I don't really understand how 'childs' fit in here. The code also
doesn't seem to be usable without patch 2 and a caller of the
new functions added in patch 2, so it is rather impossible to review.
Also:
1) why is SEV/TDX so different from other cases that need bounce
buffering to treat it different and we can't work on a general
scalability improvement
2) per previous discussions at how swiotlb itself works, it is
clear that another option is to just make pages we DMA to
shared with the hypervisor. Why don't we try that at least
for larger I/O?
