On 04.06.2011, at 16:46, Ingo Molnar wrote: > > * Alexander Graf <agraf@xxxxxxx> 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. e1000 maps its NVRAM on coalesced mmio - which is completely ok. > 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 ... Yup, which is why it's very sparsely used in qemu :). Basically, it's only e1000 and vga, both of which are heavily used and tested drivers. Alex -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
