Re: [PATCH 5/6] kho: Relocate vmalloc preservation structure to KHO ABI header

[Pratyush Yadav <pratyush@kernel.org>]

On Mon, Jan 05 2026, Mike Rapoport wrote:

> From: Jason Miu <jasonmiu@google.com>
>
> The `struct kho_vmalloc` defines the in-memory layout for preserving
> vmalloc regions across kexec. This layout is a contract between kernels
> and part of the KHO ABI.
>
> To reflect this relationship, the related structs and helper macros are
> relocated to the ABI header, `include/linux/kho/abi/kexec_handover.h`.
> This move places the structure's definition under the protection of the
> KHO_FDT_COMPATIBLE version string.
>
> The structure and its components are now also documented within the
> ABI header to describe the contract and prevent ABI breaks.
>
> Signed-off-by: Jason Miu <jasonmiu@google.com>
> Co-developed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
[...]
> +/* Helper macro to define a union for a serializable pointer. */
> +#define DECLARE_KHOSER_PTR(name, type)	\
> +	union {                        \
> +		u64 phys;              \
> +		type ptr;              \
> +	} name
> +
> +/* Stores the physical address of a serializable pointer. */
> +#define KHOSER_STORE_PTR(dest, val)               \
> +	({                                        \
> +		typeof(val) v = val;              \
> +		typecheck(typeof((dest).ptr), v); \
> +		(dest).phys = virt_to_phys(v);    \
> +	})
> +
> +/* Loads the stored physical address back to a pointer. */
> +#define KHOSER_LOAD_PTR(src)						\
> +	({                                                                   \
> +		typeof(src) s = src;                                         \
> +		(typeof((s).ptr))((s).phys ? phys_to_virt((s).phys) : NULL); \
> +	})

Nit: not a fan of exposing code internals to the ABI header. But without
this the definition of kho_vmalloc_hdr won't make any sense to someone
reading the doc without looking at the code. Dunno if we can do anything
better though...

> +
> +/*
> + * This header is embedded at the beginning of each `kho_vmalloc_chunk`
> + * and contains a pointer to the next chunk in the linked list,
> + * stored as a physical address for handover.
> + */
> +struct kho_vmalloc_hdr {
> +	DECLARE_KHOSER_PTR(next, struct kho_vmalloc_chunk *);
> +};
> +
> +#define KHO_VMALLOC_SIZE				\
> +	((PAGE_SIZE - sizeof(struct kho_vmalloc_hdr)) / \
> +	 sizeof(u64))
> +
> +/*
> + * Each chunk is a single page and is part of a linked list that describes
> + * a preserved vmalloc area. It contains the header with the link to the next
> + * chunk and an array of physical addresses of the pages that make up the
> + * preserved vmalloc area.

Perhaps also mention that the array is 0-terminated?

Looks good otherwise.

[...]

-- 
Regards,
Pratyush Yadav