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

[Ard Biesheuvel <ardb@kernel.org>]

(cc Ilias)

Note to akpm: please drop this series for now.

On Fri, 22 Aug 2025 at 04:00, Evangelos Petrongonas <epetron@amazon.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@kernel.org>
> Signed-off-by: Evangelos Petrongonas <epetron@amazon.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?


> diff --git a/drivers/firmware/efi/efi-init.c b/drivers/firmware/efi/efi-init.c
> index a00e07b853f2..a65c2d5b9e7b 100644
> --- a/drivers/firmware/efi/efi-init.c
> +++ b/drivers/firmware/efi/efi-init.c
> @@ -12,6 +12,7 @@
>  #include <linux/efi.h>
>  #include <linux/fwnode.h>
>  #include <linux/init.h>
> +#include <linux/kexec_handover.h>
>  #include <linux/memblock.h>
>  #include <linux/mm_types.h>
>  #include <linux/of.h>
> @@ -164,12 +165,32 @@ static __init void reserve_regions(void)
>                 pr_info("Processing EFI memory map:\n");
>
>         /*
> -        * Discard memblocks discovered so far: if there are any at this
> -        * point, they originate from memory nodes in the DT, and UEFI
> -        * uses its own memory map instead.
> +        * Discard memblocks discovered so far except for KHO scratch
> +        * regions. Most memblocks at this point originate from memory nodes
> +        * in the DT and UEFI uses its own memory map instead. However, if
> +        * KHO is enabled, scratch regions, which are good known memory
> +        * must be preserved.
>          */
>         memblock_dump_all();
> -       memblock_remove(0, PHYS_ADDR_MAX);
> +
> +       if (is_kho_boot()) {
> +               struct memblock_region *r;
> +
> +               /* Remove all non-KHO regions */
> +               for_each_mem_region(r) {
> +                       if (!memblock_is_kho_scratch(r)) {
> +                               memblock_remove(r->base, r->size);
> +                               r--;
> +                       }
> +               }
> +       } else {
> +               /*
> +                * KHO is disabled. Discard memblocks discovered so far:
> +                * if there are any at this point, they originate from memory
> +                * nodes in the DT, and UEFI uses its own memory map instead.
> +                */
> +               memblock_remove(0, PHYS_ADDR_MAX);
> +       }
>
>         for_each_efi_memory_desc(md) {
>                 paddr = md->phys_addr;
> --
> 2.47.3
>
>
>
>
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