On Tue, 2011-07-19 at 21:53 -0500, Anthony Liguori wrote:
> QEMU does use it and it's quite important. Coalesced MMIO is really
> about write caching MMIO exits. It only works with devices that have
> registers where writing has no side effects. Moreover, it only really
> works well when there are lots and lots of writes to these registers
> simultaneously.
>
> Couple that with the fact that the buffer is a fixed size and it's
> really not flexible enough to be useful for a wide variety of devices.
>
> But for VGA planar mode writes, it works wonders. It would be terrible
> to totally lose it. That said, I'm not at all convinced it's useful for
> much other than VGA planar mode.
Why was the coalesced approach taken in the first place? When I tried
using it for VGA in /tools/kvm it just seemed to me like a builtin
virtio-memory transport.
Thats why I think planar VGA would be fine if we deprecate coalesced
mmio in favor of either socket ioeventfds or a new virtio-memory device.