linux-kernel - Re: RFC: Restricting userspace interfaces for CXL fabric management

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Date: Thu, 25 Apr 2024 09:18:53 -0700
From: Dan Williams <dan.j.williams@...el.com>
To: Jonathan Cameron <Jonathan.Cameron@...wei.com>, Dan Williams
	<dan.j.williams@...el.com>
CC: <linux-cxl@...r.kernel.org>, Sreenivas Bagalkote
	<sreenivas.bagalkote@...adcom.com>, Brett Henning
	<brett.henning@...adcom.com>, Harold Johnson <harold.johnson@...adcom.com>,
	Sumanesh Samanta <sumanesh.samanta@...adcom.com>,
	<linux-kernel@...r.kernel.org>, Davidlohr Bueso <dave@...olabs.net>, "Dave
 Jiang" <dave.jiang@...el.com>, Alison Schofield <alison.schofield@...el.com>,
	Vishal Verma <vishal.l.verma@...el.com>, Ira Weiny <ira.weiny@...el.com>,
	<linuxarm@...wei.com>, <linux-api@...r.kernel.org>, Lorenzo Pieralisi
	<lpieralisi@...nel.org>, "Natu, Mahesh" <mahesh.natu@...el.com>
Subject: Re: RFC: Restricting userspace interfaces for CXL fabric management

Jonathan Cameron wrote:
[..]
> > Also, the assertion that these kernels will be built with
> > CONFIG_SECURITY_LOCKDOWN_LSM=n and likely CONFIG_STRICT_DEVMEM=n, then
> > the entire user-mode driver ABI is available for use. CXL commands are
> > simple polled mmio, does Linux really benefit from carrying drivers in
> > the kernel that the kernel itself does not care about?
> 
> Sure we could it in userspace...  It's bad engineering, limits the design
> to polling only and uses a bunch of interfaces we put a lot of effort into
> telling people not to use except for debug.
> 
> I really don't see the advantage in pushing a project/group of projects
> all of which are picking the upstream kernel up directly, to do a dirty
> hack. We loose all the advantages of a proper well maintained kernel
> driver purely on the argument that one use model is not the same as
> this one.  Sensible security lockdown requirements is fine (along
> with all the other kernel features that must be disable for that
> to work), making open kernel development on for a large Linux
> market harder is not.

The minimum requirement for justifying an in kernel driver is that
something else in the kernel consumes that facility. So, again, I want
to get back to specifics what else in the kernel is going to leverage
the Switch CCI mailbox?

The generic-Type-3-device mailbox has an in kernel driver because the
kernel has need to send mailbox commands internally and it is
fundamental to RAS and provisioning flows that the kernel have this
coordination. What are the motivations for an in-band Switch CCI command
submission path?

It could be the case that you have a self-evident example in mind that I
have thus far failed to realize.