Re: [PATCH v4 09/19] vfio/pci: Accept device fd for hot reset

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Feb 24, 2023 at 03:43:37AM +0000, Liu, Yi L wrote:
> > From: Tian, Kevin <kevin.tian@intel.com>
> > Sent: Friday, February 24, 2023 10:48 AM
> > 
> > > From: Jason Gunthorpe
> > > Sent: Friday, February 24, 2023 10:36 AM
> > >
> > > On Fri, Feb 24, 2023 at 02:21:33AM +0000, Tian, Kevin wrote:
> > >
> > > > Yi, while you are incorporating this change please also update the
> > > > uapi header. Rename 'group_fds[]' to 'fds[]' and add comment to
> > > > explain that it could be an array of group fds or a single iommufd.
> > >
> > > Upon reflection we can probably make it even simpler and just have a 0
> > > length fd array mean to use the iommufd the vfio_device is already
> > > associated with
> > >
> > > And the check for correctness can be simplified to simply see if each
> > > vfio_device in the dev_set is attached to the same iommufd ctx
> > > pointer instead of searching the xarray.
> 
> How about the hot reset info path? We can still keep reporting the
> current information to userspace. Isn't it?

Yeah, but I wonder if it is useful
 
> another tricky question. If user passess iommufd down for reset
> in the vfio iommufd compatible mode, should we support it as
> well?

I would say if the 0 fds mode is used and the current vfio_Device does
not have an iommufd ctx then fail.

That is the only requirement, however it got that ctx doesn't matter.

> > Locking is fine since dev_set->lock is already held in the reset path.
> 
> dev_set->lock is held prior to call bind_iommufd, so I agree locking
> is ok.

As long as the vdev's iommufd ctx and opencount cannot change under
the devset lock, which I think is the case. It should be documented
though in the vfio core code, as it is a bit subtle what the devset
lock actually covers.

Jason