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[35.199.49.16]) by smtp.gmail.com with ESMTPSA id 71dfb90a1353d-5ec4a5f26c7sm304662e0c.9.2026.10.27.09.21.11 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 27 Oct 2026 09:21:12 -0700 (PDT) Date: Tue, 27 Oct 2026 12:21:11 -0400 From: Pasha Tatashin To: David Matlack Cc: Pasha Tatashin , tarunsahu@google.com, Andrew Morton , joro@8bytes.org, Mike Rapoport , Pratyush Yadav , maz@kernel.org, arnd@arndb.de, jgg@nvidia.com, robin.murphy@arm.com, loganodell@google.com, oupton@kernel.org, kevin.tian@intel.com, alex@shazbot.org, dwmw2@infradead.org, pbonzini@redhat.com, seanjc@google.com, will@kernel.org, Alexander Graf , Bjorn Helgaas , baolu.lu@linux.intel.com, linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org, sagis@google.com, linux-mm@kvack.org, kvmarm@lists.linux.dev, iommu@lists.linux.dev, linux-pci@vger.kernel.org, kexec@lists.infradead.org, skhawaja@google.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH v2 3/4] liveupdate: Replace compatibility string with a 2-bit pair feature header Message-ID: References: X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On 10-26 20:49, David Matlack wrote: > I still think it's not advisable to define name##_wrap since anyone > could accidentally start using it on the incoming path and we wouldn't > notice at first (especially since same kernel Live Update tests would > keep working totally fine). I also don't think asserting the combined > size buys us anything. The header size is just a function of the > number of features, and what we are trying to protect against is > structs embedded in the payload changing size. Fair point. > > So I would just do: > > /* Marks struct name as a KHO serialized payload with a kho_ser_hdr. */ > #define DECLARE_KHO_SER(name, payload_size) \ > static_assert(sizeof(struct name) == (payload_size)) > > DECLARE_KHO_SER(luo_memfd_ser, 16); My main requirement is that DECLARE_KHO_SER() in include/linux/kho/abi/ must be enforced by the compiler rather than being just a bare static_assert() that someone could forget to add. Since kho/abi/ headers contain both versioned structs and unversioned sub-structs, it must be mandatory to declare versioned structs in kho/abi/ with their static_assert size check before the kho_ser_*() helpers can be used on them. So, how about adding enum names like this, that are going to be required in accessor functions later: #define DECLARE_KHO_SER(name, payload_size, nr_feats) \ static_assert(sizeof(struct name) == (payload_size), \ "struct " #name " size mismatch"); \ enum { \ __KHO_SER_SIZE_##name = (payload_size), \ __KHO_SER_NR_FEATS_##name = (nr_feats), \ } > Plus helpers that allocate and initialize the header and return the > payload as a typed pointer. Both the outgoing and incoming paths find > the payload the same way, by computing its offset from the header, > rather than through a struct with a fixed payload offset: > > /* outgoing */ > hdr = kho_ser_alloc(luo_memfd_ser, LUO_MEMFD_NR_FEATS); > ser = kho_ser_payload(hdr, luo_memfd_ser); > > /* incoming */ > hdr = phys_to_virt(args->serialized_data); > ser = kho_ser_payload(hdr, luo_memfd_ser); These would become, where without enum, the init would fail: #define kho_ser_init(buf, name) \ __kho_ser_init(buf, __KHO_SER_SIZE_##name, \ __KHO_SER_NR_FEATS_##name) And payload can also check the existance of DECLARE_KHO_SER, using (expr1, expr2): #define kho_ser_payload(hdr, name) \ ((void)__KHO_SER_SIZE_##name, \ (struct name *)__kho_ser_payload(hdr)) We can also add an optional kho_ser_alloc(name) to allocate, init, and preserve.