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[2003:d8:2f2d:2400:4052:3b5:fff9:4ed0]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-453599c9854sm9720885e9.1.2025.06.18.02.27.18 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 18 Jun 2025 02:27:19 -0700 (PDT) Message-ID: <5a55d95e-5e32-4239-a445-be13228ea80b@redhat.com> Date: Wed, 18 Jun 2025 11:27:17 +0200 Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: 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> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit 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). -- Cheers, David / dhildenb