Message-ID: <CAM_iQpWB1Jq88qDd+z7j+y1fEBRtV86LVHneLemHdvq28g5Dag@mail.gmail.com>
Date: Sat, 27 Sep 2025 13:06:49 -0700
From: Cong Wang <xiyou.wangcong@...il.com>
To: Jiaxun Yang <jiaxun.yang@...goat.com>
Cc: linux-kernel@...r.kernel.org, pasha.tatashin@...een.com, 
	Cong Wang <cwang@...tikernel.io>, Andrew Morton <akpm@...ux-foundation.org>, 
	Baoquan He <bhe@...hat.com>, Alexander Graf <graf@...zon.com>, Mike Rapoport <rppt@...nel.org>, 
	Changyuan Lyu <changyuanl@...gle.com>, kexec@...ts.infradead.org, linux-mm@...ck.org, 
	multikernel@...ts.linux.dev
Subject: Re: [RFC Patch 0/7] kernel: Introduce multikernel architecture support

Hi Jiaxun,

On Thu, Sep 25, 2025 at 8:48 AM Jiaxun Yang <jiaxun.yang@...goat.com> wrote:
>
>
>
> > 2025年9月19日 06:25，Cong Wang <xiyou.wangcong@...il.com> 写道：
> >
> > This patch series introduces multikernel architecture support, enabling
> > multiple independent kernel instances to coexist and communicate on a
> > single physical machine. Each kernel instance can run on dedicated CPU
> > cores while sharing the underlying hardware resources.
>
> Hi Cong,
>
> Sorry for chime in here, and thanks for brining replicated-kernel back to the life.

I have to clarify: in my design, kernel is not replicated. It is the opposite,
I intend to have diversified kernels for highly customization for each
application.

>
> I have some experience on original Popcorn Linux [1] [2], which seems to be the
> root of most code in this series, please see my comments below.
>
> >
> > The multikernel architecture provides several key benefits:
> > - Improved fault isolation between different workloads
> > - Enhanced security through kernel-level separation
>
> I’d agree with Stefen’s comments [3], an "isolation” solution is critical for adaptation
> of multikernel OS, given that multi-tenant system is almost everywhere.
>
> Also allowing other kernel to inject IPI without any restriction can impose DOS attack
> risk.

This is true. Like I mentioned, this is also a good opportunity to invite
hardware (CPU) vendors to catch up with software, for example, they
could provide hardware-filtering for IPI via MSR.

If we look at how virtualization evolves, it is the hardware follows software.
VMCS comes after Xen or KVM, VPDA comes after virtio.

>
> > - Better resource utilization than traditional VM (KVM, Xen etc.)
> > - Potential zero-down kernel update with KHO (Kernel Hand Over)
> >
> > Architecture Overview:
> > The implementation leverages kexec infrastructure to load and manage
> > multiple kernel images, with each kernel instance assigned to specific
> > CPU cores. Inter-kernel communication is facilitated through a dedicated
> > IPI framework that allows kernels to coordinate and share information
> > when necessary.
> >
> > Key Components:
> > 1. Enhanced kexec subsystem with dynamic kimage tracking
> > 2. Generic IPI communication framework for inter-kernel messaging
>
> I actually have concerns over inter-kernel communication. The origin Popcorn
> IPI protocol, which seems to be inherited here, was designed as a prototype,
> without much consideration on the ecosystem. It would be nice if we can reused
> existing infra design for inter kernel communication.

Popcorn does the opposite: it still stays with a single image which is
essentially against isolation. In fact, I also read its latest paper this year,
I don't see any essential change on this big direction:
https://www.ssrg.ece.vt.edu/papers/asplos25.pdf

This is why fundamentally Popcorn is not suitable for isolation. Please
don't get me wrong: I am not questioning its usefulness, it is just simply
two opposite directions. I wish people best luck on the heterogeneous
ISA design, and I hope major CPU vendors will catch up with you too. :)

>
> I would suggest look into OpenAMP [4] and remoteproc subsystem in kernel. They
> already have mature solutions on communication between different kernels over coherent
> memory and mailboxes (rpmsg [5] co). They also defined ELF extensions to pass side band
> information for other kernel images.

Thanks for the pointers. Jim Huang also shared his idea on remoteproc
at LinuxCon this year. After evaluations, I found remoteproc may not be
as good as IPI. Remoteproc is designed for heterogeneous systems with
different architectures, adding unnecessary abstraction layers.

>
> Linaro folks are also working on a new VirtIO transport called virtio-msg [6], [7], which is designed
> with Linux-Linux hardware partitioning scenario in mind.

I think there is still a fundamental difference between static partitioning.
and elastic resource allocation.

Static partitioning can be achieved as a default case of dynamic allocation
when resources remain unchanged, but the reverse is not possible.

Hope this makes sense to you.

Regards,
Cong Wang

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