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[2003:d8:2f2d:2400:4052:3b5:fff9:4ed0]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a568b61071sm16688257f8f.90.2025.06.18.04.14.25 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 18 Jun 2025 04:14:27 -0700 (PDT) Message-ID: <38101158-4475-4885-83e7-654045ca0f9b@redhat.com> Date: Wed, 18 Jun 2025 13:14:24 +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> <40a5903b-f747-4eab-8959-06ddd6e88f82@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: rspam11 X-Rspam-User: X-Rspamd-Queue-Id: 40CE3A0003 X-Stat-Signature: h94eu691eo3sjafieet3cdbn5ey3gfri X-HE-Tag: 1750245271-101748 X-HE-Meta: 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 mYiMTq32 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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 18.06.25 12:42, Xiaoyao Li wrote: > On 6/18/2025 5:59 PM, David Hildenbrand wrote: >> 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. > > So there is nothing prevent userspace from accessing it after a range is > converted to private via KVM_GMEM_CONVERT_PRIVATE since the whole file > can be mmaped()? > > If so, then for TDX case, userspace can change the TD-owner bit of the > private part by accessing it and later guest access will poison it and > trigger #MC. If the #MC is only delivered to the PCPU that triggers it, > it just leads to the TD guest being killed. If the #MC is broadcasted, > it affects other in the system. > > I just give it a try on real TDX system with in-place conversion. The TD > is killed due to SIGBUS (host kernel handles the #MC and sends the > SIGBUS). It seems OK if only the TD guest being affected due to > userspace accesses the private memory. But I'm not sure if there is any > corner case that will affect the host. I suggest you go ahead and read all about in-place conversion support, and how it all relates to the #MC problem you mention here. Long story short: SIGBUS is triggered by the fault handler, not by the #MC, because private pages cannot be faulted in and accessed. -- Cheers, David / dhildenb