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[35.245.167.64]) by smtp.gmail.com with ESMTPSA id a1e0cc1a2514c-99125931983sm6616412241.2.2026.10.28.10.44.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 28 Oct 2026 10:44:28 -0700 (PDT) Date: Wed, 28 Oct 2026 13:44:27 -0400 From: Pasha Tatashin To: Pratyush Yadav Cc: David Matlack , Pasha Tatashin , tarunsahu@google.com, Andrew Morton , joro@8bytes.org, Mike Rapoport , 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: <2vxz8q3hoicx.fsf@kernel.org> 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=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <2vxz8q3hoicx.fsf@kernel.org> On 10-28 18:17, Pratyush Yadav wrote: > On Mon, Oct 26 2026, David Matlack wrote: > > > On 2026-10-25 04:38 PM, Pasha Tatashin wrote: > >> On Fri, Oct 23, 2026 at 9:50 AM David Matlack wrote: > > > [...] > >> The reason I care about keeping the header compact is that we are > >> defining a foundational kernel ABI and want to use this header across > >> almost every serialized data structure so we can evolve them later > >> without breaking compatibility. While some top-level structs are on > >> sparse folios today, other serialized structs are tightly packed, and > >> we are looking to add KHO slab support in the future where tightly > >> packed allocations will be common. > > > > My intuition is we will have a small number of headers per preserved > > file and per preserved FLB. One for the top-level struct and then > > optionally a few more for sub-structs like arch-specific data. That is > > why I am less worried 8 bytes per header, even if we fix the folio > > problem. > > > > Maybe where our visions differ is when there are collections of > > serialized structs, like KHO blocks. For collections I don't think the > > elements will each have their own headers. Otherwise the elements could > > be different sizes. > > kho_block today doesn't store entry_size in the serialized payload. It > relies on its caller to do so. With extensible structures, this can get > messy. Sure, the owner of the kho_block_set can track the size somewhere > in its payload, but perhaps we should store the size in > kho_block_header_ser to begin with? I agree, and we will discuss versioned vs. owned serialized data structures separately. With this series, we establish a common framework: size/feature flags and accessors, and we update most of our serialized structs to include them. All "shared" structs (e.g., KHO, FLB, radix-tree, block_sets, etc.) should have their own versioning. kho_block_header_ser should have this header. Pasha > > Anyway, not a problem this series needs to solve, but something we > should eventually do. > > [...] > > -- > Regards, > Pratyush Yadav