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Peter Anvin" , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Jonathan Corbet , Shuah Khan , Shuah Khan , Vishal Annapurve , Andrew Morton , Chris Li , Kairui Song , Kemeng Shi , Nhat Pham , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Youngjun Park , Qi Zheng , Shakeel Butt , Kiryl Shutsemau , Baoquan He , Jason Gunthorpe , John Hubbard , Peter Xu , tarunsahu@google.com, Randy Dunlap , Lorenzo Stoakes , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, linux-coco@lists.linux.dev, Xiaoyao Li Content-Type: text/plain; charset="us-ascii" X-Rspam-User: X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 8759E2000A X-Stat-Signature: wb7zcbmeasrh58qiyykacddhntz9fotw X-HE-Tag: 1788355836-274308 X-HE-Meta: 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 Imh2cjZH 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Sep 02, 2026, Binbin Wu wrote: > On 9/2/2026 2:26 AM, Sean Christopherson wrote: > > On Tue, Sep 01, 2026, Fuad Tabba wrote: > >> Hi Sean, Ackerley, > >> > >> On Mon, 31 Aug 2026 at 01:25, Ackerley Tng via B4 Relay > >> wrote: > >> ... > >>> + kvm.gmem_in_place_conversion= > >>> + [KVM] Controls whether KVM enables in-place conversion > >>> + support for guest_memfd and tracks the private/shared > >>> + state of memory per guest_memfd instead of per VM. > >>> + > >>> + If enabled, KVM enables the KVM_SET_MEMORY_ATTRIBUTES2 > >>> + ioctl on guest_memfd file descriptors and disables the > >>> + legacy VM-scoped KVM_SET_MEMORY_ATTRIBUTES ioctl for > >>> + private memory state tracking. Only the > >>> + KVM_MEMORY_ATTRIBUTE_PRIVATE attribute moves to > >>> + per-guest_memfd tracking; other attributes remain > >>> + per-VM. > >>> + > >>> + This parameter toggles KVM's in-place conversion > >>> + capability support. Whether a VMM uses separate backends > >>> + or out-of-place memory management is determined by > >>> + userspace VMM design. > >>> + > >>> + Note, this parameter is only available when > >>> + CONFIG_KVM_VM_MEMORY_ATTRIBUTES=y. When > >>> + CONFIG_KVM_VM_MEMORY_ATTRIBUTES is not set, in-place > >>> + conversion is unconditionally enabled. > >>> + > >>> + Default is Y (on). > >> > >> The parameter is only settable when CONFIG_KVM_VM_MEMORY_ATTRIBUTES=y, and > >> in that config gmem_in_place_conversion initialises to > >> !IS_ENABLED(CONFIG_KVM_VM_MEMORY_ATTRIBUTES), i.e. N, not Y. Is the doc line > >> wrong, or is N-by-default not what was intended here? > > > > The doc is wrong. The intent is to maintain backwards compatibility with existing > > deployments if KVM_VM_MEMORY_ATTRIBUTES=y. > > > > Will it result in the situation that kernel builds with CONFIG_KVM_VM_MEMORY_ATTRIBUTES > enabled by default for backwards compatibility, which requires users to explicitly enable > gmem_in_place_conversion by the kernel option? Then it effectively disables > gmem_in_place_conversion by default. Yes. My intent/expectation is that the only reason to build with CONFIG_KVM_VM_MEMORY_ATTRIBUTES=y is for backwards compatibility, and so it makes sense to have the existing behavior be the default. If a deployment only wants the new behavior, then it can simply build with CONFIG_KVM_VM_MEMORY_ATTRIBUTES=n. Folks that want only the new behavior but are using a distro/pre-built kernel will need to manually enable in-place conversion, but that's not too onerous, and it's simply not feasible to provide defaults that work for every possible scenario. > But I guess it's OK for CSPs since they know what they want. Yep.