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KVM: Allow and advertise support for host mmap() on guest_memfd files

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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 <david@redhat.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Shivank Garg <shivankg@amd.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20250729225455.670324-22-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Fuad Tabba
2025-07-29 15:54:52 -07:00
committed by Paolo Bonzini
parent 32e200bd6e
commit 3d3a04fad2
4 changed files with 19 additions and 1 deletions
Download Patch File Download Diff File Expand all files Collapse all files

9
Documentation/virt/kvm/api.rst
View File

@@ -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

2
include/uapi/linux/kvm.h
View File

@@ -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;

7
virt/kvm/guest_memfd.c
View File

@@ -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;

2
virt/kvm/kvm_main.c
View File

@@ -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;