linux-kernel - Re: Re: [PATCH v3 2/2] efi: Support booting with kexec handover (KHO)

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Message-ID: <CAMj1kXHTJxBMFX6J-QwcPRojLGMQsTNOH5Bz9kHk7CFdt1JApw@mail.gmail.com>
Date: Thu, 4 Sep 2025 11:39:02 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Evangelos Petrongonas <epetron@...zon.de>
Cc: Ilias Apalodimas <ilias.apalodimas@...aro.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	bhe@...hat.com, changyuanl@...gle.com, graf@...zon.com, 
	kexec@...ts.infradead.org, linux-efi@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, nh-open-source@...zon.com, 
	rppt@...nel.org
Subject: Re: Re: [PATCH v3 2/2] efi: Support booting with kexec handover (KHO)

On Thu, 4 Sept 2025 at 11:36, Evangelos Petrongonas <epetron@...zon.de> wrote:
>
> On Thu, 4 Sep 2025 09:19:21 +0200, Ard Biesheuvel <ardb@...nel.org> wrote:
> > On Sat, 23 Aug 2025 at 23:47, Ard Biesheuvel <ardb@...nel.org> wrote:
> > >
> > > (cc Ilias)
> > >
> > > Note to akpm: please drop this series for now.
> > >
> > > On Fri, 22 Aug 2025 at 04:00, Evangelos Petrongonas <epetron@...zon.de> wrote:
> > > >
> > > > When KHO (Kexec HandOver) is enabled, it sets up scratch memory regions
> > > > early during device tree scanning. After kexec, the new kernel
> > > > exclusively uses this region for memory allocations during boot up to
> > > > the initialization of the page allocator
> > > >
> > > > However, when booting with EFI, EFI's reserve_regions() uses
> > > > memblock_remove(0, PHYS_ADDR_MAX) to clear all memory regions before
> > > > rebuilding them from EFI data. This destroys KHO scratch regions and
> > > > their flags, thus causing a kernel panic, as there are no scratch
> > > > memory regions.
> > > >
> > > > Instead of wholesale removal, iterate through memory regions and only
> > > > remove non-KHO ones. This preserves KHO scratch regions, which are
> > > > good known memory, while still allowing EFI to rebuild its memory map.
> > > >
> > > > Acked-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
> > > > Signed-off-by: Evangelos Petrongonas <epetron@...zon.de>
> > > > ---
> > > > Changes in v3:
> > > >         - Improve the code comments, by stating that the scratch regions are
> > > >         good known memory
> > > >
> > > > Changes in v2:
> > > >         - Replace the for loop with for_each_mem_region
> > > >         - Fix comment indentation
> > > >         - Amend commit message to specify that scratch regions
> > > >         are known good regions
> > > >
> > > >  drivers/firmware/efi/efi-init.c | 29 +++++++++++++++++++++++++----
> > > >  1 file changed, 25 insertions(+), 4 deletions(-)
> > > >
> > >
> > > I'd rather drop the memblock_remove() entirely if possible. Could we
> > > get some insight into whether memblocks are generally already
> > > populated at this point during the boot?
> > >
> > >
> >
> > Ping?
>
> Hey Ard I was AFK travelling. I am back now and will get to it.
> PS: Keen to meet you later today in the KVM Forum.
>

Yes, let's catch up!