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[99.254.114.190]) by smtp.gmail.com with ESMTPSA id af79cd13be357-7b2f3a813a7sm673871985a.100.2024.11.06.13.21.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 06 Nov 2024 13:21:15 -0800 (PST) Date: Wed, 6 Nov 2024 16:21:14 -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: <1730468875-249970-2-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: Received-SPF: pass client-ip=170.10.129.124; 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_H2=-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 03:59:23PM -0500, 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. > > > 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. > > Reserve judgement until you see the full patch. The helper is a > non-trivial subroutine and IMO it improves readability. Also the > cpr find/save hooks are confined to the subroutine. I thought we can use the same code path to process "aux ramblock" and all kinds of other RAM_SHARED ramblocks. IIUC cpr find/save should apply there too, but maybe I missed something. > > > Should we fallback to > > ram_block_add() below, just like a RAM_SHARED? > > I thought we all discussed and agreed that the allocation should be performed > above ram_block_add. David's suggested patch does it here also. I was not closely followed all the discussions happened.. so I could have missed something indeed. One thing I want to double check is cpr will still make things like below work, right? -object memory-backend-ram,share=on [1] IIUC with the old code this won't create fd, so to make cpr work (and also what I was trying to say in the previous email..) is we could silently start to create memfds for these, which means we need to first teach qemu_anon_ram_alloc() on creating memfd for RAM_SHARED and cache these fds (which should hopefully keep the same behavior as before). Then for aux ramblocks like ROMs, as long as it sets RAM_SHARED properly in qemu_ram_alloc_internal() (but only when aux-share-mem=on, for sure..), then the rest code path in ram_block_add() should be the same as when user specified share=on in [1]. Anyway, if both of you agreed on it, I am happy to wait and read the whole patch. Side note: I'll still use a few days for other things, but I'll get back to read this whole series before next week.. btw, this series does not depend on precreate phase now, am I right? > > - Steve > > > 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