Re: [PATCH v4 00/19] kho, liveupdate: Add feature header for fine-grained ABI compatibility

["Pasha Tatashin" <pasha.tatashin@soleen.com>]

On Tue Nov 10, 2026 at 12:01 AM EST, Tarun Sahu wrote:
> This v4 of feature flags support for LUO and KHO is no longer an RFC.
> This has gone through multiple design changes over the past few months,
> and I thank everyone for providing their suggestions and reviewing it
> promptly. Below I talk briefly about the evolution of this series to set
> context and try to make it easy for everyone to review.
>
> Initially, Logan proposed the feature flags in RFC v1 [1], which tried
> to solve the problem of dependency on the compatible string. Feature
> flags introduced finer-grained compatibility. This series had three
> fields: active, supported, and required. The number of flags was limited
> and didn't have flexibility, which reduced the complexity of the patch,
> but put a limitation on the extensibility of the features.
>
> After LPC and a few design discussions, I proposed RFC v2 [2], which
> changed the flags to optional and required and converted the encoding to
> 2 bits per feature instead of having two separate variables. But This still
> has the number of flags limited and fixed the feature header size to
> 128 bytes. Though each serialized structure is being allocated in
> multiples of PAGE_SIZE (folio order = 0, 1, 2...), where 128 bytes is
> not a problem, it still carries two fundamental issues: limited feature
> flags, and a fixed 128-byte memory overhead for each preserved file. In
> the future, as there are plans for moving to KHO slab, the allocation
> for serialized structures can be significantly reduced and this 128-byte
> size will be a problem again.
>
> Based on the RFC v2 discussion, this v4 (v3 [3] was intentionally ignored)
> moves to extensible feature flags with a variable-length flags[] array,
> avoiding the fixed 128-byte overhead and feature-count limit.
>
> Along with this, as discussed on RFC v2, v4 uses separate bit ranges for
> required and optional flags (1 bit per feature). It builds on the
> consideration that a feature in its lifetime will either be required or
> optional; it can't be toggled into the other. In the future, if such a
> need arises, a new feature flag can be introduced to replicate the
> feature as optional or required, opposite to its ancestor flag.
>
> This new layout and encoding in memory is as follows:
>
> struct kho_ser_hdr {
> 	u32 size;
> 	u16 nr_required;
> 	u16 nr_optional;
> 	u64 flags[];
> }
>
> where,
> 	flags[i] = (BIT_MAP(optional_features) << nr_required) |
> 		   BIT_MAP(required_features)
>
> This series introduces the feature header in the KHO subsystem, and it
> is used by serialized structures across KHO and LUO. A few of the
> serialized structures embed other serialized structures directly. To
> keep the relative offsets of fields to be same across kernels, we cannot
> have an embedded serialized structure's size changed; otherwise, direct
> casting will not be possible. To solve this, this series converts those
> embedded structures to KHOSER_PTR, which points to the memory just after
> the parent serialized structure, and allocation and preservation happen
> for the total memory (parent_ser_size + embedded_ser_size).
>
> <Iamparent>_ser {
> 	u64 a;
> 	u64 b;
> 	struct <Iamembedded>_ser e_ser;
> 	u64 d;
> }
> 	CONVERTS TO ===>
>
> <Iamparent>_ser {
> 	u64 a;
> 	u64 b;
> 	DECLARE_KHOSER_PTR(e_ser, <Iamembedded>_ser);
> 	u64 d;
> }
>
> There are only two such instances: kho_vmalloc and luo_file_set_ser.
>
> This series introduces kho_ser_hdr for the following structures:
>
>   - struct kho_kexec_metadata
>   - struct kho_vmalloc
>   - struct kho_block_header_ser
>   - struct luo_ser
>   - struct luo_session_ser
>   - struct luo_file_set_ser
>   - struct luo_file_ser
>   - struct luo_flb_header_ser
>   - struct luo_flb_ser
>   - struct pci_ser
>   - struct pci_dev_ser
>   - struct memfd_luo_ser
>   - struct memfd_luo_folio_ser
>
> If you notice, this also includes the PCI ser structures. PCI LUO
> support is merged in liveupdate/next, and this series is developed on
> top of it.
>
> Each serialized structure is preceded in memory by struct kho_ser_hdr
> and registered via DECLARE_KHO_SER(name, payload_size, nr_required,
> nr_optional). Both struct kho_kexec_metadata and struct pci_dev_ser are
> explicitly marked with __aligned(8) because their largest member is a
> 32-bit integer, and 8-byte alignment is required by DECLARE_KHO_SER() so

I do not believe we need __aligned(8), because the struct immediately 
follows the newly introduced kho_ser_hdr, which is 8-byte aligned. 
However, we should verify that the combined size (header + payload) 
is 8-byte aligned.

> trailing feature payloads remain 8-byte aligned across 32-bit and 64-bit
> kernels.
>
> Also, Introduce DECLARE_KHO_SER_HDR(ser_name, fhdr) which is just the
> wrapper for struct kho_ser_hdr fhdr, This makes it clear
> To which ser_name, this fhdr belongs to.
>
>
> Series structure:
>   - Patch 1 removes __packed from KHO, LUO, and memfd ABI structures and
>     asserts their natural size and 8-byte alignment under CONFIG_64BIT.
>   - Patch 2 records entry_size in struct kho_block_header_ser so the
>     incoming kernel derives the entry stride from the block header.
>   - Patch 3 converts callers that embedded struct kho_vmalloc by value
>     (memfd_luo and test_kho) to reference it via DECLARE_KHOSER_PTR so
>     struct kho_vmalloc can grow independently.
>   - Patch 4 converts struct luo_session_ser to reference
>     struct luo_file_set_ser via DECLARE_KHOSER_PTR instead of embedding
>     it by value so both structures can grow independently.
>   - Patch 5 introduces struct kho_ser_hdr, DECLARE_KHO_SER(),
>     DECLARE_KHO_SER_HDR(), and the serialization/validation helpers in
>     include/linux/kho/abi/feature.h.
>   - Patches 6-18 add the feature header to each of the 13 KHO, LUO, PCI,
>     and memfd serialized structures in independent patches.
>   - Patch 19 adds a KUnit test suite (kho_feature) covering the feature
>     header helpers.
>
> [1] https://lore.kernel.org/all/20260903023452.721732-1-loganodell@google.com/
> [2] https://lore.kernel.org/all/20261021152527.375070-1-tarunsahu@google.com/
> [3] https://lore.kernel.org/all/20261026174217.603114-1-tarunsahu@google.com/
>
> Tarun Sahu (19):
>   liveupdate, kho: Remove __packed from KHO, LUO and memfd ABI
>     structures
>   kho: block: Record the entry size in the block header
>   kho, memfd_luo: Reference struct kho_vmalloc via serialized pointer
>   liveupdate: Reference struct luo_file_set_ser via serialized pointer
>   kho: Add feature header for fine-grained ABI compatibility
>   kho: kexec_metadata: Add the feature header
>   kho: vmalloc: Add the feature header
>   kho: block: Add the feature header to blocks
>   liveupdate: Add feature header to struct luo_ser
>   liveupdate: Add feature header to struct luo_session_ser
>   liveupdate: Add feature header to struct luo_file_set_ser
>   liveupdate: Add feature header to struct luo_file_ser
>   liveupdate: Add feature header to struct luo_flb_header_ser
>   liveupdate: Add feature header to struct luo_flb_ser
>   PCI: liveupdate: Add feature header to struct pci_ser
>   PCI: liveupdate: Add feature header to struct pci_dev_ser
>   mm: memfd_luo: Add feature header to struct memfd_luo_ser
>   mm: memfd_luo: Add feature header to struct memfd_luo_folio_ser
>   kho: Add KUnit test for the feature header helpers
>
>  MAINTAINERS                            |   1 +
>  drivers/pci/liveupdate.c               |  99 +++----
>  include/linux/kexec_handover.h         |  15 +-
>  include/linux/kho/abi/block.h          |  21 +-
>  include/linux/kho/abi/feature.h        | 372 +++++++++++++++++++++++++
>  include/linux/kho/abi/kexec_handover.h |  11 +-
>  include/linux/kho/abi/kexec_metadata.h |  17 +-
>  include/linux/kho/abi/luo.h            | 102 ++++---
>  include/linux/kho/abi/memfd.h          |  27 +-
>  include/linux/kho/abi/pci.h            |  66 ++---
>  include/linux/kho_block.h              |   8 +-
>  kernel/liveupdate/kexec_handover.c     |  98 ++++---
>  kernel/liveupdate/kho_block.c          | 172 +++++++++---
>  kernel/liveupdate/luo_core.c           |  43 +--
>  kernel/liveupdate/luo_file.c           |  58 ++--
>  kernel/liveupdate/luo_flb.c            |  69 +++--
>  kernel/liveupdate/luo_internal.h       |   9 +-
>  kernel/liveupdate/luo_session.c        |  55 +++-
>  lib/Kconfig.debug                      |  13 +
>  lib/test_kho.c                         |  38 ++-
>  lib/tests/Makefile                     |   1 +
>  lib/tests/kho_feature_kunit.c          | 184 ++++++++++++
>  mm/memfd_luo.c                         | 183 ++++++++----
>  23 files changed, 1271 insertions(+), 391 deletions(-)
>  create mode 100644 include/linux/kho/abi/feature.h
>  create mode 100644 lib/tests/kho_feature_kunit.c

-- 
Pasha