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[2003:cb:c71b:7e00:9339:4017:7111:82d0]) by smtp.gmail.com with ESMTPSA id p32-20020a05600c1da000b0041478834562sm118842wms.0.2024.03.22.10.16.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 22 Mar 2024 10:16:17 -0700 (PDT) Message-ID: Date: Fri, 22 Mar 2024 18:16:14 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: folio_mmapped To: Sean Christopherson Cc: Vishal Annapurve , Quentin Perret , Matthew Wilcox , Fuad Tabba , kvm@vger.kernel.org, kvmarm@lists.linux.dev, pbonzini@redhat.com, chenhuacai@kernel.org, mpe@ellerman.id.au, anup@brainfault.org, paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, viro@zeniv.linux.org.uk, brauner@kernel.org, akpm@linux-foundation.org, xiaoyao.li@intel.com, yilun.xu@intel.com, chao.p.peng@linux.intel.com, jarkko@kernel.org, amoorthy@google.com, dmatlack@google.com, yu.c.zhang@linux.intel.com, isaku.yamahata@intel.com, mic@digikod.net, vbabka@suse.cz, ackerleytng@google.com, mail@maciej.szmigiero.name, michael.roth@amd.com, wei.w.wang@intel.com, liam.merwick@oracle.com, isaku.yamahata@gmail.com, kirill.shutemov@linux.intel.com, suzuki.poulose@arm.com, steven.price@arm.com, quic_mnalajal@quicinc.com, quic_tsoni@quicinc.com, quic_svaddagi@quicinc.com, quic_cvanscha@quicinc.com, quic_pderrin@quicinc.com, quic_pheragu@quicinc.com, catalin.marinas@arm.com, james.morse@arm.com, yuzenghui@huawei.com, oliver.upton@linux.dev, maz@kernel.org, will@kernel.org, keirf@google.com, linux-mm@kvack.org References: <7470390a-5a97-475d-aaad-0f6dfb3d26ea@redhat.com> <40f82a61-39b0-4dda-ac32-a7b5da2a31e8@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: 9k9hjkfkhdii3ggen58zxhxd3798qjs7 X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 75F6E10001E X-Rspam-User: X-HE-Tag: 1711127783-750739 X-HE-Meta: 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 OEOTe3Mg 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 19.03.24 16:04, Sean Christopherson wrote: > On Tue, Mar 19, 2024, David Hildenbrand wrote: >> On 19.03.24 01:10, Sean Christopherson wrote: >>> Performance is a secondary concern. If this were _just_ about guest performance, >>> I would unequivocally side with David: the guest gets to keep the pieces if it >>> fragments a 1GiB page. >>> >>> The main problem we're trying to solve is that we want to provision a host such >>> that the host can serve 1GiB pages for non-CoCo VMs, and can also simultaneously >>> run CoCo VMs, with 100% fungibility. I.e. a host could run 100% non-CoCo VMs, >>> 100% CoCo VMs, or more likely, some sliding mix of the two. Ideally, CoCo VMs >>> would also get the benefits of 1GiB mappings, that's not the driving motiviation >>> for this discussion. >> >> Supporting 1 GiB mappings there sounds like unnecessary complexity and >> opening a big can of worms, especially if "it's not the driving motivation". >> >> If I understand you correctly, the scenario is >> >> (1) We have free 1 GiB hugetlb pages lying around >> (2) We want to start a CoCo VM >> (3) We don't care about 1 GiB mappings for that CoCo VM, > > We care about 1GiB mappings for CoCo VMs. My comment about performance being a > secondary concern was specifically saying that it's the guest's responsilibity > to play nice with huge mappings if the guest cares about its performance. For > guests that are well behaved, we most definitely want to provide a configuration > that performs as close to non-CoCo VMs as we can reasonably make it. How does the guest know the granularity? I suspect it's just implicit knowledge that "PUD granularity might be nice". > > And we can do that today, but it requires some amount of host memory to NOT be > in the HugeTLB pool, and instead be kept in reserved so that it can be used for > shared memory for CoCo VMs. That approach has many downsides, as the extra memory > overhead affects CoCo VM shapes, our ability to use a common pool for non-CoCo > and CoCo VMs, and so on and so forth. Right. But avoiding memory waste as soon as hugetlb is involved (and we have two separate memfds for private/shared memory) is not feasible. > >> but hguetlb pages is all we have. >> (4) We want to be able to use the 1 GiB hugetlb page in the future. > > ... > >>> The other big advantage that we should lean into is that we can make assumptions >>> about guest_memfd usage that would never fly for a general purpose backing stores, >>> e.g. creating a dedicated memory pool for guest_memfd is acceptable, if not >>> desirable, for (almost?) all of the CoCo use cases. >>> >>> I don't have any concrete ideas at this time, but my gut feeling is that this >>> won't be _that_ crazy hard to solve if commit hard to guest_memfd _not_ being >>> general purposes, and if we we account for conversion scenarios when designing >>> hugepage support for guest_memfd. >> >> I'm hoping guest_memfd won't end up being the wild west of hacky MM ideas ;) > > Quite the opposite, I'm saying we should be very deliberate in how we add hugepage > support and others features to guest_memfd, so that guest_memfd doesn't become a > hacky mess. Good. > > And I'm saying say we should stand firm in what guest_memfd _won't_ support, e.g. > swap/reclaim and probably page migration should get a hard "no". I thought people wanted to support at least page migration in the future? (for example, see the reply from Will) > > In other words, ditch the complexity for features that are well served by existing > general purpose solutions, so that guest_memfd can take on a bit of complexity to > serve use cases that are unique to KVM guests, without becoming an unmaintainble > mess due to cross-products. And I believed that was true until people started wanting to mmap() this thing and brought GUP into the picture ... and then talk about HGM and all that. *shivers* -- Cheers, David / dhildenb