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

[Baolu Lu]

On 2/28/25 11:23, Nicolin Chen wrote:
> 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@xxxxxxxxxx>
> > > Signed-off-by: Nicolin Chen<nicolinc@xxxxxxxxxx>
> > > ---
> > >    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@xxxxxxxxxx/
>
> > 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?

I have no idea, just feeling that we could leave room for tomorrow.