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(-)
>
> <snip>
>
> 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);
I think if GUEST_MEMFD_FLAG_INIT_SHARED is not set, we should call
kvm_gmem_offset_range_clear_shared(); so that there is always some
shareability defined for all offsets in a file. Otherwise, when reading
shareability, we'd have to check against GUEST_MEMFD_FLAG_INIT_SHARED
to find out what to initialize it to.
> + 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;
