Re: [PATCH v4 04/13] liveupdate: register luo_ser as KHO subtree

[Pratyush Yadav <pratyush@kernel.org>]

On Sat, May 30 2026, Pasha Tatashin wrote:

> Entirely remove the LUO FDT wrapper since the FDT only carries the
> compatible string and the pointer to the centralized struct luo_ser.
> Instead, register the struct luo_ser via the KHO raw subtree
> API, placing the compatibility string inside the structure itself.
>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> ---
>  include/linux/kho/abi/luo.h  | 57 +++++++++---------------
>  kernel/liveupdate/luo_core.c | 85 +++++++++++-------------------------
>  2 files changed, 46 insertions(+), 96 deletions(-)
>
> diff --git a/include/linux/kho/abi/luo.h b/include/linux/kho/abi/luo.h
> index 1b2f865a771a..9a4fe491812b 100644
> --- a/include/linux/kho/abi/luo.h
> +++ b/include/linux/kho/abi/luo.h
> @@ -10,11 +10,11 @@
>   *
>   * Live Update Orchestrator uses the stable Application Binary Interface
>   * defined below to pass state from a pre-update kernel to a post-update
> - * kernel. The ABI is built upon the Kexec HandOver framework and uses a
> - * Flattened Device Tree to describe the preserved data.
> + * kernel. The ABI is built upon the Kexec HandOver framework and registers
> + * the central `struct luo_ser` via the KHO raw subtree API.
>   *
> - * This interface is a contract. Any modification to the FDT structure, node
> - * properties, compatible strings, or the layout of the `__packed` serialization
> + * This interface is a contract. Any modification to the structure fields,
> + * compatible strings, or the layout of the `__packed` serialization
>   * structures defined here constitutes a breaking change. Such changes require
>   * incrementing the version number in the relevant `_COMPATIBLE` string to
>   * prevent a new kernel from misinterpreting data from an old kernel.
> @@ -23,31 +23,15 @@
>   * however, backward/forward compatibility is only guaranteed for kernels
>   * supporting the same ABI version.
>   *
> - * FDT Structure Overview:
> + * KHO Structure Overview:
>   *   The entire LUO state is encapsulated within a single KHO entry named "LUO".
> - *   This entry contains an FDT with the following layout:
> - *
> - *   .. code-block:: none
> - *
> - *     / {
> - *         compatible = "luo-v2";
> - *         luo-abi-header = <phys_addr_of_luo_ser>;
> - *     };
> - *
> - * Main LUO Node (/):
> - *
> - *   - compatible: "luo-v2"
> - *     Identifies the overall LUO ABI version.
> - *   - luo-abi-header: u64
> - *     The physical address of `struct luo_ser`.
> + *   This entry contains the `struct luo_ser` structure.
>   *
>   * Serialization Structures:
> - *   The FDT properties point to memory regions containing arrays of simple,
> - *   `__packed` structures. These structures contain the actual preserved state.
> - *
>   *   - struct luo_ser:
>   *     The central ABI structure that contains the overall state of the LUO.
> - *     It includes the liveupdate-number and pointers to sessions and FLBs.
> + *     It includes the compatibility string, the liveupdate-number, and pointers
> + *     to sessions and FLBs.
>   *
>   *   - struct luo_session_header_ser:
>   *     Header for the session array. Contains the total page count of the
> @@ -78,26 +62,27 @@
>  #ifndef _LINUX_KHO_ABI_LUO_H
>  #define _LINUX_KHO_ABI_LUO_H
>  
> +#include <linux/align.h>
>  #include <uapi/linux/liveupdate.h>
>  
>  /*
> - * The LUO FDT hooks all LUO state for sessions, fds, etc.
> + * The LUO state is registered under this KHO entry name.
>   */
> -#define LUO_FDT_SIZE		PAGE_SIZE
> -#define LUO_FDT_KHO_ENTRY_NAME	"LUO"
> -#define LUO_FDT_COMPATIBLE	"luo-v2"
> -#define LUO_FDT_ABI_HEADER	"luo-abi-header"
> +#define LUO_KHO_ENTRY_NAME	"LUO"
> +#define LUO_ABI_COMPATIBLE	"luo-v3"
> +#define LUO_ABI_COMPAT_LEN	ALIGN(sizeof(LUO_ABI_COMPATIBLE), 8)

The length of the compatible field will change depending on the length
of the string. While that is technically fine since a new ABI version is
allowed to change the layout, it feels odd. I think it would be better
if we define a static size here, say 64 bytes. This way you can avoid
all the weirdness that can happen when you move from one version to
another.

>  
>  /**
>   * struct luo_ser - Centralized LUO ABI header.
> + * @compatible:     Compatibility string identifying the LUO ABI version.
>   * @liveupdate_num: A counter tracking the number of successful live updates.
>   * @sessions_pa:    Physical address of the first session block header.
>   * @flbs_pa:        Physical address of the FLB header.
>   *
> - * This structure is the root of all preserved LUO state. It is pointed to by
> - * the "luo-abi-header" property in the LUO FDT.
> + * This structure is the root of all preserved LUO state.
>   */
>  struct luo_ser {
> +	char compatible[LUO_ABI_COMPAT_LEN];
>  	u64 liveupdate_num;
>  	u64 sessions_pa;
>  	u64 flbs_pa;
[...]
> @@ -94,40 +91,29 @@ static int __init luo_early_startup(void)
>  		return 0;
>  	}
>  
> -	/* Retrieve LUO subtree, and verify its format. */
> -	err = kho_retrieve_subtree(LUO_FDT_KHO_ENTRY_NAME, &fdt_phys, NULL);
> +	/* Retrieve LUO state from KHO. */
> +	err = kho_retrieve_subtree(LUO_KHO_ENTRY_NAME, &luo_ser_phys, &len);
>  	if (err) {
>  		if (err != -ENOENT) {
> -			pr_err("failed to retrieve FDT '%s' from KHO: %pe\n",
> -			       LUO_FDT_KHO_ENTRY_NAME, ERR_PTR(err));
> +			pr_err("failed to retrieve LUO state '%s' from KHO: %pe\n",
> +			       LUO_KHO_ENTRY_NAME, ERR_PTR(err));
>  			return err;
>  		}
>  
>  		return 0;
>  	}
>  
> -	luo_global.fdt_in = phys_to_virt(fdt_phys);
> -	err = fdt_node_check_compatible(luo_global.fdt_in, 0,
> -					LUO_FDT_COMPATIBLE);
> -	if (err) {
> -		pr_err("FDT '%s' is incompatible with '%s' [%d]\n",
> -		       LUO_FDT_KHO_ENTRY_NAME, LUO_FDT_COMPATIBLE, err);
> -
> +	if (len < sizeof(*luo_ser)) {

len != sizeof(*luo_ser) here?

> +		pr_err("LUO state is too small (%zu < %zu)\n", len, sizeof(*luo_ser));
>  		return -EINVAL;
>  	}
>  
> -	header_size = 0;
> -	ptr = fdt_getprop(luo_global.fdt_in, 0, LUO_FDT_ABI_HEADER, &header_size);
> -	if (!ptr || header_size != sizeof(u64)) {
> -		pr_err("Unable to get ABI header '%s' [%d]\n",
> -		       LUO_FDT_ABI_HEADER, header_size);
> -
> +	luo_ser = phys_to_virt(luo_ser_phys);
> +	if (strncmp(luo_ser->compatible, LUO_ABI_COMPATIBLE, LUO_ABI_COMPAT_LEN)) {
> +		pr_err("LUO state is incompatible with '%s'\n", LUO_ABI_COMPATIBLE);
>  		return -EINVAL;
>  	}
>  
> -	luo_ser_pa = get_unaligned((u64 *)ptr);
> -	luo_ser = phys_to_virt(luo_ser_pa);
> -
>  	luo_global.liveupdate_num = luo_ser->liveupdate_num;
>  	pr_info("Retrieved live update data, liveupdate number: %lld\n",
>  		luo_global.liveupdate_num);
[...]

-- 
Regards,
Pratyush Yadav