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

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Message-ID: <CAMj1kXGUOFN_W_8c3a6BnsXDy4xsF+9DUV3wcPr2s7-1K_xUUQ@mail.gmail.com>
Date: Mon, 8 Sep 2025 07:50:38 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Evangelos Petrongonas <epetron@...zon.de>
Cc: akpm@...ux-foundation.org, bhe@...hat.com, changyuanl@...gle.com, 
	graf@...zon.com, ilias.apalodimas@...aro.org, 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: Re: [PATCH v3 2/2] efi: Support booting with kexec handover (KHO)

On Thu, 4 Sept 2025 at 14:59, Evangelos Petrongonas <epetron@...zon.de> wrote:
>
> On Thu, 4 Sep 2025 11:39:02 +0200, Ard Biesheuvel <ardb@...nel.org> wrote:
> > 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!
> >
> >
>
> I did some testing on qemu with memblock and EFI debug enabled
>
> (`memblock=debug efi=debug`) and no KHO.
> We see that `memblock_dump_all()` in `reserve_regions()` outputs:
> ```
> [    0.000000] MEMBLOCK configuration:
> [    0.000000]  memory size = 0x0000000200000000 reserved size = 0x000000000db5383e
> [    0.000000]  memory.cnt  = 0x7
> [    0.000000]  memory[0x0]     [0x0000000040000000-0x000000023c76ffff], 0x00000001fc770000 bytes on node 0 flags: 0x0
> ...
> [    0.000000]  reserved.cnt  = 0xf
> [    0.000000]  reserved[0x0]   [0x00000000fe000000-0x00000000ffffffff], 0x0000000002000000 bytes flags: 0x20
> ```
>
> Moreover checking the code, the boot flow  (at least on arm64)
> populates memblocks from DT memory nodes via
> `early_init_dt_add_memory_arch()` before `efi_init()` is called
>
> `setup_arch()` -> `setup_machine_fdt()` -> `early_init_dt_scan()` ->
> `early_init_dt_scan_memory()` -> `early_init_dt_add_memory_arch()` ->
> `memblock_add()`
>
> As a result, it seems that memblocks ARE populated when calling the
> `reserve_regions()`. So looks like  we still need the
> `memblock_remove()` (?)
>

Indeed.

For the series,

Reviewed-by: Ard Biesheuvel <ardb@...nel.org>