linux-kernel - Re: [PATCH v4 05/14] kexec: Add Kexec HandOver (KHO) generation helpers

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Message-ID: <20250211124943.GC3754072@nvidia.com>
Date: Tue, 11 Feb 2025 08:49:43 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Pasha Tatashin <pasha.tatashin@...een.com>
Cc: Mike Rapoport <rppt@...nel.org>, 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>,
	"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 05/14] kexec: Add Kexec HandOver (KHO) generation
 helpers

On Mon, Feb 10, 2025 at 03:58:00PM -0500, Pasha Tatashin wrote:
> >
> > > +What:                /sys/kernel/kho/dt_max
> > > +Date:                December 2023
> > > +Contact:     Alexander Graf <graf@...zon.com>
> > > +Description:
> > > +             KHO needs to allocate a buffer for the DT that gets
> > > +             generated before it knows the final size. By default, it
> > > +             will allocate 10 MiB for it. You can write to this file
> > > +             to modify the size of that allocation.
> >
> > Seems gross, why can't it use a non-contiguous page list to generate
> > the FDT? :\
> 
> We will consider some of these ideas in the future version. I like the
> idea of using preserved memory to carry sparse KHO tree: i.e FDT over
> sparse memory, maybe use the anchor page to describe how it should be
> vmapped into a virtually contiguous tree in the next kernel?

Yeah, but this is now permanent uAPI that has to be kept forever. I
think you should not add this when there are enough ideas on how to
completely avoid it.

Jason