On Sun, 2009-05-10 at 13:38 -0500, Anthony Liguori wrote: > Gregory Haskins wrote: > > > > 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 this is not the > right place to focus wrt optimizations. > > And because there's no hardware assistance, there simply isn't a > hypercall instruction. Are PFs the fastest type of exits? Probably not > but I honestly have no idea. I'm sure Hollis does though. Memory load from the guest context (for instruction decoding) is a *very* poorly performing path on most PowerPC, even considering server PowerPC with hardware virtualization support. No, I don't have any data for you, but switching the hardware MMU contexts requires some heavyweight synchronization instructions. > Page faults are going to have tremendously different performance > characteristics on PPC too because it's a software managed TLB. There's > no page table lookup like there is on x86. To clarify, software-managed TLBs are only found in embedded PowerPC. Server and classic PowerPC use hash tables, which are a third MMU type. -- 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
