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: <CAAi7L5djEJCVzWWjDMO7WKbXgx6Geba6bku=Gjj2DnBtysQC4A@mail.gmail.com>
Date: Thu, 14 Aug 2025 15:27:31 +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 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.