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

[Mike Rapoport <rppt@kernel.org>]

On Fri, Aug 08, 2025 at 04:36:51PM +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 while still
> allowing EFI to rebuild its memory map.

It's worth mentioning that scratch areas are "good known memory" :)
 
> Signed-off-by: Evangelos Petrongonas <epetron@amazon.de>
> ---
> 
> Reproduction/Verification Steps
> The issue and the fix can be reproduced/verified by booting a VM with
> EFI and attempting to perform a KHO enabled kexec. The fix
> was developed/tested on arm64.
> 
>  drivers/firmware/efi/efi-init.c | 31 +++++++++++++++++++++++++++----
>  1 file changed, 27 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/firmware/efi/efi-init.c b/drivers/firmware/efi/efi-init.c
> index a00e07b853f22..2f08b1ab764f6 100644
> --- a/drivers/firmware/efi/efi-init.c
> +++ b/drivers/firmware/efi/efi-init.c
> @@ -164,12 +164,35 @@ 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.
>  	 */
>  	memblock_dump_all();
> -	memblock_remove(0, PHYS_ADDR_MAX);
> +
> +	if (IS_ENABLED(CONFIG_MEMBLOCK_KHO_SCRATCH)) {

It's better to condition this on kho_get_fdt() that means that we are
actually doing a handover.

> +		struct memblock_region *reg;
> +		phys_addr_t start, size;
> +		int i;
> +
> +		/* Remove all non-KHO regions */
> +		for (i = memblock.memory.cnt - 1; i >= 0; i--) {

Please use for_each_mem_region() 

> +			reg = &memblock.memory.regions[i];
> +			if (!memblock_is_kho_scratch(reg)) {
> +				start = reg->base;
> +				size = reg->size;
> +				memblock_remove(start, size);
> +			}
> +		}
> +	} 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;
> -- 
> 2.43.0

-- 
Sincerely yours,
Mike.