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

[Ira Weiny <ira.weiny@intel.com>]

Michał Cłapiński wrote:
> On Tue, Jul 1, 2025 at 2:05 PM Michał Cłapiński <mclapinski@google.com> wrote:
> >
> > On Wed, Jun 25, 2025 at 11:16 PM Ira Weiny <ira.weiny@intel.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