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[2003:cb:c706:3100:19a8:d898:8e69:6aff]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-42158110ab4sm27224425e9.11.2024.06.05.09.02.06 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 05 Jun 2024 09:02:07 -0700 (PDT) Message-ID: Date: Wed, 5 Jun 2024 18:02:06 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 1/1] vhost-user: add shmem mmap request To: Stefan Hajnoczi , Stefan Hajnoczi Cc: Albert Esteve , qemu-devel@nongnu.org, mst@redhat.com, slp@redhat.com, jasowang@redhat.com References: <20240530152223.780232-1-aesteve@redhat.com> <20240530152223.780232-2-aesteve@redhat.com> <20240604185416.GB90471@fedora.redhat.com> <20240605142849.GC135899@fedora.redhat.com> Content-Language: en-US From: David Hildenbrand Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=david@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 05.06.24 17:19, Stefan Hajnoczi wrote: > On Wed, 5 Jun 2024 at 10:29, Stefan Hajnoczi wrote: >> >> On Wed, Jun 05, 2024 at 10:13:32AM +0200, Albert Esteve wrote: >>> On Tue, Jun 4, 2024 at 8:54 PM Stefan Hajnoczi wrote: >>> >>>> On Thu, May 30, 2024 at 05:22:23PM +0200, Albert Esteve wrote: >>>>> Add SHMEM_MAP/UNMAP requests to vhost-user. >>>>> >>>>> This request allows backends to dynamically map >>>>> fds into a shared memory region indentified by >>>> >>>> Please call this "VIRTIO Shared Memory Region" everywhere (code, >>>> vhost-user spec, commit description, etc) so it's clear that this is not >>>> about vhost-user shared memory tables/regions. >>>> >>>>> its `shmid`. Then, the fd memory is advertised >>>>> to the frontend through a BAR+offset, so it can >>>>> be read by the driver while its valid. >>>> >>>> Why is a PCI BAR mentioned here? vhost-user does not know about the >>>> VIRTIO Transport (e.g. PCI) being used. It's the frontend's job to >>>> report VIRTIO Shared Memory Regions to the driver. >>>> >>>> >>> I will remove PCI BAR, as it is true that it depends on the >>> transport. I was trying to explain that the driver >>> will use the shm_base + shm_offset to access >>> the mapped memory. >>> >>> >>>>> >>>>> Then, the backend can munmap the memory range >>>>> in a given shared memory region (again, identified >>>>> by its `shmid`), to free it. After this, the >>>>> region becomes private and shall not be accessed >>>>> by the frontend anymore. >>>> >>>> What does "private" mean? >>>> >>>> The frontend must mmap PROT_NONE to reserve the virtual memory space >>>> when no fd is mapped in the VIRTIO Shared Memory Region. Otherwise an >>>> unrelated mmap(NULL, ...) might use that address range and the guest >>>> would have access to the host memory! This is a security issue and needs >>>> to be mentioned explicitly in the spec. >>>> >>> >>> I mentioned private because it changes the mapping from MAP_SHARED >>> to MAP_PRIVATE. I will highlight PROT_NONE instead. >> >> I see. Then "MAP_PRIVATE" would be clearer. I wasn't sure whether you >> mean mmap flags or something like the memory range is no longer >> accessible to the driver. > > One more thing: please check whether kvm.ko memory regions need to be > modified or split to match the SHMEM_MAP mapping's read/write > permissions. > > The VIRTIO Shared Memory Area pages can have PROT_READ, PROT_WRITE, > PROT_READ|PROT_WRITE, or PROT_NONE. > > kvm.ko memory regions are read/write or read-only. I'm not sure what > happens when the guest accesses a kvm.ko memory region containing > mappings with permissions more restrictive than its kvm.ko memory > region. IIRC, the KVM R/O memory region requests could allow to further reduce permissions (assuming your mmap is R/W you could map it R/O into the KVM MMU), but I might remember things incorrectly. > In other words, the kvm.ko memory region would allow the > access but the Linux virtual memory configuration would cause a page > fault. > > For example, imagine a QEMU MemoryRegion containing a SHMEM_MAP > mapping with PROT_READ. The kvm.ko memory region would be read/write > (unless extra steps were taken to tell kvm.ko about the permissions). > When the guest stores to the PROT_READ page, kvm.ko will process a > fault...and I'm not sure what happens next. > > A similar scenario occurs when a PROT_NONE mapping exists within a > kvm.ko memory region. I don't remember how kvm.ko behaves when the > guest tries to access the pages. > > It's worth figuring this out before going further because it could > become tricky if issues are discovered later. I have CCed David > Hildenbrand in case he knows. One relevant piece is likely: "When the KVM_CAP_SYNC_MMU capability is available, changes in the backing of the memory region are automatically reflected into the guest. For example, an mmap() that affects the region will be made visible immediately. " We can already effectively get R/O or PROT_NONE PTEs in PROT_READ|PROT_WRITE mappings, and the KVM must be able to handle that properly -- trigegring a page fault to let core-MM resolve that. If we have a PROT_NONE VMA and the guest writes to it, we'd likely end up (to resolve the KVM MMU page fault) in hva_to_pfn_slow()->get_user_pages_unlocked(), which would return -EFAULT. Not sure if we really inject a page fault into the guest or if the KVM run would effectively fail with -EFAULT and make user space unhappy. Certainly something to play with! -- Cheers, David / dhildenb