[Qemu-devel] Avoiding nographic_timer exits

[David Gibson]

With PowerKVM, exits from KVM to qemu are even more expensive than on
x86.  One significant source of these we're finding (since we usually
work in -nographic mode) is the nographic_timer.

At present, we're using a hack to disable it, but that's obviously not
a long term solution.  From examination, it looks like the only
purpose of this timer is to flush coalesced mmios.  So it seems like
the timer should only be activated when a coalesced mmio region
actually exists, but I'm not entirely sure how to go about that.

Thinking longer term, it seems very odd that a userspace periodic
timer handles this at all.  Surely it would make more sense to use a
kernel timer within KVM, which can be activated only when there are
actually pending coalesced MMIOs in the buffer.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson