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

John Groves
2024-05-23T02:49:31Z

On 24/05/22 10:58AM, Miklos Szeredi wrote:
> On Wed, 22 May 2024 at 04:05, John Groves <John@groves.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