On Thu, Sep 15, 2022 at 10:29:07PM +0800, Chao Peng wrote:

This new extension, indicated by the new flag KVM_MEM_PRIVATE, adds two
additional KVM memslot fields private_fd/private_offset to allow
userspace to specify that guest private memory provided from the
private_fd and guest_phys_addr mapped at the private_offset of the
private_fd, spanning a range of memory_size.

The extended memslot can still have the userspace_addr(hva). When use, a
single memslot can maintain both private memory through private
fd(private_fd/private_offset) and shared memory through
hva(userspace_addr). Whether the private or shared part is visible to
guest is maintained by other KVM code.

What is anyway the appeal of private_offset field, instead of having just
1:1 association between regions and files, i.e. one memfd per region?

If this was the case, then an extended struct would not be needed in the
first place. A simple union inside the existing struct would do:

        union {
                __u64 userspace_addr,
                __u64 private_fd,
        };

BR, Jarkko