netdev - Re: [MAINTAINERS SUMMIT] Device Passthrough Considered Harmful?

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Message-ID: <20240722085317.GA31279@pendragon.ideasonboard.com>
Date: Mon, 22 Jul 2024 11:53:17 +0300
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Leon Romanovsky <leon@...nel.org>
Cc: Dan Williams <dan.j.williams@...el.com>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	ksummit@...ts.linux.dev, linux-cxl@...r.kernel.org,
	linux-rdma@...r.kernel.org, netdev@...r.kernel.org, jgg@...dia.com
Subject: Re: [MAINTAINERS SUMMIT] Device Passthrough Considered Harmful?

On Mon, Jul 22, 2024 at 10:31:19AM +0300, Leon Romanovsky wrote:
> On Sun, Jul 21, 2024 at 10:25:30PM +0300, Laurent Pinchart wrote:
> > On Tue, Jul 09, 2024 at 03:15:13PM -0700, Dan Williams wrote:
> > > James Bottomley wrote:
> > > > > The upstream discussion has yielded the full spectrum of positions on
> > > > > device specific functionality, and it is a topic that needs cross-
> > > > > kernel consensus as hardware increasingly spans cross-subsystem
> > > > > concerns. Please consider it for a Maintainers Summit discussion.
> > > > 
> > > > I'm with Greg on this ... can you point to some of the contrary
> > > > positions?
> > > 
> > > This thread has that discussion:
> > > 
> > > http://lore.kernel.org/0-v1-9912f1a11620+2a-fwctl_jgg@nvidia.com
> > > 
> > > I do not want to speak for others on the saliency of their points, all I
> > > can say is that the contrary positions have so far not moved me to drop
> > > consideration of fwctl for CXL.
> > > 
> > > Where CXL has a Command Effects Log that is a reasonable protocol for
> > > making decisions about opaque command codes, and that CXL already has a
> > > few years of experience with the commands that *do* need a Linux-command
> > > wrapper.
> > > 
> > > Some open questions from that thread are: what does it mean for the fate
> > > of a proposal if one subsystem Acks the ABI and another Naks it for a
> > > device that crosses subsystem functionality? Would a cynical hardware
> > > response just lead to plumbing an NVME admin queue, or CXL mailbox to
> > > get device-specific commands past another subsystem's objection?
> > 
> > My default answer would be to trust the maintainers of the relevant
> > subsystems (or try to convince them when you disagree :-)).
> 
> You know, trust is a two-way street. If you want to trust maintainers,
> they need to trust others as well. The situation where one maintainer
> says "I don't trust you, so I will not allow you and other X maintainers
> to do Y" is not a healthy situation.
> 
> > Not only should they know the technical implications best, they should also have
> > a good view of the whole vertical stack, and the implications of
> > pass-through for their ecosystem. 
> 
> It is wishful thinking. It is clearly not true for large subsystems
> and/or complex devices.

Are you saying that kernel communities behind large subsystems for
complex devices generally have no idea about what they're doing ? Or
that in a small number of particular cases those communities are
clueless ? Or does that apply to just the maintainer, not the whole
subsystem core developers ? I'd like to better understand the scale of
your claim here.

-- 
Regards,

Laurent Pinchart