linux-kernel - Re: [PATCH v3 0/2] libnvdimm/e820: Add a new parameter to configure many regions per e820 entry

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Message-ID: <68b0f92761d25_293b329449@iweiny-mobl.notmuch>
Date: Thu, 28 Aug 2025 19:49:43 -0500
From: Ira Weiny <ira.weiny@...el.com>
To: Michał Cłapiński <mclapinski@...gle.com>, "Ira
 Weiny" <ira.weiny@...el.com>
CC: Jonathan Corbet <corbet@....net>, Dan Williams <dan.j.williams@...el.com>,
	Vishal Verma <vishal.l.verma@...el.com>, Dave Jiang <dave.jiang@...el.com>,
	<nvdimm@...ts.linux.dev>, "Paul E. McKenney" <paulmck@...nel.org>, "Thomas
 Huth" <thuth@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>, "Steven
 Rostedt" <rostedt@...dmis.org>, "Borislav Petkov (AMD)" <bp@...en8.de>, "Ard
 Biesheuvel" <ardb@...nel.org>, Greg Kroah-Hartman
	<gregkh@...uxfoundation.org>, Pasha Tatashin <pasha.tatashin@...een.com>,
	Mike Rapoport <rppt@...nel.org>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-cxl@...r.kernel.org>
Subject: Re: [PATCH v3 0/2] libnvdimm/e820: Add a new parameter to configure
 many regions per e820 entry

Michał Cłapiński wrote:
> On Tue, Jul 1, 2025 at 2:05 PM Michał Cłapiński <mclapinski@...gle.com> wrote:
> >
> > On Wed, Jun 25, 2025 at 11:16 PM Ira Weiny <ira.weiny@...el.com> wrote:
> > >
> > > Michal Clapinski wrote:
> > > > This includes:
> > > > 1. Splitting one e820 entry into many regions.
> > > > 2. Conversion to devdax during boot.
> > > >
> > > > This change is needed for the hypervisor live update. VMs' memory will
> > > > be backed by those emulated pmem devices. To support various VM shapes
> > > > I want to create devdax devices at 1GB granularity similar to hugetlb.
> > > > Also detecting those devices as devdax during boot speeds up the whole
> > > > process. Conversion in userspace would be much slower which is
> > > > unacceptable while trying to minimize
> > >
> > > Did you explore the NFIT injection strategy which Dan suggested?[1]
> > >
> > > [1] https://lore.kernel.org/all/6807f0bfbe589_71fe2944d@dwillia2-xfh.jf.intel.com.notmuch/
> > >
> > > If so why did it not work?
> >
> > I'm new to all this so I might be off on some/all of the things.
> >
> > My issues with NFIT:
> > 1. I can either go with custom bios or acpi nfit injection. Custom
> > bios sounds rather aggressive to me and I'd prefer to avoid this. The
> > NFIT injection is done via initramfs, right? If a system doesn't use
> > initramfs at the moment, that would introduce another step in the boot
> > process. One of the requirements of the hypervisor live update project
> > is that the boot process has to be blazing fast and I'm worried
> > introducing initramfs would go against this requirement.
> > 2. If I were to create an NFIT, it would have to contain thousands of
> > entries. That would have to be parsed on every boot. Again, I'm
> > worried about the performance.
> >
> > Do you think an NFIT solution could be as fast as the simple command
> > line solution?
> 
> Hello,
> just a follow up email. I'd like to receive some feedback on this.

Apologies.  I'm not keen on adding kernel parameters so I'm curious what
you think about Mike's new driver?[1]

[1] https://lore.kernel.org/all/68b0f8a31a2b8_293b3294ae@iweiny-mobl.notmuch/

Ira