On Tue, Aug 19, 2025 at 11:22:46PM +0000, Evangelos Petrongonas 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.
>
> Signed-off-by: Evangelos Petrongonas <epet...@amazon.de>
> ---
> Changes in v2:
> - Replaced the for loop with for_each_mem_region
> - Fixed comment indentation
> - Amended commit message to specify that scratch regions
> are known good regions
>
> drivers/firmware/efi/efi-init.c | 28 ++++++++++++++++++++++++----
> 1 file changed, 24 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/firmware/efi/efi-init.c b/drivers/firmware/efi/efi-init.c
> index a00e07b853f2..99f7eecc320f 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 +165,31 @@ 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 must be preserved.
I'd add that KHO scratch regions are good know memory here as well. With
that
Acked-by: Mike Rapoport (Microsoft) <r...@kernel.org>
> */
> 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
--
Sincerely yours,
Mike.
