RE: RFC: Restricting userspace interfaces for CXL fabric management

Jonathan Cameron wrote:

Hi All,

This is has come up in a number of discussions both on list and in private,
so I wanted to lay out a potential set of rules when deciding whether or not
to provide a user space interface for a particular feature of CXL Fabric
Management.  The intent is to drive discussion, not to simply tell people
a set of rules.  I've brought this to the public lists as it's a Linux kernel
policy discussion, not a standards one.

Whilst I'm writing the RFC this my attempt to summarize a possible
position rather than necessarily being my personal view.

It's a straw man - shoot at it!

Not everyone in this discussion is familiar with relevant kernel or CXL concepts
so I've provided more info than I normally would.

Thanks for writing this up Jonathan!

[..]

2) Unfiltered userspace use of mailbox for Fabric Management - BMC kernels
==========================================================================

(This would just be a kernel option that we'd advise normal server
distributions not to turn on. Would be enabled by openBMC etc)

This is fine - there is some work to do, but the switch-cci PCI driver
will hopefully be ready for upstream merge soon. There is no filtering of
accesses. Think of this as similar to all the damage you can do via
MCTP from a BMC. Similarly it is likely that much of the complexity
of the actual commands will be left to user space tooling: 
https://gitlab.com/jic23/cxl-fmapi-tests has some test examples.

Whether Kconfig help text is strong enough to ensure this only gets
enabled for BMC targeted distros is an open question we can address
alongside an updated patch set.

It is not clear to me that this material makes sense to house in
drivers/ vs tools/ or even out-of-tree just for maintenance burden
relief of keeping the universes separated. What does the Linux kernel
project get out of carrying this in mainline alongside the inband code?

I do think the mailbox refactoring to support non-CXL use cases is
interesting, but only so far as refactoring is consumed for inband use
cases like RAS API.

(On to the one that the "debate" is about)

3) Unfiltered user space use of mailbox for Fabric Management - Distro kernels
=============================================================================
(General purpose Linux Server Distro (Redhat, Suse etc))

This is equivalent of RAW command support on CXL Type 3 memory devices.
You can enable those in a distro kernel build despite the scary config
help text, but if you use it the kernel is tainted. The result
of the taint is to add a flag to bug reports and print a big message to say
that you've used a feature that might result in you shooting yourself
in the foot.

The taint is there because software is not at first written to deal with
everything that can happen smoothly (e.g. surprise removal) It's hard
to survive some of these events, so is never on the initial feature list
for any bus, so this flag is just to indicate we have entered a world
where almost all bets are off wrt to stability.  We might not know what
a command does so we can't assess the impact (and no one trusts vendor
commands to report affects right in the Command Effects Log - which
in theory tells you if a command can result problems).

That is a secondary reason that the taint is there. Yes, it helps
upstream not waste their time on bug reports from proprietary use cases,
but the effect of that is to make "raw" command mode unattractive for
deploying solutions at scale. It clarifies that this interface is a
debug-tool that enterprise environment need not worry about.

The more salient reason for the taint, speaking only for myself as a
Linux kernel community member not for $employer, is to encourage open
collaboration. Take firmware-update for example that is a standard
command with known side effects that is inaccessible via the ioctl()
path. It is placed behind an ABI that is easier to maintain and reason
about. Everyone has the firmware update tool if they have the 'cat'
command. Distros appreciate the fact that they do not need ship yet
another vendor device-update tool, vendors get free tooling and end
users also appreciate one flow for all devices.

As I alluded here [1], I am not against innovation outside of the
specification, but it needs to be open, and it needs to plausibly become
if not a de jure standard at least a de facto standard.

[1]: https://lore.kernel.org/all/CAPcyv4gDShAYih5iWabKg_eTHhuHm54vEAei8ZkcmHnPp3B0cw@mail.gmail.com/ (local)

A concern was raised about GAE/FAST/LDST tables for CXL Fabrics
(a r3.1 feature) but, as I understand it, these are intended for a
host to configure and should not have side effects on other hosts?
My working assumption is that the kernel driver stack will handle
these (once we catch up with the current feature backlog!) Currently
we have no visibility of what the OS driver stack for a fabrics will
actually look like - the spec is just the starting point for that.
(patches welcome ;)

The various CXL upstream developers and maintainers may have
differing views of course, but my current understanding is we want
to support 1 and 2, but are very resistant to 3!

1, yes, 2, need to see the patches, and agree on 3.

General Notes
=============

One side aspect of why we really don't like unfiltered userspace access to any
of these devices is that people start building non standard hacks in and we
lose the ecosystem advantages. Forcing a considered discussion + patches
to let a particular command be supported, drives standardization.

Like I said above, I think this is not a side aspect. It is fundamental
to the viability Linux as a project. This project only works because
organizations with competing goals realize they need some common
infrastructure and that there is little to be gained by competing on the
commons.

https://lore.kernel.org/linux-cxl/CAPcyv4gDShAYih5iWabKg_eTHhuHm54vEAei8ZkcmHnPp3B0cw@mail.gmail.com/ (local)
provides some history on vendor specific extensions and why in general we
won't support them upstream.

Oh, you linked my writeup... I will leave the commentary I added here in case
restating it helps.

To address another question raised in an earlier discussion:
Putting these Fabric Management interfaces behind guard rails of some type
(e.g. CONFIG_IM_A_BMC_AND_CAN_MAKE_A_MESS) does not encourage the risk
of non standard interfaces, because we will be even less likely to accept
those upstream!

If anyone needs more details on any aspect of this please ask.
There are a lot of things involved and I've only tried to give a fairly
minimal illustration to drive the discussion. I may well have missed
something crucial.

You captured it well, and this is open source so I may have missed
something crucial as well.