linux-kernel - Re: [RFC PATCH 6/6] virtio-msg: Add support for loopback bus

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Message-ID: <1ba0e803-cbca-43d0-be9d-79fef4e54201@kernel.org>
Date: Wed, 30 Jul 2025 11:49:14 +0200
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Viresh Kumar <viresh.kumar@...aro.org>, linux-kernel@...r.kernel.org,
 "Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>,
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez
 <eperezma@...hat.com>
Cc: Arnd Bergmann <arnd@...nel.org>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Alex Bennée <alex.bennee@...aro.org>,
 Bill Mills <bill.mills@...aro.org>, Rob Herring <robh@...nel.org>,
 Saravana Kannan <saravanak@...gle.com>, devicetree@...r.kernel.org,
 virtualization@...ts.linux.dev, Sudeep Holla <sudeep.holla@....com>,
 Bertrand Marquis <bertrand.marquis@....com>,
 "Edgar E . Iglesias" <edgar.iglesias@....com>,
 Arnaud Pouliquen <arnaud.pouliquen@...s.st.com>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>
Subject: Re: [RFC PATCH 6/6] virtio-msg: Add support for loopback bus

On 30/07/2025 11:29, Viresh Kumar wrote:
> Add a loopback bus implementation for the virtio-msg transport.
> 
> This bus simulates a backend that echoes messages to itself, allowing
> testing and development of virtio-msg without requiring an actual remote
> backend or transport hardware.
> 
> The loopback bus requires a reserved memory region for its operation.
> All DMA-coherent and streaming DMA allocations are restricted to this
> region, enabling safe mapping into user space and helping validate the
> memory-sharing model.
> 
> The reserved-memory region must be named "vmsglb" in the device tree.

No, because I can name regions how I wish. You cannot add undocumented
ABI and claim in commit msg, that the commit msg is the ABI.

Either you have ABI documented or you cannot rely on implicit ABI, thus
remove above restriction.

This is really poor choice to add such hidden ABI and it already broke
several users in the past.

Best regards,
Krzysztof