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 <[email protected]>
> ---
> 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
