linux-kernel - Re: [RFC PATCH 00/20] Introduce the famfs shared-memory file system

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Date: Wed, 22 May 2024 21:49:28 -0500
From: John Groves <John@...ves.net>
To: Miklos Szeredi <miklos@...redi.hu>
Cc: Amir Goldstein <amir73il@...il.com>, John Groves <jgroves@...ron.com>, 
	Jonathan Corbet <corbet@....net>, Dan Williams <dan.j.williams@...el.com>, 
	Vishal Verma <vishal.l.verma@...el.com>, Dave Jiang <dave.jiang@...el.com>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	Matthew Wilcox <willy@...radead.org>, linux-cxl@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, nvdimm@...ts.linux.dev, 
	john@...alactic.com, Dave Chinner <david@...morbit.com>, 
	Christoph Hellwig <hch@...radead.org>, dave.hansen@...ux.intel.com, gregory.price@...verge.com, 
	Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [RFC PATCH 00/20] Introduce the famfs shared-memory file system

On 24/05/22 10:58AM, Miklos Szeredi wrote:
> On Wed, 22 May 2024 at 04:05, John Groves <John@...ves.net> wrote:
> > I'm happy to help with that if you care - ping me if so; getting a VM running
> > in EFI mode is not necessary if you reserve the dax memory via memmap=, or
> > via libvirt xml.
> 
> Could you please give an example?
> 
> I use a raw qemu command line with a -kernel option and a root fs
> image (not a disk image with a bootloader).

That's not the way I'm running VMs, but... I presume you know how to add
kernel command line arguments to VMs that you run this way?

- memmap=<size>!<hpa_offset> will reserve a pretend pmem device at <hpa_offset>
- memmap=<size>$<hpa_offset> will reserve a pretend dax device at <hpa_offset>

Both of the above will work regardless of whether the VM is in EFI mode.
The '$' is harder to escape through grub; and the pmem device can be converted
to devdax via 'ndctl reconfigure-device --mode=devdax...'. A dax device would
likely also need to be put in devdax mode (as the default seems to be 
system-ram mode).  

Incomplete documentation (that you have probably already seen) is at [1]

I can dig deeper if needed.

Otherwise the feedback in this thread makes sense to me and I'm planning to 
start hacking on famfs patches Thursday. Watch this space ;)

Regards,
John

[1] https://github.com/cxl-micron-reskit/famfs/blob/master/markdown/vm-configuration.md