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[2003:cb:c735:1900:ac8b:7ae5:991f:54fc]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38f258b43c9sm31401784f8f.10.2025.02.24.01.49.28 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 24 Feb 2025 01:49:28 -0800 (PST) Message-ID: Date: Mon, 24 Feb 2025 10:49:27 +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> <40859ece-0850-40cb-b8b9-28d0d76aefde@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 10:35, Albert Esteve wrote: > On Mon, Feb 24, 2025 at 10:16 AM David Hildenbrand wrote: >> >> 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. > > You mean the SET_MEM_TABLE message after the vhost_commit is sent to > all vhost-user devices? I was not sure, as I was testing with a single > device, that would be great, and simplify the patch a lot. Yes, all vhost-user devices should be updated. -- Cheers, David / dhildenb