linux-kernel - Re: [PATCH RFC 0/8] nvme: Add Controller Data Queue to the nvme driver

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Message-ID: <20250721062600.GA28867@lst.de>
Date: Mon, 21 Jul 2025 08:26:00 +0200
From: Christoph Hellwig <hch@....de>
To: Joel Granados <joel.granados@...nel.org>
Cc: Christoph Hellwig <hch@....de>, Keith Busch <kbusch@...nel.org>,
	Jens Axboe <axboe@...nel.dk>, Sagi Grimberg <sagi@...mberg.me>,
	Klaus Jensen <k.jensen@...sung.com>, linux-nvme@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 0/8] nvme: Add Controller Data Queue to the nvme
 driver

On Fri, Jul 18, 2025 at 01:33:34PM +0200, Joel Granados wrote:
> > to win a point.  So for CDQs to be useful in Linux we'll need the
> > proper live migration still under heavy development.  With that I'd
> Do you mean in the specification body or patch series in the mailing
> lists?

Actual code.  As I said I very much expect CDQ creation and usage
to be kernel driven for live migration.

> > very much expect the kernel to manage the CDQs just like any other
> > queue, and not a random user ioctl.
> This is a great segue to a question: If CDQ is like any other queue,
> what is the best way of handling the lack of CDQ submission queues?
> Something like snooping all submissions for these CDQs and triggering a
> CDQ consume on every submission?

I don't understand this question and proposed answer at all all.

> I went with the ioctl as the faster way to get it to work;

Get _what_ to work?

> I might
> explore what having it as just another queue would look like.
> 
> > So what would be the use case for a user controlled CDQ?
> Do you mean a hypothetical list besides LM in NVME 2.2?

As out line in the last two mails I don't see how live migration would
work with user controlled CDQs.  Maybe I'm wrong, but nothing in this
thread seems to even try to explain how that would work.