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[2003:cb:c70d:be00:66fa:83a6:8803:977e]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-434aa7a49dasm20458365e9.9.2024.11.27.04.40.44 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 27 Nov 2024 04:40:44 -0800 (PST) Message-ID: Date: Wed, 27 Nov 2024 13:40:43 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 1/5] vhost-user: Add VIRTIO Shared Memory map request To: Albert Esteve Cc: qemu-devel@nongnu.org, =?UTF-8?Q?Alex_Benn=C3=A9e?= , slp@redhat.com, hi@alyssa.is, mst@redhat.com, jasowang@redhat.com, stefanha@redhat.com, Stefano Garzarella , stevensd@chromium.org References: <20240912145335.129447-1-aesteve@redhat.com> <20240912145335.129447-2-aesteve@redhat.com> <44de54b5-8336-4d84-96f0-36f809f48eff@redhat.com> <6714e754-8d2d-4751-8b3b-dd7ce469a469@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: -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_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, 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 27.11.24 13:31, Albert Esteve wrote: > On Wed, Nov 27, 2024 at 1:18 PM David Hildenbrand wrote: >> >> On 27.11.24 13:10, David Hildenbrand wrote: >>> On 27.11.24 11:50, David Hildenbrand wrote: >>>> >>>>>> RAM memory region/ RAMBlock that has properly set flags/fd/whatssoever >>>>>> and map whatever you want in there. >>>>>> >>>>>> Likely you would need a distinct RAMBlock/RAM memory region per mmap(), >>>>>> and would end up mmaping implicitly via qemu_ram_mmap(). >>>>>> >>>>>> Then, your shared region would simply be an empty container into which >>>>>> you map these RAM memory regions. >>>>> >>>> >>>> Hi, >>>> >>>>> Hi, sorry it took me so long to get back to this. Lately I have been >>>>> testing the patch and fixing bugs, and I am was going to add some >>>>> tests to be able to verify the patch without having to use a backend >>>>> (which is what I am doing right now). >>>>> >>>>> But I wanted to address/discuss this comment. I am not sure of the >>>>> actual problem with the current approach (I am not completely aware of >>>>> the concern in your first paragraph), but I see other instances where >>>>> qemu mmaps stuff into a MemoryRegion. >>>> >>>> I suggest you take a look at the three relevant MAP_FIXED users outside >>>> of user emulation code. >>>> >>>> (1) hw/vfio/helpers.c: We create a custom memory region + RAMBlock with >>>> memory_region_init_ram_device_ptr(). This doesn't mmap(MAP_FIXED) >>>> into any existing RAMBlock. >>>> >>>> (2) system/physmem.c: I suggest you take a close look at >>>> qemu_ram_remap() and how it is one example of how RAMBlock >>>> properties describe exactly what is mmaped. >>>> >>>> (3) util/mmap-alloc.c: Well, this is the code that performs the mmap(), >>>> to bring a RAMBlock to life. See qemu_ram_mmap(). >>>> >>>> There is some oddity hw/xen/xen-mapcache.c; XEN mapcache seems to manage >>>> guest RAM without RAMBlocks. >>>> >>>>> Take into account that the >>>>> implementation follows the definition of shared memory region here: >>>>> https://docs.oasis-open.org/virtio/virtio/v1.3/csd01/virtio-v1.3-csd01.html#x1-10200010 >>>>> Which hints to a memory region per ID, not one per required map. So >>>>> the current strategy seems to fit it better. >>>> >>>> I'm confused, we are talking about an implementation detail here? How is >>>> that related to the spec? >>>> >>>>> >>>>> Also, I was aware that I was not the first one attempting this, so I >>>>> based this code in previous attempts (maybe I should give credit in >>>>> the commit now that I think of): >>>>> https://gitlab.com/virtio-fs/qemu/-/blob/qemu5.0-virtiofs-dax/hw/virtio/vhost-user-fs.c?ref_type=heads#L75 >>>>> As you can see, it pretty much follows the same strategy. >>>> >>>> So, people did some hacky things in a QEMU fork 6 years ago ... :) That >>>> cannot possibly be a good argument why we should have it like that in QEMU. >>>> >>>>> And in my >>>>> examples I have been able to use this to video stream with multiple >>>>> queues mapped into the shared memory (used to capture video frames), >>>>> using the backend I mentioned above for testing. So the concept works. >>>>> I may be wrong with this, but for what I understood looking at the >>>>> code, crosvm uses a similar strategy. Reserve a memory block and use >>>>> for all your mappings, and use an allocator to find a free slot. >>>>> >>>> >>>> Again, I suggest you take a look at what a RAMBlock is, and how it's >>>> properties describe the containing mmap(). >>>> >>>>> And if I were to do what you say, those distinct RAMBlocks should be >>>>> created when the device starts? What would be their size? Should I >>>>> create them when qemu receives a request to mmap? How would the driver >>>>> find the RAMBlock? >>>> >>>> You'd have an empty memory region container into which you will map >>>> memory regions that describe the memory you want to share. >>>> >>>> mr = g_new0(MemoryRegion, 1); >>>> memory_region_init(mr, OBJECT(TODO), "vhost-user-shm", region_size); >>>> >>>> >>>> Assuming you are requested to mmap an fd, you'd create a new >>>> MemoryRegion+RAMBlock that describes the memory and performs the mmap() >>>> for you: >>>> >>>> map_mr = g_new0(MemoryRegion, 1); >>>> memory_region_init_ram_from_fd(map_mr, OBJECT(TODO), "TODO", map_size, >>>> RAM_SHARED, map_fd, map_offs, errp); >>>> >>>> To then map it into your container: >>>> >>>> memory_region_add_subregion(mr, offset_within_container, map_mr); >>>> >>>> >>>> To unmap, you'd first remove the subregion, to then unref the map_mr. >>>> >>>> >>>> >>>> The only alternative would be to do it like (1) above: you perform all >>>> of the mmap() yourself, and create it using >>>> memory_region_init_ram_device_ptr(). This will set RAM_PREALLOC on the >>>> RAMBlock and tell QEMU "this is special, just disregard it". The bad >>>> thing about RAM_PREALLOC will be that it will not be compatible with >>>> vfio, not communicated to other vhost-user devices etc ... whereby what >>>> I describe above would just work with them. >>>> >>> >>> FWIW, I took another look at this patch and I cannot really make sense >>> of what you are doing. >>> >>> In virtio_new_shmem_region(), you allocate a region, but I don't see how >>> it would ever get initialized? >>> >>> In vhost_user_backend_handle_shmem_map(), you then assume that it is >>> suddenly a RAM memory region? For example, that >>> memory_region_get_ram_ptr() returns something reasonable. >>> >>> Likely, you really just want to initialize that MR using >>> memory_region_init_ram_from_fd(), and have that just perform the proper >>> mmap() for you and setup the RAMBlock? >> >> In patch #4 I find: memory_region_init_ram_ptr(vdev->shmem_list[i].mr ... >> >> Which does what I describe as the alternative. That makes it clearer >> that you are not operating on arbitrary RAMBlocks. So that should indeed >> work. >> >> The way you structured these patches really is suboptimal for review. > > Firstly, thank you so much for taking another look at the patch after > so much time. I understand it may be challenging given the time it > passed since I sent this version. Also, I rushed this version trying > to get to settle the spec first while I was working on something else, > so the code was untested and has quite a few bugs. I am mentioning it > just to note that I think this patch should not get a fine grain > review in its current status (although Stefan kindly did review it). I > fixed all (at least most) of the bugs in what would be the next > version, and it is what I use for testing. When I say I proved this > code and works, I am mostly referring to my local changes. As it is > here, this won't work. > > That said, I understand what you mean, I will try to refactor / > reorder the patch to make it easier to review. If we could make it clearer that we are operating on RAM_PREALLOC MRs/RAMBlocks in when handling shmem_map/shmem_unmap requests somehow, that would already take quite some magic out of this code. :) Having that said, I now understand why you want RAM_PREALLOC, because you might only activate/deactivate some parts within a RAMBlock. -- Cheers, David / dhildenb