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Wed, 09 Sep 2026 17:58:29 -0700 (PDT) Date: Wed, 9 Sep 2026 17:58:28 -0700 In-Reply-To: <20260905012403.GX4157646@nvidia.com> Mime-Version: 1.0 References: <20260903023452.721732-1-loganodell@google.com> <20260904160009.GV4157646@nvidia.com> <20260905012403.GX4157646@nvidia.com> Message-ID: Subject: Re: [RFC PATCH 0/3] liveupdate: Move to feature flags for LUO and memfd ABI compatibility From: Sean Christopherson To: Jason Gunthorpe Cc: David Matlack , Logan Odell , arnd@arndb.de, pasha.tatashin@soleen.com, rppt@kernel.org, pratyush@kernel.org, graf@amazon.com, akpm@linux-foundation.org, pbonzini@redhat.com, maz@kernel.org, oupton@kernel.org, bhelgaas@google.com, alex@shazbot.org, kevin.tian@intel.com, dwmw2@infradead.org, baolu.lu@linux.intel.com, joro@8bytes.org, will@kernel.org, robin.murphy@arm.com, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-mm@kvack.org, kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-pci@vger.kernel.org, iommu@lists.linux.dev Content-Type: text/plain; charset="us-ascii" X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260909_175831_294076_6ED2B6F9 X-CRM114-Status: GOOD ( 26.03 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Fri, Sep 04, 2026, Jason Gunthorpe wrote: > On Fri, Sep 04, 2026 at 10:24:21PM +0000, David Matlack wrote: > > > The proposal here (which is inspired by the KVM UAPI) is to ensure every > > LUO ABI struct has 2 properties: > > > > 1. A field to encode options/features (e.g. u64 flags). > > 2. A way way to grow without breaking backward compatibility (e.g. so > > we can add new fields). > > > > Each flag can mean whatever it needs to. e.g. It can indicate the > > precence of one or more fields (i.e. new fields in the struct), or it > > can mean a field now has a different meaning (i.e. union in the struct). > > > > This would enable adding support for new features without breaking > > backward compatibility. Downstream users would have to ensure their > > kernel does not start using a new feature while it can still rollback to > > a version that does not support the new feature. > > This was never the biggest problem. The main issue was the functional > behaviors of the kernel that cannot be represented simply as data in a > struct with some flag bits. > > Like for instance kernel A supports memfd folio sizes far larger than > kernel B because we fixed MAX_ORDER. You can't fix that just with > simplistic flags. Can you elaborate on why the folio sizes matter? Honest question, because I don't understand why the serialization format wouldn't express things as "N contiguous pages starting at PFN X". Then the implementation would rebuild its folios as appropriate. I could see things like HugeTLB not working if someone booted the kernel with support for only 1GiB pages and then tried to feed it payload with sub-1GiB ranges. But to me, those sorts of things fall into the "well yeah, don't do that" category.