Messages in this thread | | | Date | Tue, 17 Sep 2019 18:58:02 +0800 | From | Tiwei Bie <> | Subject | Re: [RFC v4 0/3] vhost: introduce mdev based hardware backend |
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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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