Re: [PATCH v2 1/4] iommu: Add private_data_owner to iommu_domain

[Nicolin Chen <nicolinc@nvidia.com>]

On Fri, Feb 28, 2025 at 11:13:23AM +0800, Baolu Lu wrote:
> On 2/28/25 09:31, Nicolin Chen wrote:
> > Steal two bits from the 32-bit "type" in struct iommu_domain to store a
> > new tag for private data owned by either dma-iommu or iommufd.
> > 
> > Set the domain->private_data_owner in dma-iommu and iommufd. These will
> > be used to replace the sw_msi function pointer.
> > 
> > Suggested-by: Jason Gunthorpe<jgg@nvidia.com>
> > Signed-off-by: Nicolin Chen<nicolinc@nvidia.com>
> > ---
> >   include/linux/iommu.h                | 7 ++++++-
> >   drivers/iommu/dma-iommu.c            | 2 ++
> >   drivers/iommu/iommufd/hw_pagetable.c | 3 +++
> >   3 files changed, 11 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> > index e93d2e918599..4f2774c08262 100644
> > --- a/include/linux/iommu.h
> > +++ b/include/linux/iommu.h
> > @@ -209,8 +209,13 @@ struct iommu_domain_geometry {
> >   #define IOMMU_DOMAIN_PLATFORM	(__IOMMU_DOMAIN_PLATFORM)
> >   #define IOMMU_DOMAIN_NESTED	(__IOMMU_DOMAIN_NESTED)
> > +#define IOMMU_DOMAIN_DATA_OWNER_NONE (0U)
> > +#define IOMMU_DOMAIN_DATA_OWNER_DMA (1U)
> > +#define IOMMU_DOMAIN_DATA_OWNER_IOMMUFD (2U)
> > +
> >   struct iommu_domain {
> > -	unsigned type;
> > +	u32 type : 30;
> > +	u32 private_data_owner : 2;
> 
> Is there any special consideration for reserving only 2 bits for the
> private data owner? Is it possible to allocate more bits so that it
> could be more extensible for the future?

It could. This "2" is copied from Jason's suggestion:
https://lore.kernel.org/linux-iommu/20250227194749.GJ39591@nvidia.com/

> For example, current iommu core allows a kernel device driver to
> allocate a paging domain and replace the default domain for kernel DMA.
> This suggests the private data owner bits may be needed beyond iommu-dma
> and iommufd.

What's the potential "private data" in this case?

Thanks
Nicolin