Re: [PATCH] kvm tools: Add MMIO coalescing support

Ingo Molnar
2011-06-04T14:46:39Z

* Alexander Graf <agraf@suse.de> wrote:

> So the simple rule is: don't register a coalesced MMIO region for a
> region where latency matters. [...]

So my first suspicion is confirmed.

A quick look at Qemu sources shows that lots of drivers are using
coalesced_mmio without being aware of the latency effects and only
one seems to make use of qemu_flush_coalesced_mmio_buffer(). Drivers
like hw/e1000.c sure look latency critical to me.

So i maintain my initial opinion: this is a pretty dangerous
'optimization' that should be used with extreme care: i can tell it
you with pretty good authority that latency problems are much more
easy to introduce than to find and remove ...

Thanks,

Ingo