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Date:   Wed, 24 May 2023 18:41:41 -0700
From:   Nicolin Chen <nicolinc@nvidia.com>
To:     "Tian, Kevin" <kevin.tian@intel.com>
CC:     "Liu, Yi L" <yi.l.liu@intel.com>,
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Subject: Re: [PATCH v2 01/11] iommu: Add new iommu op to create domains owned
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On Wed, May 24, 2023 at 07:48:46AM +0000, Tian, Kevin wrote:

> > > > > >   *           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?

Upon a quick check, I think we could. Though it'd be slightly
mismatched with the domain_alloc op, it should be fine since
iommufd is likely to be the only caller.

So, I think we can just take the approach letting user space
try a hwpt_type and see if the ioctl would fail with -EINVAL.

Thanks
Nic