Re: [PATCH v4 02/19] kho: block: Record the entry size in the block header

[Mike Rapoport <rppt@kernel.org>]

On Tue, Nov 10, 2026 at 05:01:45AM +0000, Tarun Sahu wrote:
> Subsystems using the KHO Serialization Blocks framework currently do
> not record the serialized entry size in the block header. The entry size
> is only known to individual callers, which must track and store it in an
> owning serialized data structure. If a newer source kernel adds fields
> to an entry, the changed entry size breaks the stride when an older
> target kernel iterates over blocks.
> 
> Add an entry_size field to struct kho_block_header_ser. On the outgoing
> side, record bs->entry_size into every block header during allocation.
> During restoration, derive the entry stride directly from the block
> header instead of relying on caller-supplied dimensions. The
> kho_block_set_init() interface remains unchanged. A zero entry size,
> or one that differs between blocks of a set, is rejected during
> restoration.
> 
> This change modifies the serialized layout of struct kho_block_header_ser

Imperative voice please.

> and shifts the offset of serialized entries within preserved blocks.
> Bump KHO_FDT_COMPATIBLE from kho-v4 to kho-v5 so that pre-v5 and post-v5
> kernels cleanly reject mismatched handover state during boot. Also
> update the stale compatible string example in the documentation.
> 
> Suggested-by: Pratyush Yadav <pratyush@kernel.org>
> Signed-off-by: Tarun Sahu <tarunsahu@google.com>
> ---
>  include/linux/kho/abi/block.h          | 10 ++++----
>  include/linux/kho/abi/kexec_handover.h |  6 ++---
>  kernel/liveupdate/kho_block.c          | 32 +++++++++++++++++++++++---
>  3 files changed, 38 insertions(+), 10 deletions(-)
> 
> @@ -264,8 +274,21 @@ int kho_block_set_restore(struct kho_block_set *bs, u64 head_pa)
>  	}
>  
>  	while (next_pa) {
> +		u64 capacity;
> +
>  		ser = phys_to_virt(next_pa);
> -		if (!ser->count || ser->count > bs->count_per_block) {
> +		if (!ser->entry_size || (entry_size && ser->entry_size != entry_size)) {
> +			pr_warn("Block contains invalid entry size: %llu\n",
> +				ser->entry_size);
> +			err = -EINVAL;
> +			goto err_destroy;
> +		}
> +		entry_size = ser->entry_size;
> +		bs->entry_size = entry_size;
> +		capacity = (KHO_BLOCK_SIZE - sizeof(*ser)) / bs->entry_size;
> +		bs->count_per_block = capacity;

Nit: can't we just initialize bs->count_per_block directly and drop
'capacity' and the change of the following line?

We are not very strict with 80-chars limit in kho/luo anyway.

> +
> +		if (!ser->count || ser->count > capacity) {
>  			pr_warn("Block contains invalid entry count: %llu\n",
>  				ser->count);
>  			err = -EINVAL;

-- 
Sincerely yours,
Mike.