He does seem to be saying that kdbus is pointless since those fixes would be in userspace where it's spending most of its time. Or am I misunderstanding?
The point of kdbus is to do dbus in kernel space. The one spending lots of time in userspace overhead (not doing actually useful work) is regular dbus.
Linus is saying that kdbus is pointless because its performance gains don't come from being in-kernel, they come from the code not being a complete shit-show, and he believes the same performance should be achievable by fixing regular userspace dbus.
Well, the code review of kdbus in the rest of that thread would argue that its code is a shit-show, it just happens to be a faster shit-show than userspace dbus.
