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(p200300cbc7087900b88ec72aabbdd3d9.dip0.t-ipconnect.de. [2003:cb:c708:7900:b88e:c72a:abbd:d3d9]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-432aa6bee9asm69671895e9.19.2024.11.07.08.38.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 07 Nov 2024 08:38:27 -0800 (PST) Message-ID: Date: Thu, 7 Nov 2024 17:38:26 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH V3 01/16] machine: anon-alloc option To: Peter Xu , Steven Sistare Cc: Fabiano Rosas , Marcel Apfelbaum , Eduardo Habkost , Philippe Mathieu-Daude , Paolo Bonzini , "Daniel P. Berrange" , Markus Armbruster , qemu-devel@nongnu.org References: <45ea8a8a-928d-4703-b698-d5f910e6a224@oracle.com> <1f1a2742-0429-47d5-958f-b37575c1e4ba@redhat.com> <09701693-436c-4e1a-8206-03eb26cacab5@redhat.com> <66c05a06-dbb7-49ec-b58e-ccd917d098ea@oracle.com> <7fb32744-0512-46e2-b58d-2990f742b897@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: -23 X-Spam_score: -2.4 X-Spam_bar: -- X-Spam_report: (-2.4 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.34, 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_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_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 07.11.24 17:32, Peter Xu wrote: > On Thu, Nov 07, 2024 at 09:04:02AM -0500, Steven Sistare wrote: >> On 11/7/2024 8:05 AM, David Hildenbrand wrote: >>> On 06.11.24 21:59, Steven Sistare wrote: >>>> On 11/6/2024 3:41 PM, Peter Xu wrote: >>>>> On Wed, Nov 06, 2024 at 03:12:20PM -0500, Steven Sistare wrote: >>>>>> On 11/4/2024 4:36 PM, David Hildenbrand wrote: >>>>>>> On 04.11.24 21:56, Steven Sistare wrote: >>>>>>>> On 11/4/2024 3:15 PM, David Hildenbrand wrote: >>>>>>>>> On 04.11.24 20:51, David Hildenbrand wrote: >>>>>>>>>> On 04.11.24 18:38, Steven Sistare wrote: >>>>>>>>>>> On 11/4/2024 5:39 AM, David Hildenbrand wrote: >>>>>>>>>>>> On 01.11.24 14:47, Steve Sistare wrote: >>>>>>>>>>>>> Allocate anonymous memory using mmap MAP_ANON or memfd_create depending >>>>>>>>>>>>> on the value of the anon-alloc machine property.  This option applies to >>>>>>>>>>>>> memory allocated as a side effect of creating various devices. It does >>>>>>>>>>>>> not apply to memory-backend-objects, whether explicitly specified on >>>>>>>>>>>>> the command line, or implicitly created by the -m command line option. >>>>>>>>>>>>> >>>>>>>>>>>>> The memfd option is intended to support new migration modes, in which the >>>>>>>>>>>>> memory region can be transferred in place to a new QEMU process, by sending >>>>>>>>>>>>> the memfd file descriptor to the process.  Memory contents are preserved, >>>>>>>>>>>>> and if the mode also transfers device descriptors, then pages that are >>>>>>>>>>>>> locked in memory for DMA remain locked.  This behavior is a pre-requisite >>>>>>>>>>>>> for supporting vfio, vdpa, and iommufd devices with the new modes. >>>>>>>>>>>> >>>>>>>>>>>> A more portable, non-Linux specific variant of this will be using shm, >>>>>>>>>>>> similar to backends/hostmem-shm.c. >>>>>>>>>>>> >>>>>>>>>>>> Likely we should be using that instead of memfd, or try hiding the >>>>>>>>>>>> details. See below. >>>>>>>>>>> >>>>>>>>>>> For this series I would prefer to use memfd and hide the details.  It's a >>>>>>>>>>> concise (and well tested) solution albeit linux only.  The code you supply >>>>>>>>>>> for posix shm would be a good follow on patch to support other unices. >>>>>>>>>> >>>>>>>>>> Unless there is reason to use memfd we should start with the more >>>>>>>>>> generic POSIX variant that is available even on systems without memfd. >>>>>>>>>> Factoring stuff out as I drafted does look quite compelling. >>>>>>>>>> >>>>>>>>>> I can help with the rework, and send it out separately, so you can focus >>>>>>>>>> on the "machine toggle" as part of this series. >>>>>>>>>> >>>>>>>>>> Of course, if we find out we need the memfd internally instead under >>>>>>>>>> Linux for whatever reason later, we can use that instead. >>>>>>>>>> >>>>>>>>>> But IIUC, the main selling point for memfd are additional features >>>>>>>>>> (hugetlb, memory sealing) that you aren't even using. >>>>>>>>> >>>>>>>>> FWIW, I'm looking into some details, and one difference is that shmem_open() under Linux (glibc) seems to go to /dev/shmem and memfd/SYSV go to the internal tmpfs mount. There is not a big difference, but there can be some difference (e.g., sizing of the /dev/shm mount). >>>>>>>> >>>>>>>> Sizing is a non-trivial difference.  One can by default allocate all memory using memfd_create. >>>>>>>> To do so using shm_open requires configuration on the mount.  One step harder to use. >>>>>>> >>>>>>> Yes. >>>>>>> >>>>>>>> >>>>>>>> This is a real issue for memory-backend-ram, and becomes an issue for the internal RAM >>>>>>>> if memory-backend-ram has hogged all the memory. >>>>>>>> >>>>>>>>> Regarding memory-backend-ram,share=on, I assume we can use memfd if available, but then fallback to shm_open(). >>>>>>>> >>>>>>>> Yes, and if that is a good idea, then the same should be done for internal RAM >>>>>>>> -- memfd if available and fallback to shm_open. >>>>>>> >>>>>>> Yes. >>>>>>> >>>>>>>> >>>>>>>>> I'm hoping we can find a way where it just all is rather intuitive, like >>>>>>>>> >>>>>>>>> "default-ram-share=on": behave for internal RAM just like "memory-backend-ram,share=on" >>>>>>>>> >>>>>>>>> "memory-backend-ram,share=on": use whatever mechanism we have to give us "anonymous" memory that can be shared using an fd with another process. >>>>>>>>> >>>>>>>>> Thoughts? >>>>>>>> >>>>>>>> Agreed, though I thought I had already landed at the intuitive specification in my patch. >>>>>>>> The user must explicitly configure memory-backend-* to be usable with CPR, and anon-alloc >>>>>>>> controls everything else.  Now we're just riffing on the details: memfd vs shm_open, spelling >>>>>>>> of options and words to describe them. >>>>>>> >>>>>>> Well, yes, and making it all a bit more consistent and the "machine option" behave just like "memory-backend-ram,share=on". >>>>>> >>>>>> Hi David and Peter, >>>>>> >>>>>> I have implemented and tested the following, for both qemu_memfd_create >>>>>> and qemu_shm_alloc.  This is pseudo-code, with error conditions omitted >>>>>> for simplicity. >>>>> >>>>> I'm ok with either shm or memfd, as this feature only applies to Linux >>>>> anyway.  I'll leave that part to you and David to decide. >>>>> >>>>>> >>>>>> Any comments before I submit a complete patch? >>>>>> >>>>>> ---- >>>>>> qemu-options.hx: >>>>>>       ``aux-ram-share=on|off`` >>>>>>           Allocate auxiliary guest RAM as an anonymous file that is >>>>>>           shareable with an external process.  This option applies to >>>>>>           memory allocated as a side effect of creating various devices. >>>>>>           It does not apply to memory-backend-objects, whether explicitly >>>>>>           specified on the command line, or implicitly created by the -m >>>>>>           command line option. >>>>>> >>>>>>           Some migration modes require aux-ram-share=on. >>>>>> >>>>>> qapi/migration.json: >>>>>>       @cpr-transfer: >>>>>>            ... >>>>>>            Memory-backend objects must have the share=on attribute, but >>>>>>            memory-backend-epc is not supported.  The VM must be started >>>>>>            with the '-machine aux-ram-share=on' option. >>>>>> >>>>>> Define RAM_PRIVATE >>>>>> >>>>>> Define qemu_shm_alloc(), from David's tmp patch >>>>>> >>>>>> ram_backend_memory_alloc() >>>>>>       ram_flags = backend->share ? RAM_SHARED : RAM_PRIVATE; >>>>>>       memory_region_init_ram_flags_nomigrate(ram_flags) >>>>> >>>>> Looks all good until here. >>>>> >>>>>> >>>>>> qemu_ram_alloc_internal() >>>>>>       ... >>>>>>       if (!host && !(ram_flags & RAM_PRIVATE) && current_machine->aux_ram_share) >>>>> >>>>> Nitpick: could rely on flags-only, rather than testing "!host", AFAICT >>>>> that's equal to RAM_PREALLOC. >>>> >>>> IMO testing host is clearer and more future proof, regardless of how flags >>>> are currently used.  If the caller passes host, then we should not allocate >>>> memory here, full stop. >>>> >>>>> Meanwhile I slightly prefer we don't touch >>>>> anything if SHARED|PRIVATE is set. >>>> >>>> OK, if SHARED is already set I will not set it again. >>> >>> We only have to make sure that stuff like qemu_ram_is_shared() will continue working as expected. >>> >>> What I think we should do: >>> >>> We should probably assert that nobody passes in SHARED|PRIVATE. And we can use PRIVATE only as a parameter to the function, but never actually set it on the ramblock. >>> >>> If someone passes in PRIVATE, we don't include it in block->flags. (RMA_SHARED remains cleared) >>> >>> If someone passes in SHARED, we do set it in block->flags. >>> If someone passes PRIVATE|SHARED, we assert. >>> >>> If someone passes in nothing: we set block->flags to SHARED with aux_ram_share=on. Otherwise, we do nothing (RAM_SHARED remains cleared) >>> >>> If that's also what you had in mind, great. >> >> Yes, my patch does that, but it also sets RAM_PRIVATE on the ramblock. >> I will undo the latter. > > David: why do we need to drop PRIVATE in ramblock flags? I thought it was > pretty harmless. I suppose things like qemu_ram_is_shared() will even keep > working as before? > > It looks ok to remove it too, but it adds logics that doesn't seem > necessary to me, so just to double check if I missed something.. A finished ramblock is only boolean "shared" vs. "not shared/private". A single flag (RAM_SHARED) can express that clearly. Consequently there is less to get wrong when using RAM_PRIVATE only as a flag to the creation function (and documenting that!). To make RAM_PRIVATE consistent we might have to tweak all other RAMBlock creation functions to set RAM_PRIVATE in the !RAM_SHARED case, and I don't think that is wroth the trouble. -- Cheers, David / dhildenb