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AJvYcCUMFg0kuVAVXCOglFfTh+mHdDMAnu3qWrwDz66zxtBvc2jRY3+0IJkHtdhuPzihWVCNrPxgtROHXg==@kvack.org X-Gm-Message-State: AOJu0YzVdAq7jL1n/7atHkKKUKcL8MgTWB7zD4gZphSdN/16ESjAE6EF qjN6L5ZhRTpM9yqEzyhMHh2Nwv0UwZE5FFCgH0ByvQfZwEbdA41dApk+R1YtPY8t+ChB6W8R19B lWfcuGQ== X-Google-Smtp-Source: AGHT+IGkVA9V8/ZO7g4shvg2UQ6jCg/0h7Ce9RZescp986cCMnBbbliYZnYSjUAkEJw0pSkUoLPPHW3zZ24= X-Received: from pjtu6.prod.google.com ([2002:a17:90a:c886:b0:311:d79d:e432]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:5242:b0:311:df4b:4b82 with SMTP id 98e67ed59e1d1-31e5078f52bmr4726762a91.4.1753281831499; Wed, 23 Jul 2025 07:43:51 -0700 (PDT) Date: Wed, 23 Jul 2025 07:43:50 -0700 In-Reply-To: Mime-Version: 1.0 References: <505a30a3-4c55-434c-86a5-f86d2e9dc78a@intel.com> <1fe0f46a-152a-4b5b-99e2-2a74873dafdc@intel.com> Message-ID: Subject: Re: [PATCH v15 14/21] KVM: x86: Enable guest_memfd mmap for default VM type From: Sean Christopherson To: Vishal Annapurve Cc: Xiaoyao Li , Fuad Tabba , kvm@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mm@kvack.org, kvmarm@lists.linux.dev, pbonzini@redhat.com, chenhuacai@kernel.org, mpe@ellerman.id.au, anup@brainfault.org, paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, viro@zeniv.linux.org.uk, brauner@kernel.org, willy@infradead.org, akpm@linux-foundation.org, yilun.xu@intel.com, chao.p.peng@linux.intel.com, jarkko@kernel.org, amoorthy@google.com, dmatlack@google.com, isaku.yamahata@intel.com, mic@digikod.net, vbabka@suse.cz, ackerleytng@google.com, mail@maciej.szmigiero.name, david@redhat.com, michael.roth@amd.com, wei.w.wang@intel.com, liam.merwick@oracle.com, isaku.yamahata@gmail.com, kirill.shutemov@linux.intel.com, suzuki.poulose@arm.com, steven.price@arm.com, quic_eberman@quicinc.com, quic_mnalajal@quicinc.com, quic_tsoni@quicinc.com, quic_svaddagi@quicinc.com, quic_cvanscha@quicinc.com, quic_pderrin@quicinc.com, quic_pheragu@quicinc.com, catalin.marinas@arm.com, james.morse@arm.com, yuzenghui@huawei.com, oliver.upton@linux.dev, maz@kernel.org, will@kernel.org, qperret@google.com, keirf@google.com, roypat@amazon.co.uk, shuah@kernel.org, hch@infradead.org, jgg@nvidia.com, rientjes@google.com, jhubbard@nvidia.com, fvdl@google.com, hughd@google.com, jthoughton@google.com, peterx@redhat.com, pankaj.gupta@amd.com, ira.weiny@intel.com Content-Type: text/plain; charset="us-ascii" X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: C053740007 X-Stat-Signature: ys3s3qc3citspgyie15zquudctkko84o X-HE-Tag: 1753281832-420776 X-HE-Meta: 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 P2AcKSW+ 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Jul 23, 2025, Vishal Annapurve wrote: > 2) Userspace brings a mappable guest_memfd to back guest private > memory (passed as guest_memfd field in the > KVM_USERSPACE_MEMORY_REGION2): KVM will always fault in all guest > faults via guest_memfd so if userspace brings in VMAs that point to > different physical memory then there would be a discrepancy between > what guest and userspace/KVM (going through HVAs) sees for shared > memory ranges. I am not sure if KVM needs to enforce anything here, We agreed (I think in a guest_memfd call?) that KVM won't enforce anything, because trying to do so for uaccesses, e.g. via __kvm_read_guest_page(), would require grabbing mmap_lock in hot paths, i.e. would be a complete non-starter. So yeah, it's the VMM's responsibility to not be stupid.