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AFNElJ8C/N+OD3U33cPcwdWT+XESOa4nd0Ms/5OxfaTEDkxzEhZ5BHB6lYbjm71NlPXB8K2BqBB5SdFt+g==@kvack.org X-Gm-Message-State: AOJu0YzEyUYn4NKJMjW9wD+0oOqOmjfSTxM6cNl1eZb7B3mVLSQsiiG/ wA7KluopiepAd/OwM5qakylKxf6LPmDmVMC3lCTTxnwu8/u8Xr4vkbPlCpmCbMz8Ro42Wpv3+qq ETlrc6w== X-Received: from pfbhk7.prod.google.com ([2002:a05:6a00:8787:b0:82f:f2d:5b4d]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a00:2d8d:b0:82c:1cd0:2f5e with SMTP id d2e1a72fcca58-82f8c96eb30mr20038023b3a.44.1776789068848; Tue, 21 Apr 2026 09:31:08 -0700 (PDT) Date: Tue, 21 Apr 2026 09:31:07 -0700 In-Reply-To: <20260410151746.61150-11-kalyazin@amazon.com> Mime-Version: 1.0 References: <20260410151746.61150-1-kalyazin@amazon.com> <20260410151746.61150-11-kalyazin@amazon.com> Message-ID: Subject: Re: [PATCH v12 10/16] KVM: guest_memfd: Add flag to remove from direct map From: Sean Christopherson To: Nikita Kalyazin Cc: "kvm@vger.kernel.org" , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , "kvmarm@lists.linux.dev" , "linux-fsdevel@vger.kernel.org" , "linux-mm@kvack.org" , "bpf@vger.kernel.org" , "linux-kselftest@vger.kernel.org" , "kernel@xen0n.name" , "linux-riscv@lists.infradead.org" , "linux-s390@vger.kernel.org" , "loongarch@lists.linux.dev" , "linux-pm@vger.kernel.org" , "pbonzini@redhat.com" , "corbet@lwn.net" , "maz@kernel.org" , "oupton@kernel.org" , "joey.gouly@arm.com" , "suzuki.poulose@arm.com" , "yuzenghui@huawei.com" , "catalin.marinas@arm.com" , "will@kernel.org" , "tglx@kernel.org" , "mingo@redhat.com" , "bp@alien8.de" , "dave.hansen@linux.intel.com" , "x86@kernel.org" , "hpa@zytor.com" , "luto@kernel.org" , "peterz@infradead.org" , "willy@infradead.org" , "akpm@linux-foundation.org" , "david@kernel.org" , "lorenzo.stoakes@oracle.com" , "vbabka@kernel.org" , "rppt@kernel.org" , "surenb@google.com" , "mhocko@suse.com" , "ast@kernel.org" , "daniel@iogearbox.net" , "andrii@kernel.org" , "martin.lau@linux.dev" , "eddyz87@gmail.com" , "song@kernel.org" , "yonghong.song@linux.dev" , "john.fastabend@gmail.com" , "kpsingh@kernel.org" , "sdf@fomichev.me" , "haoluo@google.com" , "jolsa@kernel.org" , "jgg@ziepe.ca" , "jhubbard@nvidia.com" , "peterx@redhat.com" , "jannh@google.com" , "pfalcato@suse.de" , "skhan@linuxfoundation.org" , "riel@surriel.com" , "ryan.roberts@arm.com" , "jgross@suse.com" , "yu-cheng.yu@intel.com" , "kas@kernel.org" , "coxu@redhat.com" , "ackerleytng@google.com" , "yosry@kernel.org" , "ajones@ventanamicro.com" , "maobibo@loongson.cn" , "tabba@google.com" , "prsampat@amd.com" , "wu.fei9@sanechips.com.cn" , "mlevitsk@redhat.com" , "jmattson@google.com" , "jthoughton@google.com" , "agordeev@linux.ibm.com" , "alex@ghiti.fr" , "aou@eecs.berkeley.edu" , "borntraeger@linux.ibm.com" , "chenhuacai@kernel.org" , "baolu.lu@linux.intel.com" , "dev.jain@arm.com" , "gor@linux.ibm.com" , "hca@linux.ibm.com" , "palmer@dabbelt.com" , "pjw@kernel.org" , "shijie@os.amperecomputing.com" , "svens@linux.ibm.com" , "thuth@redhat.com" , "yang@os.amperecomputing.com" , "Liam.Howlett@oracle.com" , "urezki@gmail.com" , "zhengqi.arch@bytedance.com" , "gerald.schaefer@linux.ibm.com" , "jiayuan.chen@shopee.com" , "lenb@kernel.org" , "pavel@kernel.org" , "rafael@kernel.org" , "yangyicong@hisilicon.com" , "vannapurve@google.com" , "jackmanb@google.com" , "patrick.roy@linux.dev" , Jack Thomson , Takahiro Itazuri , Derek Manwaring , Nikita Kalyazin Content-Type: text/plain; charset="us-ascii" X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 8A82C2000E X-Stat-Signature: sgqj4jdow9g6bkhhyia339fuoki3fg5a X-Rspam-User: X-HE-Tag: 1776789070-94735 X-HE-Meta: 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 MJCc8I6V 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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 Fri, Apr 10, 2026, Nikita Kalyazin wrote: > From: Patrick Roy > > Add GUEST_MEMFD_FLAG_NO_DIRECT_MAP flag for KVM_CREATE_GUEST_MEMFD() > ioctl. When set, guest_memfd folios will be removed from the direct map > after preparation, with direct map entries only restored when the folios > are freed. > > To ensure these folios do not end up in places where the kernel cannot > deal with them, set AS_NO_DIRECT_MAP on the guest_memfd's struct > address_space if GUEST_MEMFD_FLAG_NO_DIRECT_MAP is requested. > > Note that this flag causes removal of direct map entries for all > guest_memfd folios independent of whether they are "shared" or "private" > (although current guest_memfd only supports either all folios in the > "shared" state, or all folios in the "private" state if > GUEST_MEMFD_FLAG_MMAP is not set). The usecase for removing direct map > entries of also the shared parts of guest_memfd are a special type of > non-CoCo VM where, host userspace is trusted to have access to all of > guest memory, but where Spectre-style transient execution attacks > through the host kernel's direct map should still be mitigated. In this > setup, KVM retains access to guest memory via userspace mappings of > guest_memfd, which are reflected back into KVM's memslots via > userspace_addr. This is needed for things like MMIO emulation on x86_64 > to work. > > Direct map entries are zapped right before guest or userspace mappings > of gmem folios are set up, e.g. in kvm_gmem_fault_user_mapping() or > kvm_gmem_get_pfn() [called from the KVM MMU code]. ... > +#define KVM_GMEM_FOLIO_NO_DIRECT_MAP BIT(0) > + > +static bool kvm_gmem_folio_no_direct_map(struct folio *folio) > +{ > + return ((u64)folio->private) & KVM_GMEM_FOLIO_NO_DIRECT_MAP; > +} > + > +static int kvm_gmem_folio_zap_direct_map(struct folio *folio) > +{ > + int r = 0; > + > + VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio); > + > + if (WARN_ON_ONCE(!(GMEM_I(folio_inode(folio))->flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP))) > + return -EINVAL; > + > + if (kvm_gmem_folio_no_direct_map(folio)) > + goto out; > + > + r = folio_zap_direct_map(folio); > + if (!r) > + folio->private = (void *)((u64)folio->private | KVM_GMEM_FOLIO_NO_DIRECT_MAP); > + > +out: > + return r; > +} > + > +static void kvm_gmem_folio_restore_direct_map(struct folio *folio) > +{ > + folio_restore_direct_map(folio); > + folio->private = (void *)((u64)folio->private & ~KVM_GMEM_FOLIO_NO_DIRECT_MAP); > +} Making guest_memfd responsible for zapping and restoring the direct map on a per- folio basis feels wrong given the addition of AS_NO_DIRECT_MAP. I especially don't like that the "rules" for when an AS_NO_DIRECT_MAP folio has a direct map will vary based on the owner, and even within an owner (e.g. guest_memfd) will be ad hoc. E.g. as per the series to add guest_memfd write() support[*]: When direct map removal is implemented [2] - write() will not be allowed to access pages that have already been removed from direct map - on completion, write() will remove the populated pages from direct map That's pretty gross ABI, because with KVM_GMEM_FOLIO_NO_DIRECT_MAP, userspace can write() exactly once. To re-write memory, I assume userspace would need to do a PUNCH_HOLE or truncate. What's preventing us from handling this automagically in e.g. filemap_add_folio() and filemap_remove_folio()? Then the usage rules are pretty straightforward: the kernel must *always* assume the direct map is invalid for folios from AS_NO_DIRECT_MAP mappings. Then if KVM needs to utilize a kernel mapping, e.g. in kvm_gmem_populate(), KVM could use dedicated variants of kmap_local_xxx() to deal with a local mapping for a folio/page without a direct map. Or, KVM could simply disallow the specific sequence that would require KVM to do the memcpy (I'm pretty sure we can do that with in-place shared=>private conversion support). I realize that could throw a big wrench into write() performance, but IMO, before merging either series, we need a complete story for exactly how this will all fit together, in a maintainable fashion and with sane ABI. [*] https://lore.kernel.org/all/20251114151828.98165-1-kalyazin@amazon.com