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[2003:d8:2f18:7500:202e:b0f1:76d6:f9af]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a88c7fb20esm13693987f8f.36.2025.07.01.07.44.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 01 Jul 2025 07:44:25 -0700 (PDT) Message-ID: <0cdc7890-aade-4fa5-ad72-24cde6c7bce9@redhat.com> Date: Tue, 1 Jul 2025 16:44:22 +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 10/18] KVM: x86/mmu: Handle guest page faults for guest_memfd with shared memory To: Ackerley Tng , Shivank Garg , Fuad Tabba Cc: Sean Christopherson , 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, 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, vannapurve@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, ira.weiny@intel.com References: <20250611133330.1514028-1-tabba@google.com> <20250611133330.1514028-11-tabba@google.com> <434ab5a3-fedb-4c9e-8034-8f616b7e5e52@amd.com> <923b1c02-407a-4689-a047-dd94e885b103@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit >>> I support this approach. >> >> Agreed. Let's get this in with the changes requested by Sean applied. >> >> How to use GUEST_MEMFD_FLAG_MMAP in combination with a CoCo VM with >> legacy mem attributes (-> all memory in guest_memfd private) could be >> added later on top, once really required. >> >> As discussed, CoCo VMs that want to support GUEST_MEMFD_FLAG_MMAP will >> have to disable legacy mem attributes using a new capability in stage-2. >> > > I rewatched the guest_memfd meeting on 2025-06-12. We do want to > support the use case where userspace wants to have mmap (e.g. to set > mempolicy) but does not want to allow faulting into the host. > > On 2025-06-12, the conclusion was that the problem will be solved once > guest_memfd supports shareability, and that's because userspace can set > shareability to GUEST, so the memory can't be faulted into the host. > > On 2025-06-26, Sean said we want to let userspace have an extra layer of > protection so that memory cannot be faulted in to the host, ever. IOW, > we want to let userspace say that even if there is a stray > private-to-shared conversion, *don't* allow faulting memory into the > host. > > The difference is the "extra layer of protection", which should remain > in effect even if there are (stray/unexpected) private-to-shared > conversions to guest_memfd or to KVM. Here's a direct link to the point > in the video where Sean brought this up [1]. I'm really hoping I didn't > misinterpret this! > > Let me look ahead a little, since this involves use cases already > brought up though I'm not sure how real they are. I just want to make > sure that in a few patch series' time, we don't end up needing userspace > to use a complex bunch of CAPs and FLAGs. > > In this series (mmap support, V12, patch 10/18) [2], to allow > KVM_X86_DEFAULT_VMs to use guest_memfd, I added a `fault_from_gmem()` > helper, which is defined as follows (before the renaming Sean requested): > > +static inline bool fault_from_gmem(struct kvm_page_fault *fault) > +{ > + return fault->is_private || kvm_gmem_memslot_supports_shared(fault->slot); > +} > > The above is changeable, of course :). The intention is that if the > fault is private, fault from guest_memfd. If GUEST_MEMFD_FLAG_MMAP is > set (KVM_MEMSLOT_GMEM_ONLY will be set on the memslot), fault from > guest_memfd. > > If we defer handling GUEST_MEMFD_FLAG_MMAP in combination with a CoCo VM > with legacy mem attributes to the future, this helper will probably > become > > -static inline bool fault_from_gmem(struct kvm_page_fault *fault) > +static inline bool fault_from_gmem(struct kvm *kvm, struct kvm_page_fault *fault) > +{ > - return fault->is_private || kvm_gmem_memslot_supports_shared(fault->slot); > + return fault->is_private || (kvm_gmem_memslot_supports_shared(fault->slot) && > + !kvm_arch_disable_legacy_private_tracking(kvm)); > +} > > And on memslot binding we check > > if kvm_arch_disable_legacy_private_tracking(kvm) and not GUEST_MEMFD_FLAG_MMAP > return -EINVAL; > > 1. Is that what yall meant? My understanding: CoCo VMs will initially (stage-1) only support !GUEST_MEMFD_FLAG_MMAP. With stage-2, CoCo VMs will support GUEST_MEMFD_FLAG_MMAP only with kvm_arch_disable_legacy_private_tracking(). Non-CoCo VMs will only support GUEST_MEMFD_FLAG_MMAP. (no concept of private) > > 2. Does this kind of not satisfy the "extra layer of protection" > requirement (if it is a requirement)? > > A legacy CoCo VM using guest_memfd only for private memory (shared > memory from say, shmem) and needing to set mempolicy would > > * Set GUEST_MEMFD_FLAG_MMAP > * Leave KVM_CAP_DISABLE_LEGACY_PRIVATE_TRACKING defaulted to false > > but still be able to send conversion ioctls directly to guest_memfd, > and then be able to fault guest_memfd memory into the host. In that configuration, I would expect that all memory in guest_memfd is private and remains private. guest_memfd without memory attributes cannot support in-place conversion. How to achieve that might be interesting: the capability will affect guest_memfd behavior? > > 3. Now for a use case I've heard of (feel free to tell me this will > never be supported or "we'll deal with it if it comes"): On a > non-CoCo VM, we want to use guest_memfd but not use mmap (and the > initial VM image will be written using write() syscall or something > else). > > * Set GUEST_MEMFD_FLAG_MMAP to false > * Leave KVM_CAP_DISABLE_LEGACY_PRIVATE_TRACKING defaulted to false > (it's a non-CoCo VM, weird to do anything to do with private > tracking) > > And now we're stuck because fault_from_gmem() will return false all > the time and we can't use memory from guest_memfd. I think I discussed that with Sean: we would have GUEST_MEMFD_FLAG_WRITE that will imply everything that GUEST_MEMFD_FLAG_MMAP would imply, except the actual mmap() support. -- Cheers, David / dhildenb