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: <CAAi7L5cq9OqRGdyZ07rHhsA8GRh2xVXYGh7n20UoTCRfQK03WA@mail.gmail.com>
Date: Tue, 1 Jul 2025 14:05:59 +0200
From: Michał Cłapiński <mclapinski@...gle.com>
To: 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

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?

[snip]