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[2003:cb:c735:1900:ac8b:7ae5:991f:54fc]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38f258ddbbdsm31212962f8f.37.2025.02.24.01.16.44 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 24 Feb 2025 01:16:44 -0800 (PST) Message-ID: <40859ece-0850-40cb-b8b9-28d0d76aefde@redhat.com> Date: Mon, 24 Feb 2025 10:16:43 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 0/9] vhost-user: Add SHMEM_MAP/UNMAP requests To: Albert Esteve Cc: qemu-devel@nongnu.org, slp@redhat.com, stevensd@chromium.org, =?UTF-8?Q?Alex_Benn=C3=A9e?= , Stefano Garzarella , stefanha@redhat.com, hi@alyssa.is, mst@redhat.com, jasowang@redhat.com References: <20250217164012.246727-1-aesteve@redhat.com> From: David Hildenbrand Content-Language: en-US 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: -25 X-Spam_score: -2.6 X-Spam_bar: -- X-Spam_report: (-2.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.446, 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_H2=-0.01, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 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 24.02.25 09:54, Albert Esteve wrote: > On Mon, Feb 17, 2025 at 9:01 PM David Hildenbrand wrote: >> >> On 17.02.25 17:40, Albert Esteve wrote: >>> Hi all, >>> >> >> Hi, >> >> looks like our debugging session was successfu :) >> >> One question below. >> >>> v3->v4 >>> - Change mmap strategy to use RAM blocks >>> and subregions. >>> - Add new bitfield to qmp feature map >>> - Followed most review comments from >>> last iteration. >>> - Merged documentation patch again with >>> this one. Makes more sense to >>> review them together after all. >>> - Add documentation for MEM_READ/WRITE >>> messages. >>> >>> The goal of this patch is to support >>> dynamic fd-backed memory maps initiated >>> from vhost-user backends. >>> There are many devices that could already >>> benefit of this feature, e.g., >>> virtiofs or virtio-gpu. >>> >>> After receiving the SHMEM_MAP/UNMAP request, >>> the frontend creates the RAMBlock form the >>> fd and maps it by adding it as a subregion >>> of the shared memory region container. >>> >>> The VIRTIO Shared Memory Region list is >>> declared in the `VirtIODevice` struct >>> to make it generic. >>> >>> TODO: There was a conversation on the >>> previous version around adding tests >>> to the patch (which I have acknowledged). >>> However, given the numerous changes >>> that the patch already has, I have >>> decided to send it early and collect >>> some feedback while I work on the >>> tests for the next iteration. >>> Given that I have been able to >>> test the implementation with >>> my local setup, I am more or less >>> confident that, at least, the code >>> is in a relatively sane state >>> so that no reviewing time is >>> wasted on broken patches. >>> >>> This patch also includes: >>> - SHMEM_CONFIG frontend request that is >>> specifically meant to allow generic >>> vhost-user-device frontend to be able to >>> query VIRTIO Shared Memory settings from the >>> backend (as this device is generic and agnostic >>> of the actual backend configuration). >>> >>> - MEM_READ/WRITE backend requests are >>> added to deal with a potential issue when having >>> multiple backends sharing a file descriptor. >>> When a backend calls SHMEM_MAP it makes >>> accessing to the region fail for other >>> backend as it is missing from their translation >>> table. So these requests are a fallback >>> for vhost-user memory translation fails. >> >> Can you elaborate what the issue here is? >> >> Why would SHMEM_MAP make accessing the region fail for other backends -- >> what makes this missing from their translation? > > This issue was raised by Stefan Hajnoczi in one of the first > iterations of this patchset, based upon previous David Gilbert's work > on the virtiofs DAX Window. > > Let me paste here some of his remarks: > > """ > Other backends don't see these mappings. If the guest submits a vring > descriptor referencing a mapping to another backend, then that backend > won't be able to access this memory. > """ > [...] > """ > A bit more detail: > > Device A has a VIRTIO Shared Memory Region. An application mmaps that > memory (examples: guest userspace driver using Linux VFIO, a guest > kernel driver that exposes the memory to userspace via mmap, or guest > kernel DAX). The application passes that memory as an I/O buffer to > device B (e.g. O_DIRECT disk I/O). > > The result is that device B's vhost-user backend receives a vring > descriptor that points to a guest memory address in device A's VIRTIO > Shared Memory Region. Since device B does not have this memory in its > table, it cannot translate the address and the device breaks. > """ > > I have not triggered the issue myself. So the idea is that the next > patch will *definitively* include some testing for the commits that I > cannot verify with my local setup. Hah! But isn't that exact problem which is now solved by our rework? Whatever is mapped in the VIRTIO Shared Memory Region will be communicated to all other vhost-user devices. So they should have that memory in their map and should be able to access it. The only thing vhost-user devices cannot access are IIRC ram_device_ptr memory regions (e.g., from vfio devices). But that is independent shared memory regions. -- Cheers, David / dhildenb