On Tue, Sep 17, 2019 at 11:32:03AM +0800, Jason Wang wrote:
> On 2019/9/17 上午9:02, Tiwei Bie wrote:
> > This RFC is to demonstrate below ideas,
> >
> > a) Build vhost-mdev on top of the same abstraction defined in
> > the virtio-mdev series [1];
> >
> > b) Introduce /dev/vhost-mdev to do vhost ioctls and support
> > setting mdev device as backend;
> >
> > Now the userspace API looks like this:
> >
> > - Userspace generates a compatible mdev device;
> >
> > - Userspace opens this mdev device with VFIO API (including
> > doing IOMMU programming for this mdev device with VFIO's
> > container/group based interface);
> >
> > - Userspace opens /dev/vhost-mdev and gets vhost fd;
> >
> > - Userspace uses vhost ioctls to setup vhost (userspace should
> > do VHOST_MDEV_SET_BACKEND ioctl with VFIO group fd and device
> > fd first before doing other vhost ioctls);
> >
> > Only compile test has been done for this series for now.
>
>
> Have a hard thought on the architecture:
Thanks a lot! Do appreciate it!
>
> 1) Create a vhost char device and pass vfio mdev device fd to it as a
> backend and translate vhost-mdev ioctl to virtio mdev transport (e.g
> read/write). DMA was done through the VFIO DMA mapping on the container that
> is attached.
Yeah, that's what we are doing in this series.
>
> We have two more choices:
>
> 2) Use vfio-mdev but do not create vhost-mdev device, instead, just
> implement vhost ioctl on vfio_device_ops, and translate them into
> virtio-mdev transport or just pass ioctl to parent.
Yeah. Instead of introducing /dev/vhost-mdev char device, do
vhost ioctls on VFIO device fd directly. That's what we did
in RFC v3.
>
> 3) Don't use vfio-mdev, create a new vhost-mdev driver, during probe still
> try to add dev to vfio group and talk to parent with device specific ops
If my understanding is correct, this means we need to introduce
a new VFIO device driver to replace the existing vfio-mdev driver
in our case. Below is a quick draft just to show my understanding:
#include <linux/init.h>
#include <linux/module.h>
#include <linux/device.h>
#include <linux/kernel.h>
#include <linux/slab.h>
#include <linux/vfio.h>
#include <linux/mdev.h>
#include "mdev_private.h"
/* XXX: we need a proper way to include below vhost header. */
#include "../../vhost/vhost.h"
static int vfio_vhost_mdev_open(void *device_data)
{
if (!try_module_get(THIS_MODULE))
return -ENODEV;
/* ... */
vhost_dev_init(...);
return 0;
}
static void vfio_vhost_mdev_release(void *device_data)
{
/* ... */
module_put(THIS_MODULE);
}
static long vfio_vhost_mdev_unlocked_ioctl(void *device_data,
unsigned int cmd, unsigned long arg)
{
struct mdev_device *mdev = device_data;
struct mdev_parent *parent = mdev->parent;
/*
* Use vhost ioctls.
*
* We will have a different parent_ops design.
* And potentially, we can share the same parent_ops
* with virtio_mdev.
*/
switch (cmd) {
case VHOST_GET_FEATURES:
parent->ops->get_features(mdev, ...);
break;
/* ... */
}
return 0;
}
static ssize_t vfio_vhost_mdev_read(void *device_data, char __user *buf,
size_t count, loff_t *ppos)
{
/* ... */
return 0;
}
static ssize_t vfio_vhost_mdev_write(void *device_data, const char __user *buf,
size_t count, loff_t *ppos)
{
/* ... */
return 0;
}
static int vfio_vhost_mdev_mmap(void *device_data, struct vm_area_struct *vma)
{
/* ... */
return 0;
}
static const struct vfio_device_ops vfio_vhost_mdev_dev_ops = {
.name = "vfio-vhost-mdev",
.open = vfio_vhost_mdev_open,
.release = vfio_vhost_mdev_release,
.ioctl = vfio_vhost_mdev_unlocked_ioctl,
.read = vfio_vhost_mdev_read,
.write = vfio_vhost_mdev_write,
.mmap = vfio_vhost_mdev_mmap,
};
static int vfio_vhost_mdev_probe(struct device *dev)
{
struct mdev_device *mdev = to_mdev_device(dev);
/* ... */
return vfio_add_group_dev(dev, &vfio_vhost_mdev_dev_ops, mdev);
}
static void vfio_vhost_mdev_remove(struct device *dev)
{
/* ... */
vfio_del_group_dev(dev);
}
static struct mdev_driver vfio_vhost_mdev_driver = {
.name = "vfio_vhost_mdev",
.probe = vfio_vhost_mdev_probe,
.remove = vfio_vhost_mdev_remove,
};
static int __init vfio_vhost_mdev_init(void)
{
return mdev_register_driver(&vfio_vhost_mdev_driver, THIS_MODULE);
}
module_init(vfio_vhost_mdev_init)
static void __exit vfio_vhost_mdev_exit(void)
{
mdev_unregister_driver(&vfio_vhost_mdev_driver);
}
module_exit(vfio_vhost_mdev_exit)
>
> So I have some questions:
>
> 1) Compared to method 2, what's the advantage of creating a new vhost char
> device? I guess it's for keep the API compatibility?
One benefit is that we can avoid doing vhost ioctls on
VFIO device fd.
>
> 2) For method 2, is there any easy way for user/admin to distinguish e.g
> ordinary vfio-mdev for vhost from ordinary vfio-mdev?
I think device-api could be a choice.
> I saw you introduce
> ops matching helper but it's not friendly to management.
The ops matching helper is just to check whether a given
vfio-device is based on a mdev device.
>
> 3) A drawback of 1) and 2) is that it must follow vfio_device_ops that
> assumes the parameter comes from userspace, it prevents support kernel
> virtio drivers.
>
> 4) So comes the idea of method 3, since it register a new vhost-mdev driver,
> we can use device specific ops instead of VFIO ones, then we can have a
> common API between vDPA parent and vhost-mdev/virtio-mdev drivers.
As the above draft shows, this requires introducing a new
VFIO device driver. I think Alex's opinion matters here.
Thanks,
Tiwei
>
> What's your thoughts?
>
> Thanks
>
>
> >
> > RFCv3: https://patchwork.kernel.org/patch/11117785/
> >
> > [1] https://lkml.org/lkml/2019/9/10/135
> >
> > Tiwei Bie (3):
> > vfio: support getting vfio device from device fd
> > vfio: support checking vfio driver by device ops
> > vhost: introduce mdev based hardware backend
> >
> > drivers/vfio/mdev/vfio_mdev.c | 3 +-
> > drivers/vfio/vfio.c | 32 +++
> > drivers/vhost/Kconfig | 9 +
> > drivers/vhost/Makefile | 3 +
> > drivers/vhost/mdev.c | 462 +++++++++++++++++++++++++++++++
> > drivers/vhost/vhost.c | 39 ++-
> > drivers/vhost/vhost.h | 6 +
> > include/linux/vfio.h | 11 +
> > include/uapi/linux/vhost.h | 10 +
> > include/uapi/linux/vhost_types.h | 5 +
> > 10 files changed, 573 insertions(+), 7 deletions(-)
> > create mode 100644 drivers/vhost/mdev.c
> >
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