RE: [PATCH v2 01/11] iommu: Add new iommu op to create domains owned by userspace

[Tian, Kevin]

> From: Nicolin Chen <nicolinc@xxxxxxxxxx>
> Sent: Wednesday, May 24, 2023 1:24 PM
> 
> On Wed, May 24, 2023 at 05:02:19AM +0000, Tian, Kevin wrote:
> 
> > > From: Nicolin Chen <nicolinc@xxxxxxxxxx>
> > > Sent: Saturday, May 20, 2023 2:45 AM
> > >
> > > On Fri, May 19, 2023 at 08:47:45AM +0000, Tian, Kevin wrote:
> > > > > From: Liu, Yi L <yi.l.liu@xxxxxxxxx>
> > > > > Sent: Thursday, May 11, 2023 10:39 PM
> > > > > @@ -229,6 +238,15 @@ struct iommu_iotlb_gather {
> > > > >   *           after use. Return the data buffer if success, or ERR_PTR on
> > > > >   *           failure.
> > > > >   * @domain_alloc: allocate iommu domain
> > > > > + * @domain_alloc_user: allocate user iommu domain
> > > > > + * @domain_alloc_user_data_len: return the required length of the
> user
> > > > > data
> > > > > + *                              to allocate a specific type user iommu domain.
> > > > > + *                              @hwpt_type is defined as enum
> iommu_hwpt_type
> > > > > + *                              in include/uapi/linux/iommufd.h. The returned
> > > > > + *                              length is the corresponding sizeof driver data
> > > > > + *                              structures in include/uapi/linux/iommufd.h.
> > > > > + *                              -EOPNOTSUPP would be returned if the input
> > > > > + *                              @hwpt_type is not supported by the driver.
> > > >
> > > > Can this be merged with earlier @hw_info callback? That will already
> > > > report a list of supported hwpt types. is there a problem to further
> > > > describe the data length for each type in that interface?
> > >
> > > Yi and I had a last minute talk before he sent this version
> > > actually... This version of hw_info no longer reports a list
> > > of supported hwpt types. We previously did that in a bitmap,
> > > but we found that a bitmap will not be sufficient eventually
> > > if there are more than 64 hwpt_types.
> > >
> > > And this domain_alloc_user_data_len might not be necessary,
> > > because in this version the IOMMUFD core doesn't really care
> > > about the actual data_len since it copies the data into the
> > > ucmd_buffer, i.e. we would probably only need a bool op like
> > > "hwpt_type_is_supported".
> > >
> >
> > Or just pass to the @domain_alloc_user ops which should fail
> > if the type is not supported?
> 
> The domain_alloc_user returns NULL, which then would be turned
> into an ENOMEM error code. It might be confusing from the user
> space perspective. Having an op at least allows the user space
> to realize that something is wrong with the input structure?
> 

this is a new callback. any reason why it cannot be defined to
allow returning ERR_PTR?