Re: [RFC 2/2] Set steering-tag directly for PCIe P2P memory access

[Zhiping Zhang <zhipingz@meta.com>]

On Wed, 28 Jan 2026 12:57:39 -0400, Jason Gunthorpe wrote:

> > Thanks Jason.
> >
> > We already have an end-to-end workflow around dmah (perftest → rdma-core → kernel)
> > to carry the TPH hint, across multiple patch sets. References:
> >  https://github.com/linux-rdma/perftest/commit/98bfb3679a1e71ec96df6a6d6c8124ac66ebce25
> >  https://github.com/linux-rdma/rdma-core/pull/1623/commits
> >  https://lore.kernel.org/all/cover.1752752567.git.leon@kernel.org/
> >
> > Given that, I’d like to minimize churn and use the existing dmah-based flow, while
> > addressing the CPU-memory protection concern you raised. Would you be open to
> > reconsidering this approach?
>
> You would need to initialize the dmah from the dmabuf and then lock it
> to only be usable with that dmabuf. It doesn't avoid any dmabuf work,
> it just makes the whole flow more convoluted.
>
> Jason

Thanks, I see your point. I’ll send a new revision that doesn’t use dmah.

Zhiping