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

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Message-ID: <20251117160028.GA17968@ziepe.ca>
Date: Mon, 17 Nov 2025 12:00:28 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Zhiping Zhang <zhipingz@...a.com>
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>
Subject: Re: [RFC 2/2] Set steering-tag directly for PCIe P2P memory access

On Thu, Nov 13, 2025 at 01:37:12PM -0800, Zhiping Zhang wrote:
> RDMA: Set steering-tag value directly in DMAH struct for DMABUF MR
> 
> This patch enables construction of a dma handler (DMAH) with the P2P memory type
> and a direct steering-tag value. It can be used to register a RDMA memory
> region with DMABUF for the RDMA NIC to access the other device's memory via P2P.
> 
> Signed-off-by: Zhiping Zhang <zhipingz@...a.com>
> ---
>  .../infiniband/core/uverbs_std_types_dmah.c   | 28 +++++++++++++++++++
>  drivers/infiniband/core/uverbs_std_types_mr.c |  3 ++
>  drivers/infiniband/hw/mlx5/dmah.c             |  5 ++--
>  .../net/ethernet/mellanox/mlx5/core/lib/st.c  | 12 +++++---
>  include/linux/mlx5/driver.h                   |  4 +--
>  include/rdma/ib_verbs.h                       |  2 ++
>  include/uapi/rdma/ib_user_ioctl_cmds.h        |  1 +
>  7 files changed, 46 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/infiniband/core/uverbs_std_types_dmah.c b/drivers/infiniband/core/uverbs_std_types_dmah.c
> index 453ce656c6f2..1ef400f96965 100644
> --- a/drivers/infiniband/core/uverbs_std_types_dmah.c
> +++ b/drivers/infiniband/core/uverbs_std_types_dmah.c
> @@ -61,6 +61,27 @@ static int UVERBS_HANDLER(UVERBS_METHOD_DMAH_ALLOC)(
>  		dmah->valid_fields |= BIT(IB_DMAH_MEM_TYPE_EXISTS);
>  	}
>  
> +	if (uverbs_attr_is_valid(attrs, UVERBS_ATTR_ALLOC_DMAH_DIRECT_ST_VAL)) {
> +		ret = uverbs_copy_from(&dmah->direct_st_val, attrs,
> +				       UVERBS_ATTR_ALLOC_DMAH_DIRECT_ST_VAL);
> +		if (ret)
> +			goto err;

This should not come from userspace, the dmabuf exporter should
provide any TPH hints as part of the attachment process.

We are trying not to allow userspace raw access to the TPH values, so
this is not a desirable UAPI here.

Jason