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SubjectRe: [RFC v3] vhost: introduce mdev based hardware vhost backend
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 <tiwei.bie@intel.com>

....

> +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

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