linux-kernel - Re: [PATCH v4 00/14] kexec: introduce Kexec HandOver (KHO)

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Message-ID: <CAM_iQpW+K-Q7qCZ_EH+BsF64aevK0tE3_kO=hma7sWa4TcWDSw@mail.gmail.com>
Date: Sat, 8 Feb 2025 16:51:44 -0800
From: Cong Wang <xiyou.wangcong@...il.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: linux-kernel@...r.kernel.org, Alexander Graf <graf@...zon.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Andy Lutomirski <luto@...nel.org>, 
	Anthony Yznaga <anthony.yznaga@...cle.com>, Arnd Bergmann <arnd@...db.de>, 
	Ashish Kalra <ashish.kalra@....com>, Benjamin Herrenschmidt <benh@...nel.crashing.org>, 
	Borislav Petkov <bp@...en8.de>, Catalin Marinas <catalin.marinas@....com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, David Woodhouse <dwmw2@...radead.org>, 
	Eric Biederman <ebiederm@...ssion.com>, Ingo Molnar <mingo@...hat.com>, 
	James Gowans <jgowans@...zon.com>, Jonathan Corbet <corbet@....net>, 
	Krzysztof Kozlowski <krzk@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	Paolo Bonzini <pbonzini@...hat.com>, Pasha Tatashin <pasha.tatashin@...een.com>, 
	"H. Peter Anvin" <hpa@...or.com>, Peter Zijlstra <peterz@...radead.org>, Pratyush Yadav <ptyadav@...zon.de>, 
	Rob Herring <robh+dt@...nel.org>, Rob Herring <robh@...nel.org>, 
	Saravana Kannan <saravanak@...gle.com>, 
	Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>, Steven Rostedt <rostedt@...dmis.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Tom Lendacky <thomas.lendacky@....com>, 
	Usama Arif <usama.arif@...edance.com>, Will Deacon <will@...nel.org>, devicetree@...r.kernel.org, 
	kexec@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org, 
	linux-doc@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org
Subject: Re: [PATCH v4 00/14] kexec: introduce Kexec HandOver (KHO)

Hi Mike,

On Thu, Feb 6, 2025 at 5:28 AM Mike Rapoport <rppt@...nel.org> wrote:
> We introduce a metadata file that the kernels pass between each other. How
> they pass it is architecture specific. The file's format is a Flattened
> Device Tree (fdt) which has a generator and parser already included in
> Linux. When the root user enables KHO through /sys/kernel/kho/active, the
> kernel invokes callbacks to every driver that supports KHO to serialize
> its state. When the actual kexec happens, the fdt is part of the image
> set that we boot into. In addition, we keep a "scratch regions" available
> for kexec: A physically contiguous memory regions that is guaranteed to
> not have any memory that KHO would preserve.  The new kernel bootstraps
> itself using the scratch regions and sets all handed over memory as in use.
> When drivers initialize that support KHO, they introspect the fdt and
> recover their state from it. This includes memory reservations, where the
> driver can either discard or claim reservations.

I have gone through your entire patchset, if you could provide an example
of a specific driver that supports KHO it would help a lot for people to
understand and more importantly help driver developers to adopt.
Even with a simulated driver, e.g. netdevsim, it would be greatly helpful.

Thanks.