Nadav Har'El <nyh <at> math.technion.ac.il> writes: > > On Fri, Apr 20, 2012, Chegu Vinod wrote about "Re: Networking performance on > a KVM Host (with no guests)": > > Removing the "intel_iommu=on" boot time parameter in the Config 1 > > case seemed to help > > "intel_iommu=on" is essential with you're mostly running guests *and* > using device assignment. >
Yes. I do understand that... > However, unfortunately, it also has a serious performance penalty for > I/O in *host* processes: > > When intel_iommu=on, Linux (completely unrelated to KVM) adds a new level > of protection which didn't exist without an IOMMU - the network card, which > without an IOMMU could write (via DMA) to any memory location, now is > not allowed - the card can only write to memory locates which the OS > wanted it to write. Theoretically, this can protect the OS against > various kinds of attacks. But what happens now is that every time that > Linux passes a new buffer to the card, it needs to change the IOMMU > mappings. This noticably slows down I/O, unfortunately. > Hmm... So if one were to have a private link setup between two hosts to drive some traffic through between the hosts... then we can't expect to get line rate on that private NIC. Thanks much for your response ! Vinod -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
