Re: [PATCH v1 4/4] xprtrdma: Plant XID in on-the-wire RDMA offset (FRWR)

From: Jason Gunthorpe <jgg@ziepe.ca>
To: "Mora, Jorge" <Jorge.Mora@netapp.com>
Cc: Chuck Lever <chuck.lever@oracle.com>,
	Olga Kornievskaia <aglo@umich.edu>,
	linux-rdma <linux-rdma@vger.kernel.org>,
	Linux NFS Mailing List <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH v1 4/4] xprtrdma: Plant XID in on-the-wire RDMA offset (FRWR)
Date: Mon, 19 Nov 2018 15:46:15 -0700	[thread overview]
Message-ID: <20181119224615.GM4890@ziepe.ca> (raw)
In-Reply-To: <5EA42399-05C2-40D3-A5CA-7B40971AEC33@netapp.com>

On Mon, Nov 19, 2018 at 09:42:56PM +0000, Mora, Jorge wrote:
> Hello Chuck,
> 
> I am confused, is it the whole purpose of RDMA is to place the data
> directly into the memory location given by the virtual address or
> offset? What you are saying is that this offset is not the actual
> memory address and so the driver must map this offset to the actual
> address?

All RDMA on the wire addresses are 'virtual' and always go through
translation hardware to reach a physical memory address. Even if you
have a virtual address that matches your physical address there will
still be translation hardware to enforce security.

The direct placement refers to not copying data, not to using physical
memory addresses throughout the protocol design.

So it is better to use a virtual address that doesn't disclose any
information about the host's configuration to the network, such as
physical locations of kernel memory.

Jason