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

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Message-ID: <20250904125917.78112-1-epetron@amazon.de>
Date: Thu, 4 Sep 2025 12:57:51 +0000
From: Evangelos Petrongonas <epetron@...zon.de>
To: <ardb@...nel.org>
CC: <akpm@...ux-foundation.org>, <bhe@...hat.com>, <changyuanl@...gle.com>,
	<epetron@...zon.de>, <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 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()` (?)





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