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AJvYcCXuYkXtFIGJEYm/6sYJ4kMd5KklXBPgZqkQKLWMMun4ZRwuxvL88xJSqVeBatX/9k/h2AROCi3I2iIpfbB3@vger.kernel.org X-Gm-Message-State: AOJu0YyCcXnHNYspr1i44bE3iy9o3/yRGhItJbpTxl7frtjNrRE/zymQ JSgVKrAp9HR0gl/R2m7lXIpV8rDeohBO0YTxkv6Qg+zerstxHFvWh7SwWBjttwE6t3elaguDGne FHO9caZ7wbKyOtU/z5blwvX24Du+uo3cVTUS/5b5t X-Gm-Gg: ASbGncsZtYpu6In/7aQQw1hoRdsiKa3IysNan50I6NixrwSTGyR5SyweJPcY0elpFBP 1iCHQEjTprnZkXSeM8Q7eBwjSqVc29NTQyijHzmRYgkeflntRKF2cer804/GnnsMW8WdOCHYVZg lWACowdgbkpxY4UJqeFci9QIj8Eu5sqdU0fZeBux7MMyu1mcTabI06wPS+UisWrn9OFP2shC3Fx yhL X-Google-Smtp-Source: AGHT+IHZnJT8IPYNYYlq8IZWbrFrKSERazCCUgk/PYCXPT2eafZj+EWruKSZR2KdLOBmcVX/Dr9qFLZcvU+hjVe4mck= X-Received: by 2002:a17:903:234a:b0:22e:4509:cb86 with SMTP id d9443c01a7336-23ddae070d9mr2349035ad.19.1752071340735; Wed, 09 Jul 2025 07:29:00 -0700 (PDT) Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 References: <006899ccedf93f45082390460620753090c01914.camel@intel.com> <5decd42b3239d665d5e6c5c23e58c16c86488ca8.camel@intel.com> In-Reply-To: From: Vishal Annapurve Date: Wed, 9 Jul 2025 07:28:48 -0700 X-Gm-Features: Ac12FXzvLP7atWaiCB-cuaA1xwFEC8dxkhwp1kDPwptSswG_7FLCPsdSJrQ6QBQ Message-ID: Subject: Re: [RFC PATCH v2 00/51] 1G page support for guest_memfd To: Sean Christopherson Cc: Rick P Edgecombe , "pvorel@suse.cz" , "kvm@vger.kernel.org" , "catalin.marinas@arm.com" , Jun Miao , "palmer@dabbelt.com" , "pdurrant@amazon.co.uk" , "vbabka@suse.cz" , "peterx@redhat.com" , "x86@kernel.org" , "amoorthy@google.com" , "tabba@google.com" , "quic_svaddagi@quicinc.com" , "maz@kernel.org" , "vkuznets@redhat.com" , "anthony.yznaga@oracle.com" , "mail@maciej.szmigiero.name" , "quic_eberman@quicinc.com" , Wei W Wang , Fan Du , "Wieczor-Retman, Maciej" , Yan Y Zhao , "ajones@ventanamicro.com" , Dave Hansen , "paul.walmsley@sifive.com" , "quic_mnalajal@quicinc.com" , "aik@amd.com" , "usama.arif@bytedance.com" , "fvdl@google.com" , "jack@suse.cz" , "quic_cvanscha@quicinc.com" , Kirill Shutemov , "willy@infradead.org" , "steven.price@arm.com" , "anup@brainfault.org" , "thomas.lendacky@amd.com" , "keirf@google.com" , "mic@digikod.net" , "linux-kernel@vger.kernel.org" , "nsaenz@amazon.es" , "akpm@linux-foundation.org" , "oliver.upton@linux.dev" , "binbin.wu@linux.intel.com" , "muchun.song@linux.dev" , Zhiquan1 Li , "rientjes@google.com" , Erdem Aktas , "mpe@ellerman.id.au" , "david@redhat.com" , "jgg@ziepe.ca" , "hughd@google.com" , "jhubbard@nvidia.com" , Haibo1 Xu , Isaku Yamahata , "jthoughton@google.com" , "rppt@kernel.org" , "steven.sistare@oracle.com" , "jarkko@kernel.org" , "quic_pheragu@quicinc.com" , "chenhuacai@kernel.org" , Kai Huang , "shuah@kernel.org" , "bfoster@redhat.com" , "dwmw@amazon.co.uk" , Chao P Peng , "pankaj.gupta@amd.com" , Alexander Graf , "nikunj@amd.com" , "viro@zeniv.linux.org.uk" , "pbonzini@redhat.com" , "yuzenghui@huawei.com" , "jroedel@suse.de" , "suzuki.poulose@arm.com" , "jgowans@amazon.com" , Yilun Xu , "liam.merwick@oracle.com" , "michael.roth@amd.com" , "quic_tsoni@quicinc.com" , Xiaoyao Li , "aou@eecs.berkeley.edu" , Ira Weiny , "richard.weiyang@gmail.com" , "kent.overstreet@linux.dev" , "qperret@google.com" , "dmatlack@google.com" , "james.morse@arm.com" , "brauner@kernel.org" , "linux-fsdevel@vger.kernel.org" , "ackerleytng@google.com" , "pgonda@google.com" , "quic_pderrin@quicinc.com" , "roypat@amazon.co.uk" , "hch@infradead.org" , "will@kernel.org" , "linux-mm@kvack.org" Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Tue, Jul 8, 2025 at 11:55=E2=80=AFAM Sean Christopherson wrote: > > On Tue, Jul 08, 2025, Rick P Edgecombe wrote: > > On Tue, 2025-07-08 at 11:03 -0700, Sean Christopherson wrote: > > > > I think there is interest in de-coupling it? > > > > > > No? > > > > I'm talking about the intra-host migration/reboot optimization stuff. A= nd not > > doing a good job, sorry. > > > > > Even if we get to a point where multiple distinct VMs can bind to a= single > > > guest_memfd, e.g. for inter-VM shared memory, there will still need t= o be a > > > sole > > > owner of the memory. AFAICT, fully decoupling guest_memfd from a VM = would add > > > non-trivial complexity for zero practical benefit. > > > > I'm talking about moving a gmem fd between different VMs or something u= sing > > KVM_LINK_GUEST_MEMFD [0]. Not advocating to try to support it. But tryi= ng to > > feel out where the concepts are headed. It kind of allows gmem fds (or = just > > their source memory?) to live beyond a VM lifecycle. > > I think the answer is that we want to let guest_memfd live beyond the "st= ruct kvm" > instance, but not beyond the Virtual Machine. From a past discussion on = this topic[*]. > > : No go. Because again, the inode (physical memory) is coupled to the v= irtual machine > : as a thing, not to a "struct kvm". Or more concretely, the inode is c= oupled to an > : ASID or an HKID, and there can be multiple "struct kvm" objects associ= ated with a > : single ASID. And at some point in the future, I suspect we'll have mu= ltiple KVM > : objects per HKID too. > : > : The current SEV use case is for the migration helper, where two KVM ob= jects share > : a single ASID (the "real" VM and the helper). I suspect TDX will end = up with > : similar behavior where helper "VMs" can use the HKID of the "real" VM.= For KVM, > : that means multiple struct kvm objects being associated with a single = HKID. > : > : To prevent use-after-free, KVM "just" needs to ensure the helper insta= nces can't > : outlive the real instance, i.e. can't use the HKID/ASID after the owni= ng virtual > : machine has been destroyed. > : > : To put it differently, "struct kvm" is a KVM software construct that _= usually_, > : but not always, is associated 1:1 with a virtual machine. > : > : And FWIW, stashing the pointer without holding a reference would not b= e a complete > : solution, because it couldn't guard against KVM reusing a pointer. E.= g. if a > : struct kvm was unbound and then freed, KVM could reuse the same memory= for a new > : struct kvm, with a different ASID/HKID, and get a false negative on th= e rebinding > : check. > > Exactly what that will look like in code is TBD, but the concept/logic ho= lds up. I think we can simplify the role of guest_memfd in line with discussion [1]= : 1) guest_memfd is a memory provider for userspace, KVM, IOMMU. - It allows fallocate to populate/deallocate memory 2) guest_memfd supports the notion of private/shared faults. 3) guest_memfd supports memory access control: - It allows shared faults from userspace, KVM, IOMMU - It allows private faults from KVM, IOMMU 4) guest_memfd supports changing access control on its ranges between shared/private. - It notifies the users to invalidate their mappings for the ranges getting converted/truncated. Responsibilities that ideally should not be taken up by guest_memfd: 1) guest_memfd can not initiate pre-faulting on behalf of it's users. 2) guest_memfd should not be directly communicating with the underlying architecture layers. - All communication should go via KVM/IOMMU. 3) KVM should ideally associate the lifetime of backing pagetables/protection tables/RMP tables with the lifetime of the binding of memslots with guest_memfd. - Today KVM SNP logic ties RMP table entry lifetimes with how long the folios are mapped in guest_memfd, which I think should be revisited. Some very early thoughts on how guest_memfd could be laid out for the long = term: 1) guest_memfd code ideally should be built-in to the kernel. 2) guest_memfd instances should still be created using KVM IOCTLs that carry specific capabilities/restrictions for its users based on the backing VM/arch. 3) Any outgoing communication from guest_memfd to it's users like userspace/KVM/IOMMU should be via notifiers to invalidate similar to how MMU notifiers work. 4) KVM and IOMMU can implement intermediate layers to handle interaction with guest_memfd. - e.g. there could be a layer within kvm that handles: - creating guest_memfd files and associating a kvm_gmem_context with those files. - memslot binding - kvm_gmem_context will be used to bind kvm memslots with the context ranges. - invalidate notifier handling - kvm_gmem_context will be used to intercept guest_memfd callbacks and translate them to the right GPA ranges. - linking - kvm_gmem_context can be linked to different KVM instances. This line of thinking can allow cleaner separation between guest_memfd/KVM/IOMMU [2]. [1] https://lore.kernel.org/lkml/CAGtprH-+gPN8J_RaEit=3DM_ErHWTmFHeCipC6viT= 6PHhG3ELg6A@mail.gmail.com/#t [2] https://lore.kernel.org/lkml/31beeed3-b1be-439b-8a5b-db8c06dadc30@amd.c= om/ > > [*] https://lore.kernel.org/all/ZOO782YGRY0YMuPu@google.com > > > [0] https://lore.kernel.org/all/cover.1747368092.git.afranji@google.com= / > > https://lore.kernel.org/kvm/cover.1749672978.git.afranji@google.com/