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[2003:d8:2f02:2900:f54f:bad7:c5f4:9404]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4405b4f2444sm20160495e9.17.2025.04.16.05.41.13 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 16 Apr 2025 05:41:15 -0700 (PDT) Message-ID: Date: Wed, 16 Apr 2025 14:41:13 +0200 Precedence: bulk X-Mailing-List: linux-arm-msm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v7 4/9] KVM: guest_memfd: Handle in-place shared memory as guest_memfd backed memory To: Patrick Roy , Fuad Tabba , kvm@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mm@kvack.org Cc: pbonzini@redhat.com, chenhuacai@kernel.org, mpe@ellerman.id.au, anup@brainfault.org, paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, seanjc@google.com, 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, 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, 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 References: <20250318161823.4005529-1-tabba@google.com> <20250318161823.4005529-5-tabba@google.com> <8ebc66ae-5f37-44c0-884b-564a65467fe4@redhat.com> <392fc76a-5d2a-441d-99c8-532c0bbb052b@amazon.co.uk> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 16.04.25 14:30, Patrick Roy wrote: > Hi! > > On Mon, 2025-04-14 at 12:51 +0100, David Hildenbrand wrote: >> > > [...] > >> On top of that, I was wondering if we could look into doing something like >> the following. It would also allow for pulling pages out of gmem for >> existing SW-protected VMs once they enable shared memory for GMEM IIUC. >> >> >> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c >> index 08eebd24a0e18..6f878cab0f466 100644 >> --- a/arch/x86/kvm/mmu/mmu.c >> +++ b/arch/x86/kvm/mmu/mmu.c >> @@ -4495,11 +4495,6 @@ static int kvm_mmu_faultin_pfn_gmem(struct kvm_vcpu *vcpu, >> { >> int max_order, r; >> >> - if (!kvm_slot_has_gmem(fault->slot)) { >> - kvm_mmu_prepare_memory_fault_exit(vcpu, fault); >> - return -EFAULT; >> - } >> - >> r = kvm_gmem_get_pfn(vcpu->kvm, fault->slot, fault->gfn, &fault->pfn, >> &fault->refcounted_page, &max_order); >> if (r) { >> @@ -4518,8 +4513,19 @@ static int __kvm_mmu_faultin_pfn(struct kvm_vcpu *vcpu, >> struct kvm_page_fault *fault) >> { >> unsigned int foll = fault->write ? FOLL_WRITE : 0; >> + bool use_gmem = false; >> + >> + if (fault->is_private) { >> + if (!kvm_slot_has_gmem(fault->slot)) { >> + kvm_mmu_prepare_memory_fault_exit(vcpu, fault); >> + return -EFAULT; >> + } >> + use_gmem = true; >> + } else if (kvm_slot_has_gmem_with_shared(fault->slot)) { >> + use_gmem = true; >> + } >> >> - if (fault->is_private) >> + if (use_gmem) >> return kvm_mmu_faultin_pfn_gmem(vcpu, fault); >> >> foll |= FOLL_NOWAIT; >> >> >> That is, we'd not claim that things are private when they are not, but instead >> teach the code about shared memory coming from gmem. >> >> There might be some more missing, just throwing it out there if I am completely off. > > I think I arrived at the need for this as well while experimenting with > building a Firecracker version that works with my direct map removal > patches. > > With this patch series, on ARM, as soon as a memslot has a guest_memfd > associated with it, all guest faults go through kvm_gmem_get_pfn, but on > x86, they go through slot->userspace_addr by default, as > CONFIG_KVM_SW_PROTECTED_VM selects CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES. > There's no real difference between these if slot->userspace_addr can be > GUP'd, but if its a VMA of a guest_memfd without direct map entries, > faulting through slot->userspace_addr wont work. So on x86 Firecracker > has to formally set the memory attributes to private, while on ARM it > doesn't [1], which is a bit awkward. Exactly. I proposed all that (especially the interactions/expectations regarding KVM memory slots) as a topic for tomorrow's meeting. See below on my current idea. > > David, I couldn't find an implementation of > kvm_slot_has_gmem_with_shared() in the branch you shared, but would it > be something like "slot->userspace_addr points to a gmem VMA, > particularly to a VMA of the gmem that's associated with this memslot, > mapped at the same offset"? That's open for discussion, and the implementation of that will have to be clarified in the concept of this series. I would have made the KVM slot setup by user space a requirement: If we enabled "gmem allows for shared memory / mmap" for a MM, then user space *must* provide a mmap() of that guest_memfd in slot->userspace_addr. (we might also be able to use a new slot flag to specify that) With that, we can just say * In the MMU etc, avoid walking the page tables and just grab the page from guest_memfd. * Where not possible (e.g., emulator access as discussed) we can just access the page in the user space mapping Does that sound reasonable? -- Cheers, David / dhildenb