Re: [Intel-gfx] [PATCH v3 00/15] Add vfio_device cdev for iommufd support

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 14 Feb 2023 01:55:17 +0000
"Liu, Yi L" <yi.l.liu@intel.com> wrote:

> > From: Alex Williamson <alex.williamson@redhat.com>
> > Sent: Tuesday, February 14, 2023 3:47 AM
> > 
> > On Mon, 13 Feb 2023 07:13:33 -0800
> > Yi Liu <yi.l.liu@intel.com> wrote:
> >   
> > > Existing VFIO provides group-centric user APIs for userspace. Userspace
> > > opens the /dev/vfio/$group_id first before getting device fd and hence
> > > getting access to device. This is not the desired model for iommufd. Per
> > > the conclusion of community discussion[1], iommufd provides device-  
> > centric  
> > > kAPIs and requires its consumer (like VFIO) to be device-centric user
> > > APIs. Such user APIs are used to associate device with iommufd and also
> > > the I/O address spaces managed by the iommufd.
> > >
> > > This series first introduces a per device file structure to be prepared
> > > for further enhancement and refactors the kvm-vfio code to be prepared
> > > for accepting device file from userspace. Then refactors the vfio to be
> > > able to handle iommufd binding. This refactor includes the mechanism of
> > > blocking device access before iommufd bind, making vfio_device_open()  
> > be  
> > > exclusive between the group path and the cdev path. Eventually, adds the
> > > cdev support for vfio device, and makes group infrastructure optional as
> > > it is not needed when vfio device cdev is compiled.
> > >
> > > This is also a prerequisite for iommu nesting for vfio device[2].
> > >
> > > The complete code can be found in below branch, simple test done with  
> > the  
> > > legacy group path and the cdev path. Draft QEMU branch can be found  
> > at[3]  
> > >
> > > https://github.com/yiliu1765/iommufd/tree/vfio_device_cdev_v3
> > > (config CONFIG_IOMMUFD=y CONFIG_VFIO_DEVICE_CDEV=y)  
> > 
> > Even using your branch[1], it seems like this has not been tested
> > except with cdev support enabled:
> > 
> > /home/alwillia/Work/linux.git/drivers/vfio/vfio_main.c: In function
> > ‘vfio_device_add’:
> > /home/alwillia/Work/linux.git/drivers/vfio/vfio_main.c:253:48: error: ‘struct
> > vfio_device’ has no member named ‘cdev’; did you mean ‘dev’?
> >   253 |                 ret = cdev_device_add(&device->cdev, &device->device);
> >       |                                                ^~~~
> >       |                                                dev
> > /home/alwillia/Work/linux.git/drivers/vfio/vfio_main.c: In function
> > ‘vfio_device_del’:
> > /home/alwillia/Work/linux.git/drivers/vfio/vfio_main.c:262:42: error: ‘struct
> > vfio_device’ has no member named ‘cdev’; did you mean ‘dev’?
> >   262 |                 cdev_device_del(&device->cdev, &device->device);
> >       |                                          ^~~~
> >       |                                          dev  
> 
> Sorry for it. It is due to the cdev definition is under
> "#if IS_ENABLED(CONFIG_VFIO_DEVICE_CDEV)". While, in the code it
> uses "if (IS_ENABLED(CONFIG_VFIO_DEVICE_CDEV))".  I think for
> readability, it would be better to always define cdev in vfio_device,
> and keep the using of cdev in code. How about your taste?

It seems necessary unless we want to litter the code with #ifdefs.

> > Additionally the VFIO_ENABLE_GROUP Kconfig option doesn't make much
> > sense to me, it seems entirely redundant to VFIO_GROUP.  
> 
> The intention is to make the group code compiling match existing case.
> Currently, if VFIO is configured, group code is by default compiled.
> So VFIO_ENABLE_GROUP a hidden option, and VFIO_GROUP an option
> for user.  User needs to explicitly config VFIO_GROUP if VFIO_DEVICE_CDEV==y.
> If VFIO_DEVICE_CDEV==n, then no matter user configed VFIO_GROUP or not,
> the group code shall be compiled.

I understand the mechanics, I still find VFIO_ENABLE_GROUP redundant
and unnecessary.  Also, Kconfig should not allow a configuration
without either VFIO_GROUP or VFIO_DEVICE_CDEV as this is not
functional.  Deselecting VFIO_GROUP should select VFIO_DEVICE_CDEV, but
VFIO_DEVICE_CDEV should be an optional addition to VFIO_GROUP.  Thanks,

Alex