From: Fuad Tabba Date: Tue, 29 Jul 2025 22:54:52 +0000 (-0700) Subject: KVM: Allow and advertise support for host mmap() on guest_memfd files X-Git-Tag: v6.18-rc1~95^2~8^2~3 X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=3d3a04fad25a6621828518a2abe536142d2c1a7d;p=thirdparty%2Fkernel%2Fstable.git KVM: Allow and advertise support for host mmap() on guest_memfd files Now that all the x86 and arm64 plumbing for mmap() on guest_memfd is in place, allow userspace to set GUEST_MEMFD_FLAG_MMAP and advertise support via a new capability, KVM_CAP_GUEST_MEMFD_MMAP. The availability of this capability is determined per architecture, and its enablement for a specific guest_memfd instance is controlled by the GUEST_MEMFD_FLAG_MMAP flag at creation time. Update the KVM API documentation to detail the KVM_CAP_GUEST_MEMFD_MMAP capability, the associated GUEST_MEMFD_FLAG_MMAP, and provide essential information regarding support for mmap in guest_memfd. Reviewed-by: David Hildenbrand Reviewed-by: Gavin Shan Reviewed-by: Shivank Garg Reviewed-by: Xiaoyao Li Signed-off-by: Fuad Tabba Signed-off-by: Sean Christopherson Message-ID: <20250729225455.670324-22-seanjc@google.com> Signed-off-by: Paolo Bonzini --- diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst index 6aa40ee05a4ae..c17a87a0a5acb 100644 --- a/Documentation/virt/kvm/api.rst +++ b/Documentation/virt/kvm/api.rst @@ -6414,6 +6414,15 @@ most one mapping per page, i.e. binding multiple memory regions to a single guest_memfd range is not allowed (any number of memory regions can be bound to a single guest_memfd file, but the bound ranges must not overlap). +When the capability KVM_CAP_GUEST_MEMFD_MMAP is supported, the 'flags' field +supports GUEST_MEMFD_FLAG_MMAP. Setting this flag on guest_memfd creation +enables mmap() and faulting of guest_memfd memory to host userspace. + +When the KVM MMU performs a PFN lookup to service a guest fault and the backing +guest_memfd has the GUEST_MEMFD_FLAG_MMAP set, then the fault will always be +consumed from guest_memfd, regardless of whether it is a shared or a private +fault. + See KVM_SET_USER_MEMORY_REGION2 for additional details. 4.143 KVM_PRE_FAULT_MEMORY diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h index f0f0d49d25443..6efa98a57ec11 100644 --- a/include/uapi/linux/kvm.h +++ b/include/uapi/linux/kvm.h @@ -962,6 +962,7 @@ struct kvm_enable_cap { #define KVM_CAP_ARM_EL2_E2H0 241 #define KVM_CAP_RISCV_MP_STATE_RESET 242 #define KVM_CAP_ARM_CACHEABLE_PFNMAP_SUPPORTED 243 +#define KVM_CAP_GUEST_MEMFD_MMAP 244 struct kvm_irq_routing_irqchip { __u32 irqchip; @@ -1598,6 +1599,7 @@ struct kvm_memory_attributes { #define KVM_MEMORY_ATTRIBUTE_PRIVATE (1ULL << 3) #define KVM_CREATE_GUEST_MEMFD _IOWR(KVMIO, 0xd4, struct kvm_create_guest_memfd) +#define GUEST_MEMFD_FLAG_MMAP (1ULL << 0) struct kvm_create_guest_memfd { __u64 size; diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index d5b445548af42..08a6bc7d25b60 100644 --- a/virt/kvm/guest_memfd.c +++ b/virt/kvm/guest_memfd.c @@ -314,7 +314,9 @@ static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn) static bool kvm_gmem_supports_mmap(struct inode *inode) { - return false; + const u64 flags = (u64)inode->i_private; + + return flags & GUEST_MEMFD_FLAG_MMAP; } static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf) @@ -522,6 +524,9 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args) u64 flags = args->flags; u64 valid_flags = 0; + if (kvm_arch_supports_gmem_mmap(kvm)) + valid_flags |= GUEST_MEMFD_FLAG_MMAP; + if (flags & ~valid_flags) return -EINVAL; diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 4f57cb92e1091..18f29ef935437 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -4918,6 +4918,8 @@ static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg) #ifdef CONFIG_KVM_GUEST_MEMFD case KVM_CAP_GUEST_MEMFD: return 1; + case KVM_CAP_GUEST_MEMFD_MMAP: + return !kvm || kvm_arch_supports_gmem_mmap(kvm); #endif default: break;