Fuad Tabba <tabba@xxxxxxxxxx> writes:
> Not all use cases require guest_memfd() to be shared with the host when
> first created. Add a new flag, GUEST_MEMFD_FLAG_INIT_SHARED, which when
> set on KVM_CREATE_GUEST_MEMFD initializes the memory as shared with the
> host, and therefore mappable by it. Otherwise, memory is private until
> explicitly shared by the guest with the host.
>
> Signed-off-by: Fuad Tabba <tabba@xxxxxxxxxx>
> ---
> Documentation/virt/kvm/api.rst | 4 ++++
> include/uapi/linux/kvm.h | 1 +
> tools/testing/selftests/kvm/guest_memfd_test.c | 7 +++++--
> virt/kvm/guest_memfd.c | 12 ++++++++++++
> 4 files changed, 22 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 2b52eb77e29c..a5496d7d323b 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -6386,6 +6386,10 @@ 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).
>
> +If the capability KVM_CAP_GMEM_SHARED_MEM is supported, then the flags field
> +supports GUEST_MEMFD_FLAG_INIT_SHARED, which initializes the memory as shared
> +with the host, and thereby, mappable by it.
> +
> 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 117937a895da..22d7e33bf09c 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1566,6 +1566,7 @@ 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_INIT_SHARED (1UL << 0)
>
> struct kvm_create_guest_memfd {
> __u64 size;
> diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c
> index 38c501e49e0e..4a7fcd6aa372 100644
> --- a/tools/testing/selftests/kvm/guest_memfd_test.c
> +++ b/tools/testing/selftests/kvm/guest_memfd_test.c
> @@ -159,7 +159,7 @@ static void test_invalid_punch_hole(int fd, size_t page_size, size_t total_size)
> static void test_create_guest_memfd_invalid(struct kvm_vm *vm)
> {
> size_t page_size = getpagesize();
> - uint64_t flag;
> + uint64_t flag = BIT(0);
> size_t size;
> int fd;
>
> @@ -170,7 +170,10 @@ static void test_create_guest_memfd_invalid(struct kvm_vm *vm)
> size);
> }
>
> - for (flag = BIT(0); flag; flag <<= 1) {
> + if (kvm_has_cap(KVM_CAP_GMEM_SHARED_MEM))
> + flag = GUEST_MEMFD_FLAG_INIT_SHARED << 1;
> +
> + for (; flag; flag <<= 1) {
This would end up shifting the GUEST_MEMFD_FLAG_INIT_SHARED flag for
each loop.
> fd = __vm_create_guest_memfd(vm, page_size, flag);
> TEST_ASSERT(fd == -1 && errno == EINVAL,
> "guest_memfd() with flag '0x%lx' should fail with EINVAL",
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index eec9d5e09f09..32e149478b04 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -1069,6 +1069,15 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
> goto err_gmem;
> }
>
> + if (IS_ENABLED(CONFIG_KVM_GMEM_SHARED_MEM) &&
> + (flags & GUEST_MEMFD_FLAG_INIT_SHARED)) {
> + err = kvm_gmem_offset_range_set_shared(file_inode(file), 0, size >> PAGE_SHIFT);
> + if (err) {
> + fput(file);
> + goto err_gmem;
> + }
> + }
> +
> kvm_get_kvm(kvm);
> gmem->kvm = kvm;
> xa_init(&gmem->bindings);
> @@ -1090,6 +1099,9 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
> u64 flags = args->flags;
> u64 valid_flags = 0;
>
> + if (IS_ENABLED(CONFIG_KVM_GMEM_SHARED_MEM))
> + valid_flags |= GUEST_MEMFD_FLAG_INIT_SHARED;
> +
> if (flags & ~valid_flags)
> return -EINVAL;
