Re: [PATCH] vfio: Remove vfio_group dev_counter

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Aug 16, 2022 at 09:21:05AM +0800, Yi Liu wrote:
> Hi Jason,
> 
> On 2022/8/16 00:50, Jason Gunthorpe wrote:
> > This counts the number of devices attached to a vfio_group, ie the number
> > of items in the group->device_list.
> 
> yes. This dev_counter is added to ensure only singleton vfio group supports
> pin page. Although I don't think it is a good approach as it only counts the
> registered devices.

Ah, I missed explaining this, lets try again:

vfio: Remove vfio_group dev_counter

This counts the number of devices attached to a vfio_group, ie the number
of items in the group->device_list.

It has two purposes
 - To ensure the vfio_device is opened
 - To assert the group is singleton because the dirty tracking code in the
   type1 iommu has limitations

However, vfio_pin_pages() already calls vfio_assert_device_open() so the
first is taken care of, and all callers of vfio_pin_pages() use now use
vfio_register_emulated_iommu_dev() which guarentees single groups by
constrution.

So delete dev_counter and leave a note that vfio_pin_pages() can only be
used with vfio_register_emulated_iommu_dev().

Jason