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[99.254.114.190]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-6d353fef965sm76459056d6.69.2024.11.06.12.41.40 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 06 Nov 2024 12:41:41 -0800 (PST) Date: Wed, 6 Nov 2024 15:41:39 -0500 From: Peter Xu To: Steven Sistare Cc: David Hildenbrand , Fabiano Rosas , Marcel Apfelbaum , Eduardo Habkost , Philippe Mathieu-Daude , Paolo Bonzini , "Daniel P. Berrange" , Markus Armbruster , qemu-devel@nongnu.org Subject: Re: [PATCH V3 01/16] machine: anon-alloc option Message-ID: References: <1730468875-249970-1-git-send-email-steven.sistare@oracle.com> <1730468875-249970-2-git-send-email-steven.sistare@oracle.com> <78fa25f1-03dc-400c-a604-998c53e4fbce@redhat.com> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <66c05a06-dbb7-49ec-b58e-ccd917d098ea@oracle.com> Received-SPF: pass client-ip=170.10.133.124; envelope-from=peterx@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 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. Meanwhile I slightly prefer we don't touch anything if SHARED|PRIVATE is set. All combined, it could be: if (!(ram_flags & (RAM_PREALLOC | RAM_PRIVATE | RAM_SHARED))) { // ramblock to be allocated, with no share/private request, aka, // aux memory chunk... } > new_block->flags |= RAM_SHARED; > > if (!host && (new_block->flags & RAM_SHARED)) { > qemu_ram_alloc_shared(new_block); I'm not sure whether this needs its own helper. Should we fallback to ram_block_add() below, just like a RAM_SHARED? IIUC, we could start to create RAM_SHARED in qemu_anon_ram_alloc() and always cache the fd (even if we don't do that before)? > } else > new_block->fd = -1; > new_block->host = host; > } > ram_block_add(new_block); > > qemu_ram_alloc_shared() > if qemu_memfd_check() > new_block->fd = qemu_memfd_create() > else > new_block->fd = qemu_shm_alloc() > new_block->host = file_ram_alloc(new_block->fd) > ---- > > - Steve > -- Peter Xu