Subject: Re: [RFC v3] vhost: introduce mdev based hardware vhost backend

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 3 Sep 2019 07:26:03 -0400

On Wed, Aug 28, 2019 at 01:37:12PM +0800, Tiwei Bie wrote:
> Details about this can be found here:
> 
> https://lwn.net/Articles/750770/
> 
> What's new in this version
> ==========================
> 
> There are three choices based on the discussion [1] in RFC v2:
> 
> > #1. We expose a VFIO device, so we can reuse the VFIO container/group
> >     based DMA API and potentially reuse a lot of VFIO code in QEMU.
> >
> >     But in this case, we have two choices for the VFIO device interface
> >     (i.e. the interface on top of VFIO device fd):
> >
> >     A) we may invent a new vhost protocol (as demonstrated by the code
> >        in this RFC) on VFIO device fd to make it work in VFIO's way,
> >        i.e. regions and irqs.
> >
> >     B) Or as you proposed, instead of inventing a new vhost protocol,
> >        we can reuse most existing vhost ioctls on the VFIO device fd
> >        directly. There should be no conflicts between the VFIO ioctls
> >        (type is 0x3B) and VHOST ioctls (type is 0xAF) currently.
> >
> > #2. Instead of exposing a VFIO device, we may expose a VHOST device.
> >     And we will introduce a new mdev driver vhost-mdev to do this.
> >     It would be natural to reuse the existing kernel vhost interface
> >     (ioctls) on it as much as possible. But we will need to invent
> >     some APIs for DMA programming (reusing VHOST_SET_MEM_TABLE is a
> >     choice, but it's too heavy and doesn't support vIOMMU by itself).
> 
> This version is more like a quick PoC to try Jason's proposal on
> reusing vhost ioctls. And the second way (#1/B) in above three
> choices was chosen in this version to demonstrate the idea quickly.
> 
> Now the userspace API looks like this:
> 
> - VFIO's container/group based IOMMU API is used to do the
>   DMA programming.
> 
> - Vhost's existing ioctls are used to setup the device.
> 
> And the device will report device_api as "vfio-vhost".
> 
> Note that, there are dirty hacks in this version. If we decide to
> go this way, some refactoring in vhost.c/vhost.h may be needed.
> 
> PS. The direct mapping of the notify registers isn't implemented
>     in this version.
> 
> [1] https://lkml.org/lkml/2019/7/9/101
> 
> Signed-off-by: Tiwei Bie <[email-masked]>

....

> +long vhost_mdev_ioctl(struct mdev_device *mdev, unsigned int cmd,
> +		      unsigned long arg)
> +{
> +	void __user *argp = (void __user *)arg;
> +	struct vhost_mdev *vdpa;
> +	unsigned long minsz;
> +	int ret = 0;
> +
> +	if (!mdev)
> +		return -EINVAL;
> +
> +	vdpa = mdev_get_drvdata(mdev);
> +	if (!vdpa)
> +		return -ENODEV;
> +
> +	switch (cmd) {
> +	case VFIO_DEVICE_GET_INFO:
> +	{
> +		struct vfio_device_info info;
> +
> +		minsz = offsetofend(struct vfio_device_info, num_irqs);
> +
> +		if (copy_from_user(&info, (void __user *)arg, minsz)) {
> +			ret = -EFAULT;
> +			break;
> +		}
> +
> +		if (info.argsz < minsz) {
> +			ret = -EINVAL;
> +			break;
> +		}
> +
> +		info.flags = VFIO_DEVICE_FLAGS_VHOST;
> +		info.num_regions = 0;
> +		info.num_irqs = 0;
> +
> +		if (copy_to_user((void __user *)arg, &info, minsz)) {
> +			ret = -EFAULT;
> +			break;
> +		}
> +
> +		break;
> +	}
> +	case VFIO_DEVICE_GET_REGION_INFO:
> +	case VFIO_DEVICE_GET_IRQ_INFO:
> +	case VFIO_DEVICE_SET_IRQS:
> +	case VFIO_DEVICE_RESET:
> +		ret = -EINVAL;
> +		break;
> +
> +	case VHOST_MDEV_SET_STATE:
> +		ret = vhost_set_state(vdpa, argp);
> +		break;
> +	case VHOST_GET_FEATURES:
> +		ret = vhost_get_features(vdpa, argp);
> +		break;
> +	case VHOST_SET_FEATURES:
> +		ret = vhost_set_features(vdpa, argp);
> +		break;
> +	case VHOST_GET_VRING_BASE:
> +		ret = vhost_get_vring_base(vdpa, argp);
> +		break;
> +	default:
> +		ret = vhost_dev_ioctl(&vdpa->dev, cmd, argp);
> +		if (ret == -ENOIOCTLCMD)
> +			ret = vhost_vring_ioctl(&vdpa->dev, cmd, argp);
> +	}
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL(vhost_mdev_ioctl);

I don't have a problem with this approach. A small question:
would it make sense to have two fds: send vhost ioctls
on one and vfio ioctls on another?
We can then pass vfio fd to the vhost fd with a
SET_BACKEND ioctl.

What do you think?

-- 
MST