On Mon, 2009-05-11 at 09:14 -0400, Gregory Haskins wrote: > > >> for request-response, this is generally for *every* packet since > you > >> cannot exploit buffering/deferring. > >> > >> Can you back up your claim that PPC has no difference in > performance > >> with an MMIO exit and a "hypercall" (yes, I understand PPC has no > "VT" > >> like instructions, but clearly there are ways to cause a trap, so > >> presumably we can measure the difference between a PF exit and > something > >> more explicit). > >> > > > > First, the PPC that KVM supports performs very poorly relatively > > speaking because it receives no hardware assistance > > So wouldn't that be making the case that it could use as much help as > possible? I think he's referencing Ahmdal's Law here. While I'd agree, this is relevant only for the current KVM PowerPC implementations. I think it would be short-sighted to design an IO architecture around that. > > this is not the right place to focus wrt optimizations. > > Odd choice of words. I am advocating the opposite (broad solution to > many arches and many platforms (i.e. hypervisors)) and therefore I am > not "focused" on it (or really any one arch) at all per se. I am > _worried_ however, that we could be overlooking PPC (as an example) if > we ignore the disparity between MMIO and HC since other higher > performance options are not available like PIO. The goal on this > particular thread is to come up with an IO interface that works > reasonably well across as many hypervisors as possible. MMIO/PIO do > not appear to fit that bill (at least not without tunneling them over > HCs) I haven't been following this conversation at all. With that in mind... AFAICS, a hypercall is clearly the higher-performing option, since you don't need the additional memory load (which could even cause a page fault in some circumstances) and instruction decode. That said, I'm willing to agree that this overhead is probably negligible compared to the IOp itself... Ahmdal's Law again. -- Hollis Blanchard IBM Linux Technology Center -- 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
