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[2003:cb:c707:1600:a3ce:b459:ef57:7b93]) by smtp.gmail.com with ESMTPSA id l3-20020a05600c4f0300b003a4bb3f9bc6sm1837127wmq.41.2022.08.10.02.55.20 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 10 Aug 2022 02:55:21 -0700 (PDT) Message-ID: <64ab9678-c72d-b6d9-8532-346cc9c06814@redhat.com> Date: Wed, 10 Aug 2022 11:55:19 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 Subject: Re: [PATCH v7 05/14] mm/memfd: Introduce MFD_INACCESSIBLE flag Content-Language: en-US To: Chao Peng Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, linux-doc@vger.kernel.org, qemu-devel@nongnu.org, linux-kselftest@vger.kernel.org, Paolo Bonzini , Jonathan Corbet , Sean Christopherson , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86@kernel.org, "H . Peter Anvin" , Hugh Dickins , Jeff Layton , "J . Bruce Fields" , Andrew Morton , Shuah Khan , Mike Rapoport , Steven Price , "Maciej S . Szmigiero" , Vlastimil Babka , Vishal Annapurve , Yu Zhang , "Kirill A . Shutemov" , luto@kernel.org, jun.nakajima@intel.com, dave.hansen@intel.com, ak@linux.intel.com, aarcange@redhat.com, ddutile@redhat.com, dhildenb@redhat.com, Quentin Perret , Michael Roth , mhocko@suse.com, Muchun Song References: <20220706082016.2603916-1-chao.p.peng@linux.intel.com> <20220706082016.2603916-6-chao.p.peng@linux.intel.com> <203c752f-9439-b5ae-056c-27b2631dcb81@redhat.com> <20220810093741.GE862421@chaop.bj.intel.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <20220810093741.GE862421@chaop.bj.intel.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10.08.22 11:37, Chao Peng wrote: > On Fri, Aug 05, 2022 at 03:28:50PM +0200, David Hildenbrand wrote: >> On 06.07.22 10:20, Chao Peng wrote: >>> Introduce a new memfd_create() flag indicating the content of the >>> created memfd is inaccessible from userspace through ordinary MMU >>> access (e.g., read/write/mmap). However, the file content can be >>> accessed via a different mechanism (e.g. KVM MMU) indirectly. >>> >>> It provides semantics required for KVM guest private memory support >>> that a file descriptor with this flag set is going to be used as the >>> source of guest memory in confidential computing environments such >>> as Intel TDX/AMD SEV but may not be accessible from host userspace. >>> >>> The flag can not coexist with MFD_ALLOW_SEALING, future sealing is >>> also impossible for a memfd created with this flag. >> >> It's kind of weird to have it that way. Why should the user have to >> care? It's the notifier requirement to have that, no? >> >> Why can't we handle that when register a notifier? If anything is >> already mapped, fail registering the notifier if the notifier has these >> demands. If registering succeeds, block it internally. >> >> Or what am I missing? We might not need the memfile set flag semantics >> eventually and would not have to expose such a flag to user space. > > This makes sense if doable. The major concern was: is there a reliable > way to detect this (already mapped) at the time of memslot registering. If too complicated, we could simplify to "was this ever mapped" and fail for now. Hooking into shmem_mmap() might be sufficient for that to get notified about the first mmap. As an alternative, mapping_mapped() or similar *might* do what we want. -- Thanks, David / dhildenb