Re: [PATCH 0/9] Initial DMABUF support for iommufd

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Nov 13, 2025 at 11:37:12AM -0700, Alex Williamson wrote:
> > The latest series for interconnect negotation to exchange a phys_addr is:
> >  https://lore.kernel.org/r/20251027044712.1676175-1-vivek.kasireddy@intel.com
> 
> If this is in development, why are we pursuing a vfio specific
> temporary "private interconnect" here rather than building on that
> work?  What are the gaps/barriers/timeline?

I broadly don't expect to see an agreement on the above for probably
half a year, and I see no reason to hold this up for it. Many people
are asking for this P2P support to be completed in iommufd.

Further, I think the above will be easier to work on when we have this
merged as an example that can consume it in a different way. Right now
it is too theoretical, IMHO.

> I don't see any uAPI changes here, is there any visibility to userspace
> whether IOMMUFD supports this feature or is it simply a try and fail
> approach?  

So far we haven't done discoverably things beyond try and fail.

I'd be happy if the userspace folks doing libvirt or whatever came
with some requests/patches for discoverability. It is not just this
feature, but things like nesting and IOMMU driver support and so on.

> The latter makes it difficult for management tools to select
> whether to choose a VM configuration based on IOMMUFD or legacy vfio if
> p2p DMA is a requirement.  Thanks,

In alot of cases it isn't really a choice as you need iommufd to do an
accelerated vIOMMU.

But yes, it would be nice to eventually automatically use iommufd
whenever possible.

Thanks,
Jason