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

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Message-ID: <20260124011436.172058-1-zhipingz@meta.com>
Date: Fri, 23 Jan 2026 17:13:19 -0800
From: Zhiping Zhang <zhipingz@...a.com>
To: Jason Gunthorpe <jgg@...pe.ca>
CC: Leon Romanovsky <leon@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
        <linux-rdma@...r.kernel.org>, <linux-pci@...r.kernel.org>,
        <netdev@...r.kernel.org>, Keith Busch <kbusch@...nel.org>,
        Yochai Cohen
	<yochai@...dia.com>, Yishai Hadas <yishaih@...dia.com>,
        Zhiping Zhang
	<zhipingz@...a.com>
Subject: Re: [RFC 2/2] Set steering-tag directly for PCIe P2P memory access

On Tue, 13 Jan 2026 12:49:23 -0400, Jason Gunthorpe wrote:

> On Mon, Jan 12, 2026 at 11:43:13PM -0800, Zhiping Zhang wrote:
> > Got it, thanks for pointing out the security concern! To address this, I
> > propose that we still pass the TPH value when allocating the dmah, but we add
> > a verification callback in the reg_mr_dmabuf flow to the dmabuf exporter. This
> > callback will ensure that the TPH value is correctly linked to the exporting
> > device’s MMIO, and only the exporter can authorize the TPH/tag association.

> That still sounds messy because we have to protect CPU memory.

> I think you should not use dmah possibly and make it so the dmabuf
entirely supplies the TPH value.

> Jason

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?

Zhiping