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[34.127.75.64]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2eae6bddae0sm12585675ad.35.2026.10.27.10.33.44 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 27 Oct 2026 10:33:44 -0700 (PDT) Date: Tue, 27 Oct 2026 17:33:41 +0000 From: David Matlack To: Pasha Tatashin Cc: 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 2026-10-27 12:21 PM, Pasha Tatashin wrote: > 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), \ > } Sounds good. Since you're also on board with splitting required and optional features [1], this would take both counts: #define DECLARE_KHO_SER(name, payload_size, nr_required, nr_optional) \ static_assert(sizeof(struct name) == (payload_size), \ "struct " #name " size mismatch"); \ enum { \ __KHO_SER_SIZE_##name = (payload_size), \ __KHO_SER_NR_REQUIRED_##name = (nr_required), \ __KHO_SER_NR_OPTIONAL_##name = (nr_optional), \ } DECLARE_KHO_SER(memfd_luo_ser, 16, MEMFD_LUO_NR_REQUIRED, MEMFD_LUO_NR_OPTIONAL); The incoming path can also use __KHO_SER_NR_OPTIONAL_##name to validate the incoming header only contains required features that it knows how to handle: #define kho_ser_validate(hdr, name) \ __kho_ser_validate(hdr, __KHO_SER_NR_REQUIRED_##name) > > 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. +1