Re: [RFC v2 PATCH] reserve_mem: add support for static memory

[Shyam Saini <shyamsaini@linux.microsoft.com>]

Hi Everyone,

> On 25 Jun 2026 11:37, Mike Rapoport wrote:
> > Hi Shyam,
> > 
> > On Wed, Jun 24, 2026 at 06:22:33PM -0700, Shyam Saini wrote:
> > > On 21 Jun 2026 13:36, Mike Rapoport wrote:
> > > > On Thu, Jun 18, 2026 at 11:23:31PM -0700, Shyam Saini wrote:
> > > > > reserve_mem relies on dynamic memory allocation, this limits the
> > > > > usecase where memory is required to be preserved across the boots.
> > > > > Eg: ramoops memory reservation on ACPI platforms
> > > > >
> > > > > So add support to pass a pre-determined static address and reserve
> > > > > memory at a specified location. This enables use case like ramoops
> > > > > on ACPI platforms to reliably access ramoops region with previous
> > > > > boot logs.
> > > > > 
> > > > > Also skip the parsing of <align> when static address is passed.
> > > > > 
> > > > > Example syntax for static address
> > > > >  reserve_mem=4M@0x1E0000000:oops
> > > > 
> > > > reserve_mem is best effort by design because such hacks as well as memmap=
> > > > cannot guarantee this memory is actually free.
> > > > 
> > > > If you want to preserve ramoops reliably, use KHO with reserve_mem.
> > > > The first kernel will allocate memory, this memory will be preserved by KHO
> > > > and could be picked up by the second kernel.
> > > 
> > > ok, On ARM64 DTS systems, we can reserve ramoops memory in the device tree during
> > > the warm reboot.
> > 
> > The cc list actually implied x86 ;-)
> > Added arm64 folks now.
> 
> Thanks for adding ARM folks, I had just included whatever get_maintainer script
> suggested, sorry. 
> > > For an equivalent ARM64 ACPI platform, what is the recommended way to reserve
> > > and preserve that memory across the boots? 
> > 
> > I don't think it exists, but a command line option (be it memmap= or
> > reserve_mem=) does not seem the right way to me.
> > 
> > Most of the arguments that were made against adding memmap= to arm64 [1]
> > apply here.
> > 
> > If kexec is an option, KHO provides a reliable way to preserve memory
> > across boots.
> > 
> > If kexec is not an option, we should look for a generic way to specify
> > something like DT's reserved_mem for ACPI/EFI systems.
> > 
> > [1] https://lkml.kernel.org/lkml/20201118063314.22940-1-song.bao.hua@hisilicon.com/T/
> > 
> Well, kexec is one of the option for my use case, it also requires
> memory reservation during warm reboot, I think memory can be reserved in
> the firmware but this will create a dependency on firmware for Linux
> reservation.
> 
> It would be great to have a in kernel memory reservation mechanism for
> ARM64 ACPI platforms. I believe some other use cases like PMEM
> reservation would also benefit from this.
>

Following up on this, As Mike pointed that reserve_mem is best effort
reservation mechanism, so what is the recommended reliable Linux
mechanism, if any, to reserve a predetermined memory range during
early boot on ARM64/ACPI platforms for warm boot scenarios? KHO is
one option, but I'm specifically looking for a solution that preserves
the region across warm reboots.

Please let me know.

Thanks,
Shyam