulu@...hat.com, parav@...lanox.com,
rdunlap@...radead.org, hch@...radead.org, jiri@...lanox.com,
hanand@...inx.com, mhabets@...arflare.com,
maxime.coquelin@...hat.com, lingshan.zhu@...el.com,
dan.daly@...el.com, cunming.liang@...el.com, zhihong.wang@...el.com
Subject: Re: [PATCH] vhost: introduce vDPA based backend
On Tue, Feb 18, 2020 at 09:53:59AM -0400, Jason Gunthorpe wrote:
> On Fri, Jan 31, 2020 at 11:36:51AM +0800, Tiwei Bie wrote:
>
> > +static int vhost_vdpa_alloc_minor(struct vhost_vdpa *v)
> > +{
> > + return idr_alloc(&vhost_vdpa.idr, v, 0, MINORMASK + 1,
> > + GFP_KERNEL);
> > +}
>
> Please don't use idr in new code, use xarray directly
>
> > +static int vhost_vdpa_probe(struct device *dev)
> > +{
> > + struct vdpa_device *vdpa = dev_to_vdpa(dev);
> > + const struct vdpa_config_ops *ops = vdpa->config;
> > + struct vhost_vdpa *v;
> > + struct device *d;
> > + int minor, nvqs;
> > + int r;
> > +
> > + /* Currently, we only accept the network devices. */
> > + if (ops->get_device_id(vdpa) != VIRTIO_ID_NET) {
> > + r = -ENOTSUPP;
> > + goto err;
> > + }
> > +
> > + v = kzalloc(sizeof(*v), GFP_KERNEL | __GFP_RETRY_MAYFAIL);
> > + if (!v) {
> > + r = -ENOMEM;
> > + goto err;
> > + }
> > +
> > + nvqs = VHOST_VDPA_VQ_MAX;
> > +
> > + v->vqs = kmalloc_array(nvqs, sizeof(struct vhost_virtqueue),
> > + GFP_KERNEL);
> > + if (!v->vqs) {
> > + r = -ENOMEM;
> > + goto err_alloc_vqs;
> > + }
> > +
> > + mutex_init(&v->mutex);
> > + atomic_set(&v->opened, 0);
> > +
> > + v->vdpa = vdpa;
> > + v->nvqs = nvqs;
> > + v->virtio_id = ops->get_device_id(vdpa);
> > +
> > + mutex_lock(&vhost_vdpa.mutex);
> > +
> > + minor = vhost_vdpa_alloc_minor(v);
> > + if (minor < 0) {
> > + r = minor;
> > + goto err_alloc_minor;
> > + }
> > +
> > + d = device_create(vhost_vdpa.class, NULL,
> > + MKDEV(MAJOR(vhost_vdpa.devt), minor),
> > + v, "%d", vdpa->index);
> > + if (IS_ERR(d)) {
> > + r = PTR_ERR(d);
> > + goto err_device_create;
> > + }
> > +
>
> I can't understand what this messing around with major/minor numbers
> does. Without allocating a cdev via cdev_add/etc there is only a
> single char dev in existence here. This and the stuff in
> vhost_vdpa_open() looks non-functional.
I followed the code in VFIO. Please see more details below.
>
> > +static void vhost_vdpa_remove(struct device *dev)
> > +{
> > + DEFINE_WAIT_FUNC(wait, woken_wake_function);
> > + struct vhost_vdpa *v = dev_get_drvdata(dev);
> > + int opened;
> > +
> > + add_wait_queue(&vhost_vdpa.release_q, &wait);
> > +
> > + do {
> > + opened = atomic_cmpxchg(&v->opened, 0, 1);
> > + if (!opened)
> > + break;
> > + wait_woken(&wait, TASK_UNINTERRUPTIBLE, HZ * 10);
> > + } while (1);
> > +
> > + remove_wait_queue(&vhost_vdpa.release_q, &wait);
>
> *barf* use the normal refcount pattern please
>
> read side:
>
> refcount_inc_not_zero(uses)
> //stuff
> if (refcount_dec_and_test(uses))
> complete(completer)
>
> destroy side:
> if (refcount_dec_and_test(uses))
> complete(completer)
> wait_for_completion(completer)
> // refcount now permanently == 0
>
> Use a completion in driver code
>
> > + mutex_lock(&vhost_vdpa.mutex);
> > + device_destroy(vhost_vdpa.class,
> > + MKDEV(MAJOR(vhost_vdpa.devt), v->minor));
> > + vhost_vdpa_free_minor(v->minor);
> > + mutex_unlock(&vhost_vdpa.mutex);
> > + kfree(v->vqs);
> > + kfree(v);
>
> This use after-fress vs vhost_vdpa_open prior to it setting the open
> bit. Maybe use xarray, rcu and kfree_rcu ..
>
> > +static int __init vhost_vdpa_init(void)
> > +{
> > + int r;
> > +
> > + idr_init(&vhost_vdpa.idr);
> > + mutex_init(&vhost_vdpa.mutex);
> > + init_waitqueue_head(&vhost_vdpa.release_q);
> > +
> > + /* /dev/vhost-vdpa/$vdpa_device_index */
> > + vhost_vdpa.class = class_create(THIS_MODULE, "vhost-vdpa");
> > + if (IS_ERR(vhost_vdpa.class)) {
> > + r = PTR_ERR(vhost_vdpa.class);
> > + goto err_class;
> > + }
> > +
> > + vhost_vdpa.class->devnode = vhost_vdpa_devnode;
> > +
> > + r = alloc_chrdev_region(&vhost_vdpa.devt, 0, MINORMASK + 1,
> > + "vhost-vdpa");
> > + if (r)
> > + goto err_alloc_chrdev;
> > +
> > + cdev_init(&vhost_vdpa.cdev, &vhost_vdpa_fops);
> > + r = cdev_add(&vhost_vdpa.cdev, vhost_vdpa.devt, MINORMASK + 1);
> > + if (r)
> > + goto err_cdev_add;
>
> It is very strange, is the intention to create a single global char
> dev?
No. It's to create a per-vdpa char dev named
vhost-vdpa/$vdpa_device_index in dev.
I followed the code in VFIO which creates char dev
vfio/$GROUP dynamically, e.g.:
https://github.com/torvalds/linux/blob/b1da3acc781c/drivers/vfio/vfio.c#L2164-L2180
https://github.com/torvalds/linux/blob/b1da3acc781c/drivers/vfio/vfio.c#L373-L387
https://github.com/torvalds/linux/blob/b1da3acc781c/drivers/vfio/vfio.c#L1553
Is it something unwanted?
Thanks for the review.
Regards,
Tiwei
>
> If so, why is there this:
>
> +static int vhost_vdpa_open(struct inode *inode, struct file *filep)
> +{
> + struct vhost_vdpa *v;
> + struct vhost_dev *dev;
> + struct vhost_virtqueue **vqs;
> + int nvqs, i, r, opened;
> +
> + v = vhost_vdpa_get_from_minor(iminor(inode));
>
> ?
>
> If the idea is to create a per-vdpa char dev then this stuff belongs
> in vhost_vdpa_probe(), the cdev should be part of the vhost_vdpa, and
> the above should be container_of not an idr lookup.
>
> Jason
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