linux-kernel - Re: [PATCH] vhost: introduce mdev based hardware backend

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Message-ID: <05ab395e-6677-e8c3-becf-57bc1529921f@redhat.com>
Date:   Fri, 27 Sep 2019 16:47:43 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Tiwei Bie <tiwei.bie@...el.com>
Cc:     mst@...hat.com, alex.williamson@...hat.com,
        maxime.coquelin@...hat.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, dan.daly@...el.com,
        cunming.liang@...el.com, zhihong.wang@...el.com,
        lingshan.zhu@...el.com
Subject: Re: [PATCH] vhost: introduce mdev based hardware backend


On 2019/9/27 下午12:54, Tiwei Bie wrote:
> On Fri, Sep 27, 2019 at 11:46:06AM +0800, Jason Wang wrote:
>> On 2019/9/26 下午12:54, Tiwei Bie wrote:
>>> +
>>> +static long vhost_mdev_start(struct vhost_mdev *m)
>>> +{
>>> +	struct mdev_device *mdev = m->mdev;
>>> +	const struct virtio_mdev_device_ops *ops = mdev_get_dev_ops(mdev);
>>> +	struct virtio_mdev_callback cb;
>>> +	struct vhost_virtqueue *vq;
>>> +	int idx;
>>> +
>>> +	ops->set_features(mdev, m->acked_features);
>>> +
>>> +	mdev_add_status(mdev, VIRTIO_CONFIG_S_FEATURES_OK);
>>> +	if (!(mdev_get_status(mdev) & VIRTIO_CONFIG_S_FEATURES_OK))
>>> +		goto reset;
>>> +
>>> +	for (idx = 0; idx < m->nvqs; idx++) {
>>> +		vq = &m->vqs[idx];
>>> +
>>> +		if (!vq->desc || !vq->avail || !vq->used)
>>> +			break;
>>> +
>>> +		if (ops->set_vq_state(mdev, idx, vq->last_avail_idx))
>>> +			goto reset;
>> If we do set_vq_state() in SET_VRING_BASE, we won't need this step here.
> Yeah, I plan to do it in the next version.
>
>>> +
>>> +		/*
>>> +		 * In vhost-mdev, userspace should pass ring addresses
>>> +		 * in guest physical addresses when IOMMU is disabled or
>>> +		 * IOVAs when IOMMU is enabled.
>>> +		 */
>> A question here, consider we're using noiommu mode. If guest physical
>> address is passed here, how can a device use that?
>>
>> I believe you meant "host physical address" here? And it also have the
>> implication that the HPA should be continuous (e.g using hugetlbfs).
> The comment is talking about the virtual IOMMU (i.e. iotlb in vhost).
> It should be rephrased to cover the noiommu case as well. Thanks for
> spotting this.
>
>
>>> +
>>> +	switch (cmd) {
>>> +	case VHOST_MDEV_SET_STATE:
>>> +		r = vhost_set_state(m, argp);
>>> +		break;
>>> +	case VHOST_GET_FEATURES:
>>> +		r = vhost_get_features(m, argp);
>>> +		break;
>>> +	case VHOST_SET_FEATURES:
>>> +		r = vhost_set_features(m, argp);
>>> +		break;
>>> +	case VHOST_GET_VRING_BASE:
>>> +		r = vhost_get_vring_base(m, argp);
>>> +		break;
>> Does it mean the SET_VRING_BASE may only take affect after
>> VHOST_MEV_SET_STATE?
> Yeah, in this version, SET_VRING_BASE won't set the base to the
> device directly. But I plan to not delay this anymore in the next
> version to support the SET_STATUS.
>
>>> +	default:
>>> +		r = vhost_dev_ioctl(&m->dev, cmd, argp);
>>> +		if (r == -ENOIOCTLCMD)
>>> +			r = vhost_vring_ioctl(&m->dev, cmd, argp);
>>> +	}
>>> +
>>> +	mutex_unlock(&m->mutex);
>>> +	return r;
>>> +}
>>> +
>>> +static const struct vfio_device_ops vfio_vhost_mdev_dev_ops = {
>>> +	.name		= "vfio-vhost-mdev",
>>> +	.open		= vhost_mdev_open,
>>> +	.release	= vhost_mdev_release,
>>> +	.ioctl		= vhost_mdev_unlocked_ioctl,
>>> +};
>>> +
>>> +static int vhost_mdev_probe(struct device *dev)
>>> +{
>>> +	struct mdev_device *mdev = mdev_from_dev(dev);
>>> +	const struct virtio_mdev_device_ops *ops = mdev_get_dev_ops(mdev);
>>> +	struct vhost_mdev *m;
>>> +	int nvqs, r;
>>> +
>>> +	m = kzalloc(sizeof(*m), GFP_KERNEL | __GFP_RETRY_MAYFAIL);
>>> +	if (!m)
>>> +		return -ENOMEM;
>>> +
>>> +	mutex_init(&m->mutex);
>>> +
>>> +	nvqs = ops->get_queue_max(mdev);
>>> +	m->nvqs = nvqs;
>> The name could be confusing, get_queue_max() is to get the maximum number of
>> entries for a virtqueue supported by this device.
> OK. It might be better to rename it to something like:
>
> 	get_vq_num_max()
>
> which is more consistent with the set_vq_num().
>
>> It looks to me that we need another API to query the maximum number of
>> virtqueues supported by the device.
> Yeah.
>
> Thanks,
> Tiwei


One problem here:

Consider if we want to support multiqueue, how did userspace know about 
this? Note this information could be fetched from get_config() via a 
device specific way, do we want ioctl for accessing that area?

Thanks