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AFNElJ+ftrelxkTLLLGGGSbayNvyMUrPClSBw5zaO4G06HdRSTidWi5BjU9ZaEm4spCRkKHDLnUyyCGZWg==@kvack.org X-Gm-Message-State: AOJu0Ywui8xV0S/PjIJsygCwKOXpyVQOb1O7gE571BkbV0XKvZpQVbqT lSIXqy9Xovw+Kymgyb8u28yqTsCwWvVwvHgM4KyjyPoMJgrDIp6UFNlKh8VigCpLkN21d64GJjS O7BA4Fw== X-Received: from pgbbo12.prod.google.com ([2002:a05:6a02:38c:b0:c80:296b:72cc]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a20:7fa3:b0:3b4:8bc6:138 with SMTP id adf61e73a8af0-3bd4ae9cd56mr397852637.23.1782348119218; Wed, 24 Jun 2026 17:41:59 -0700 (PDT) Date: Wed, 24 Jun 2026 17:41:58 -0700 In-Reply-To: Mime-Version: 1.0 References: <20260618-gmem-inplace-conversion-v8-0-9d2959357853@google.com> <20260618-gmem-inplace-conversion-v8-24-9d2959357853@google.com> Message-ID: Subject: Re: [PATCH v8 24/46] KVM: guest_memfd: Make in-place conversion the default From: Sean Christopherson To: Ackerley Tng Cc: Yan Zhao , aik@amd.com, andrew.jones@linux.dev, binbin.wu@linux.intel.com, 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, tabba@google.com, willy@infradead.org, wyihan@google.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 , Vlastimil Babka , 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 Content-Type: text/plain; charset="us-ascii" X-Rspam-User: X-Stat-Signature: usrbicoh8ba3f3do9ie99qg8ca358uhd X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: D16AFA0003 X-HE-Tag: 1782348120-570176 X-HE-Meta: U2FsdGVkX1+CM6FGVgZn5XgXUW129rFQj+pYiM7YOb8LaRna5JpNW5PLpyAr4pyj4s5Z1IrzNE6sX4GT5kADDfqrUWtdwUddVxC+mRnKBR6zFbsvAxVOtEWPEm2GxmnNb+0F49d8y6PjOcmE0rk3FiPOWKyscNjuPi8lNWIceGwapTiPsxesgrirxBh02/BfGbB3rwz6nWl80+vqhssVIzHHzkwvLdserqol4eX1ULExB8W+TZtfvTxovsyusJEDVZ3OzJzkcL7Tgl8EuJTTvWLZ8faTMGqac4F9GhJ78ra4Srto7VXRHJbg2/90XEhsHsHSEvLUJ1ABf/3L/sTlo8YEbacaX56reUg0p42NeyMfp36nGR93gbHRaQsTXNheanyVsfQZHu8KqnUeMCmitGBb1y94wpE0e2ifGu1TOt7bZhXHzmVaca8Qq9yJX+ldWWY1M6sgs13OH6wdoSIGpYyLfOdtsE2pZeHP+gw7t+hnEd4PX+lGDaCpJc8uVtqxjo9SD3gzAZeoh+ChjEnrOmiI5j+tSfkaQi7TYqnZaiynBG7d1zv/JtHAL/3d93lvHHXFwjCn0QxlVGLivZ0RRRJt0+ReKO3UMB4J1fnINrbXCEeSPOK1MgW+yvbVO/n4DMH59jKHc9C/YJ0gs8p9bDSoPK7cv8mFImnZETPYNSApfX1YO+9D13eaDHc7pydtRAHEB6NTBHFcL4o2dpQAoh+sM6062pUgYcMWJw+FB1mcGlshUYzV28lFgz0JXqJkHoYDidb3wIg0NWlqIp1g8JkjDAWgQ1y0hFIIURU/wcT8CO1VqihEr30dECGbdDBaRxbsUgnoSGnX78Xm/bROknikV4gXJchn/IevSaa/qO7TB3g2iAh82lAfc848k9Hr0kvI275EdhwFDgeS3GXkFoKQMQCCeUYjABa+2t6xZFqdGF8eT6Tk+iPfZTziA8R/K8LR8ABHOuGkLLomzp8 eKZKggh7 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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, Jun 24, 2026, Ackerley Tng wrote: > Yan Zhao writes: > > With gmem_in_place_conversion=true, userspace can create guest_memfd without the > > MMAP flag. In such cases, shared memory is allocated from different backends. > > This means this module parameter only enables per-gmem memory attribute and does > > not guarantee that gmem in-place conversion will actually occur. KVM module params are pretty much always about what KVM supports, not what is guaranteed to happen. - enable_mmio_caching doesn't guarantee there will actually be MMIO SPTEs, because maybe the guest never accesses emulated MMIO. - enable_pmu doesn't guarantee VMs will get a PMU, because userspace may elect not to advertise one. - and so on and so forth... Yes, there's a small mental jump to get from "KVM supports in-place conversion" to "I need to set memory attributes on the guest_memfd instance, not the VM", but I don't see that as a big hurdle, certainly not in the long term. And once the VMM code is written, I really do think most people are going to care about whether or not KVM supports in-place conversion, not where PRIVATE is tracked. > > To avoid confusion, could we rename this module parameter to something more > > accurate, such as gmem_memory_attribute? > > I asked Sean about this after getting some fixes off list. Sean said > gmem_in_place_conversion is named for a host admin to use, and something > like gmem_memory_attributes is too much implementation details for the > admin. > > Sean, would you reconsider since Yan also asked? If the admin compiled > the kernel knowing what CONFIG_KVM_VM_MEMORY_ATTRIBUTES means, then the > admin would also be able to use a param like gmem_memory_attributes? No, because it's not all memory attributes, it's very specifically the PRIVATE attribute that will get moved to guest_memfd. I don't want to pick a name that will become stale and confusing when RWX attributes come along. The RWX bits will be per-VM, while PRIVATE will be per-guest_memfd.