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([2a01:599:915:8911:b13f:d972:e237:7fe2]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a0b0f18accsm1745424f8f.41.2025.05.06.07.15.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 06 May 2025 07:15:15 -0700 (PDT) Message-ID: <39ea3946-6683-462e-af5d-fe7d28ab7d00@redhat.com> Date: Tue, 6 May 2025 16:15:11 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v8 06/13] KVM: x86: Generalize private fault lookups to guest_memfd fault lookups To: Sean Christopherson , Vishal Annapurve Cc: Ackerley Tng , Fuad Tabba , kvm@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mm@kvack.org, 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, willy@infradead.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, isaku.yamahata@intel.com, mic@digikod.net, vbabka@suse.cz, 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_eberman@quicinc.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, qperret@google.com, keirf@google.com, roypat@amazon.co.uk, shuah@kernel.org, hch@infradead.org, jgg@nvidia.com, rientjes@google.com, jhubbard@nvidia.com, fvdl@google.com, hughd@google.com, jthoughton@google.com, peterx@redhat.com, pankaj.gupta@amd.com References: <386c1169-8292-43d1-846b-c50cbdc1bc65@redhat.com> <7e32aabe-c170-4cfc-99aa-f257d2a69364@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 597A8C0014 X-Stat-Signature: 1b5whcfwkqwmnskrdbdmm8xfb6zttids X-Rspam-User: X-HE-Tag: 1746540919-59819 X-HE-Meta: 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 kavjAgIM KF9OU1QwBZqN2DX8QG17zmUA44HclaW21BXGwGoExy0eGEqAUCIk3enH491ovst+5PJlZmF6JM/E0fgWhNLUj2gMwJmpb1uwk6bgaMFS/k8V4Wv01UBzYFRIVrXDf+I+nlz1pqWOZtOWNigkKDPU4kkEnqp0Jh2vqg53EIKTxI8thkpmoRgnXUeHDls2396z5SfI/ 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 06.05.25 15:58, Sean Christopherson wrote: > On Mon, May 05, 2025, Vishal Annapurve wrote: >> On Mon, May 5, 2025 at 10:17 PM Vishal Annapurve wrote: >>> >>> On Mon, May 5, 2025 at 3:57 PM Sean Christopherson wrote: >>>>> ... >>>>> And not worry about lpage_infor for the time being, until we actually do >>>>> support larger pages. >>>> >>>> I don't want to completely punt on this, because if it gets messy, then I want >>>> to know now and have a solution in hand, not find out N months from now. >>>> >>>> That said, I don't expect it to be difficult. What we could punt on is >>>> performance of the lookups, which is the real reason KVM maintains the rather >>>> expensive disallow_lpage array. >>>> >>>> And that said, memslots can only bind to one guest_memfd instance, so I don't >>>> immediately see any reason why the guest_memfd ioctl() couldn't process the >>>> slots that are bound to it. I.e. why not update KVM_LPAGE_MIXED_FLAG from the >>>> guest_memfd ioctl() instead of from KVM_SET_MEMORY_ATTRIBUTES? >>> >>> I am missing the point here to update KVM_LPAGE_MIXED_FLAG for the >>> scenarios where in-place memory conversion will be supported with >>> guest_memfd. As guest_memfd support for hugepages comes with the >>> design that hugepages can't have mixed attributes. i.e. max_order >>> returned by get_pfn will always have the same attributes for the folio >>> range. > > Oh, if this will naturally be handled by guest_memfd, then do that. I was purely > reacting to David's suggestion to "not worry about lpage_infor for the time being, > until we actually do support larger pages". > >>> Is your suggestion around using guest_memfd ioctl() to also toggle >>> memory attributes for the scenarios where guest_memfd instance doesn't >>> have in-place memory conversion feature enabled? >> >> Reading more into your response, I guess your suggestion is about >> covering different usecases present today and new usecases which may >> land in future, that rely on kvm_lpage_info for faster lookup. If so, >> then it should be easy to modify guest_memfd ioctl to update >> kvm_lpage_info as you suggested. > > Nah, I just missed/forgot that using a single guest_memfd for private and shared > would naturally need to split the folio and thus this would Just Work. Yeah, I ignored that fact as well. So essentially, this patch should be mostly good for now. Only kvm_mmu_hugepage_adjust() must be taught to not rely on fault->is_private. Once we support large folios in guest_memfd, only the "alignment" consideration might have to be taken into account. Anything else? -- Cheers, David / dhildenb