OK, so on the one hand, you think that the proposed synchronous
interface for frontswap is insufficiently extensible for other
uses (presumably including KVM). On the other hand, you agree
that using the existing I/O subsystem is unnecessarily heavyweight.
On the third hand, Nitin has answered your questions and spent
a good part of three years finding that extending the existing swap
interface to efficiently support swap-to-pseudo-RAM requires
some kind of in-kernel notification mechanism to which Linus
has already objected.
So you are instead proposing some new guest-to-host asynchronous
notification mechanism that doesn't use the existing bio
mechanism (and so presumably not irqs),
imitates or can
utilize a dma engine, and uses less cpu cycles than copying
pages. AND, for long-term maintainability, you'd like to avoid
creating a new guest-host API that does all this, even one that
is as simple and lightweight as the proposed frontswap hooks.
Does that summarize your objection well?