On Thu, Aug 21 2025, 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.
>
> Acked-by: Mike Rapoport (Microsoft) <r...@kernel.org>
> Signed-off-by: Evangelos Petrongonas <epet...@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(-)
>
> 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--;
Hmm, this caught me off-guard. I had to do a double take to realize that
memblock_remove() would decrease memblock.memory.cnt and move the whole
regions array back. A comment would have been nice here.
But then, I wouldn't want you to do a full resend of the series for this
minor nitpick. So perhaps whoever is taking this patch can add one when
applying? Either way is fine though...
Acked-by: Pratyush Yadav <praty...@kernel.org>
> + }
> + }
> + } 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;
--
Regards,
Pratyush Yadav
