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[2003:d8:2f2d:2400:4052:3b5:fff9:4ed0]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a568b62ba7sm16812291f8f.91.2025.06.18.02.59.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 18 Jun 2025 02:59:05 -0700 (PDT) Message-ID: <40a5903b-f747-4eab-8959-06ddd6e88f82@redhat.com> Date: Wed, 18 Jun 2025 11:59:02 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v12 08/18] KVM: guest_memfd: Allow host to map guest_memfd pages To: Xiaoyao Li , Sean Christopherson Cc: Fuad Tabba , Ira Weiny , kvm@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mm@kvack.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, willy@infradead.org, akpm@linux-foundation.org, 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, vannapurve@google.com, 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_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: <20250611133330.1514028-1-tabba@google.com> <20250611133330.1514028-9-tabba@google.com> <68501fa5dce32_2376af294d1@iweiny-mobl.notmuch> <701c8716-dd69-4bf6-9d36-4f8847f96e18@redhat.com> <3fb0e82b-f4ef-402d-a33c-0b12e8aa990c@redhat.com> <5ee9bbb8-d100-408c-ac07-ea9c5b603545@intel.com> <5a55d95e-5e32-4239-a445-be13228ea80b@redhat.com> <45af2c0d-a416-49bc-8011-4ec57a56d6f5@intel.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 9853F4000F X-Stat-Signature: 641mg635fnoi7o3a9mkpfka93io3ew9j X-HE-Tag: 1750240749-156144 X-HE-Meta: U2FsdGVkX19p5uZYxpbcVLVYZyg7xknaw/g8vztCtS89lrHYjb9brd4mRcayLmYx6tGHq/shIydoki1UH6RkJ7OlyckwHlzxUy5f1aFGybYK/WhqADENDAMxvh49+rXATil3bFlXdyp1vDIeL/c97H0F5zVm8UFrpxTeX38kPOQMwvSzJ4gM5UAS5n61EJHA0p8SPErQKUYVUqoIduxDhM3XJQaXE5bzTECuM9Y8yvwA+r5lkixgHNiHPudjT5ydLsstZog/XYgvjY+HSVeJAyVMs61LwgWXNN1VRFVmvW5T0JAC/0ErEVmt3BaWFNip44JAzyaS1DA/QB+hTbGigb+/FFPb7qTLKgt4mfvnKCufvnBjsqi240jU//CGIJe5YD5Zw1MY6Mj0IgjZL/2mr4Wy0Gr2pBah2Qh8/SD9BNZdiYvh/nJMFcANp7QHO7Xwk2068cyFdnI6F08xZuiwy/jXZfdUwE7EGpRPBDcnDcZUCR0o7xH6z/24Ji9qpfilIxFUXGjreq2g4kxIftC+TE2bnHjlgIxe+xfFZQv907xoNR9uEBkJKApm2qPIb5PkjFDb+SFnAjGwMm0BByw++sgT3dad0dbyiycukPfMfSPYZxNWPA7JH1E1At8uoX3yyI/7p0jdyiLFuxQVsI5D74FvqgFPHu9yykz2MGbCGfSK64xKHTx46EpQkUeAcYkdqwoGJMYG0Psq+CiY1aTMoTm1pzomX0Bwblm3oFOtQkGuQEfcLOBQwq2E35XK4pcmRagqrwLf/zlC7GH8G8cKjjF07rYaJJvuZw8RdXLZfSU2VqHR/DqK7/u4UIwBQtWIdBLmrnwn5QG0EXmQ1GsX40ruip5QYLGlIt1yYVpuxyZw/zjjF6ijTOsOhdr3K/5vXFFJAY0x+YHp8A3fVI3l68nlSbUDZBBBh7/8Fi1iQ6BAWqTT4YIL88PzWC45t5GnexL1/+/PrMKXQJIaYa+ crUouUnb AtsRYLZ9XwG9GhVqXI1F6446f4OpR3yZLLcGyo+lZP2wEM7LtMSJplFv4QAdUimNF6ct29uGVTfGb0LnYXPxHRU0VuQf3wNPT615iAPj+QhIinsRa6D+GnB7E88iXytg9SoNAo2tTXH898Sgc3+NASoxyQG2UWQi4cLIhU/adkAL/e33prPiFCWe2HVmmS6AuCll5 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 18.06.25 11:44, Xiaoyao Li wrote: > On 6/18/2025 5:27 PM, David Hildenbrand wrote: >> On 18.06.25 11:20, Xiaoyao Li wrote: >>> On 6/18/2025 4:15 PM, David Hildenbrand wrote: >>>>> If we are really dead set on having SHARED in the name, it could be >>>>> GUEST_MEMFD_FLAG_USER_MAPPABLE_SHARED or >>>>> GUEST_MEMFD_FLAG_USER_MAP_SHARED?  But >>>>> to me that's _too_ specific and again somewhat confusing given the >>>>> unfortunate >>>>> private vs. shared usage in CoCo-land.  And just playing the odds, I'm >>>>> fine taking >>>>> a risk of ending up with GUEST_MEMFD_FLAG_USER_MAPPABLE_PRIVATE or >>>>> whatever, >>>>> because I think that is comically unlikely to happen. >>>> >>>> I think in addition to GUEST_MEMFD_FLAG_MMAP we want something to >>>> express "this is not your old guest_memfd that only supports private >>>> memory". And that's what I am struggling with. >>> >>> Sorry for chiming in. >>> >>> Per my understanding, (old) guest memfd only means it's the memory that >>> cannot be accessed by userspace. There should be no shared/private >>> concept on it. >>> >>> And "private" is the concept of KVM. Guest memfd can serve as private >>> memory, is just due to the character of it cannot be accessed from >>> userspace. >>> >>> So if the guest memfd can be mmap'ed, then it become userspace >>> accessable and cannot serve as private memory. >>> >>>> Now, if you argue "support for mmap() implies support for non-private >>>> memory", I'm probably okay for that. >>> >>> I would say, support for mmap() implies cannot be used as private memory. >> >> That's not where we're heading with in-place conversion support: you >> will have private (ianccessible) and non-private (accessible) parts, and >> while guest_memfd will support mmap() only the accessible parts can >> actually be accessed (faulted in etc). > > That's OK. The guestmemfd can be fine-grained, i.e., different > range/part of it can have different access property. But one rule never > change: only the sub-range is not accessible by userspace can it be > serve as private memory. I'm sorry, I don't understand what you are getting at. You said "So if the guest memfd can be mmap'ed, then it become userspace accessable and cannot serve as private memory." and I say, with in-place conversion support you are wrong. The whole file can be mmaped(), that does not tell us anything about which parts can be private or not. -- Cheers, David / dhildenb