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X-CSE-ConnectionGUID: GBgOIUDeR5iYV7Q9G6K3HQ== X-CSE-MsgGUID: am2E//U2QSW/BdoCGDlH6g== X-IronPort-AV: E=McAfee;i="6800,10657,11893"; a="91286634" X-IronPort-AV: E=Sophos;i="6.25,257,1779174000"; d="scan'208";a="91286634" Received: from fmviesa001.fm.intel.com ([10.60.135.141]) by fmvoesa107.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 01 Sep 2026 22:56:21 -0700 X-CSE-ConnectionGUID: TKdU4rLOTfitm42CraiH0Q== X-CSE-MsgGUID: F4dTuqYWTCK6sK9qyDwvcg== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.25,257,1779174000"; d="scan'208";a="294139596" Received: from unknown (HELO [10.238.2.33]) ([10.238.2.33]) by smtpauth.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 01 Sep 2026 22:56:08 -0700 Message-ID: <33431813-fc94-48fb-9042-5cfc68af05b8@linux.intel.com> Date: Wed, 2 Sep 2026 13:56:06 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v12 23/45] KVM: Let userspace disable per-VM mem attributes, enable per-gmem attributes To: Sean Christopherson , Fuad Tabba Cc: ackerleytng@google.com, aik@amd.com, andrew.jones@linux.dev, brauner@kernel.org, chao.p.peng@linux.intel.com, david@kernel.org, jmattson@google.com, jthoughton@google.com, michael.roth@amd.com, oupton@kernel.org, pankaj.gupta@amd.com, qperret@google.com, rick.p.edgecombe@intel.com, rientjes@google.com, shivankg@amd.com, steven.price@arm.com, willy@infradead.org, wyihan@google.com, yan.y.zhao@intel.com, forkloop@google.com, pratyush@kernel.org, suzuki.poulose@arm.com, aneesh.kumar@kernel.org, liam@infradead.org, Paolo Bonzini , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. 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 References: <20260830-gmem-inplace-conversion-v12-0-85e5fd25252a@google.com> <20260830-gmem-inplace-conversion-v12-23-85e5fd25252a@google.com> Content-Language: en-US From: Binbin Wu In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 1A9DE140008 X-Stat-Signature: igk54xpna4cixhmdd5176zw73hpqfwap X-Rspam-User: X-HE-Tag: 1788328583-822329 X-HE-Meta: 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 0UHkRw32 38L45GNGmIsHUW/yyzUqnu7KN+25imYGD6covp7uMhHeN36wWs4Z8wimwDH4Ua1XNCXL/32dNgsjNcsS2LvXpEV/xOBCJ9GePQ4Y6H9zFcoYrsF7yXWLUOVKJVm2sEBb88x+EsPtqBBtSHBamFc5GnWepAO/p8XhDc88ylMz3n2IZlun8faVk4H4Q57QGpbIl0O5eXxhXu2zgOQYK84CTps7dHgso0KNN89ZuwIEQwduNU93tBQJaAbqFKl6TNvREDcV0LjO6XDj2lpg9FbMcNdzn2w== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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. But I guess it's OK for CSPs since they know what they want. >>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c >>> index 05c518c9b8078..929fd3e1a01e6 100644 >>> --- a/virt/kvm/kvm_main.c >>> +++ b/virt/kvm/kvm_main.c >>> @@ -103,7 +103,10 @@ static bool __ro_after_init allow_unsafe_mappings; >>> module_param(allow_unsafe_mappings, bool, 0444); >>> >>> #ifdef kvm_arch_has_private_mem >>> -bool __ro_after_init gmem_in_place_conversion = false; >>> +bool __ro_after_init gmem_in_place_conversion = !IS_ENABLED(CONFIG_KVM_VM_MEMORY_ATTRIBUTES); >>> +#ifdef CONFIG_KVM_VM_MEMORY_ATTRIBUTES >>> +module_param(gmem_in_place_conversion, bool, 0444); >>> +#endif >>> EXPORT_SYMBOL_FOR_KVM_INTERNAL(gmem_in_place_conversion); >>> #endif >