Commit d2042d8f authored by Sean Christopherson's avatar Sean Christopherson
Browse files

KVM: Rework KVM_CAP_GUEST_MEMFD_MMAP into KVM_CAP_GUEST_MEMFD_FLAGS 



Rework the not-yet-released KVM_CAP_GUEST_MEMFD_MMAP into a more generic
KVM_CAP_GUEST_MEMFD_FLAGS capability so that adding new flags doesn't
require a new capability, and so that developers aren't tempted to bundle
multiple flags into a single capability.

Note, kvm_vm_ioctl_check_extension_generic() can only return a 32-bit
value, but that limitation can be easily circumvented by adding e.g.
KVM_CAP_GUEST_MEMFD_FLAGS2 in the unlikely event guest_memfd supports more
than 32 flags.

Reviewed-by: default avatarAckerley Tng <ackerleytng@google.com>
Tested-by: default avatarAckerley Tng <ackerleytng@google.com>
Reviewed-by: default avatarDavid Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20251003232606.4070510-2-seanjc@google.com


Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
parent 034417c1

Documentation/virt/kvm/api.rst

+7 −3
Original line number Diff line number Diff line
@@ -6432,9 +6432,13 @@ 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.

The capability KVM_CAP_GUEST_MEMFD_FLAGS enumerates the `flags` that can be

specified via KVM_CREATE_GUEST_MEMFD.  Currently defined flags:



  ============================ ================================================

  GUEST_MEMFD_FLAG_MMAP        Enable using mmap() on the guest_memfd file

                               descriptor.

  ============================ ================================================



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

include/uapi/linux/kvm.h

+1 −1
Original line number Diff line number Diff line
@@ -962,7 +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

#define KVM_CAP_GUEST_MEMFD_FLAGS 244



struct kvm_irq_routing_irqchip {

	__u32 irqchip;

tools/testing/selftests/kvm/guest_memfd_test.c

+6 −7
Original line number Diff line number Diff line
@@ -262,19 +262,17 @@ static void test_guest_memfd_flags(struct kvm_vm *vm, uint64_t valid_flags) 


static void test_guest_memfd(unsigned long vm_type)

{

	uint64_t flags = 0;

	struct kvm_vm *vm;

	size_t total_size;

	size_t page_size;

	uint64_t flags;

	int fd;



	page_size = getpagesize();

	total_size = page_size * 4;



	vm = vm_create_barebones_type(vm_type);



	if (vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_MMAP))

		flags |= GUEST_MEMFD_FLAG_MMAP;

	flags = vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_FLAGS);



	test_create_guest_memfd_multiple(vm);

	test_create_guest_memfd_invalid_sizes(vm, flags, page_size);
@@ -328,13 +326,14 @@ static void test_guest_memfd_guest(void) 
	size_t size;

	int fd, i;



	if (!kvm_has_cap(KVM_CAP_GUEST_MEMFD_MMAP))

	if (!kvm_check_cap(KVM_CAP_GUEST_MEMFD_FLAGS))

		return;



	vm = __vm_create_shape_with_one_vcpu(VM_SHAPE_DEFAULT, &vcpu, 1, guest_code);



	TEST_ASSERT(vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_MMAP),

		    "Default VM type should always support guest_memfd mmap()");

	TEST_ASSERT(vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_FLAGS) & GUEST_MEMFD_FLAG_MMAP,

		    "Default VM type should support MMAP, supported flags = 0x%x",

		    vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_FLAGS));



	size = vm->page_size;

	fd = vm_create_guest_memfd(vm, size, GUEST_MEMFD_FLAG_MMAP);

virt/kvm/kvm_main.c

+5 −2
Original line number Diff line number Diff line
@@ -4928,8 +4928,11 @@ 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);

	case KVM_CAP_GUEST_MEMFD_FLAGS:

		if (!kvm || kvm_arch_supports_gmem_mmap(kvm))

			return GUEST_MEMFD_FLAG_MMAP;



		return 0;

#endif

	default:

		break;