From: Avi Kivity Subject: Re: [PATCH 0/16 v6] PCI: Linux kernel SR-IOV support Date: Sun, 16 Nov 2008 18:04:55 +0200 Message-ID: <492044A7.3080107@redhat.com> References: <20081106154351.GA30459@kroah.com> <894107.30288.qm@web45108.mail.sp1.yahoo.com> <20081106164919.GA4099@kroah.com> <0199E0D51A61344794750DC57738F58E5E26F996C4@GVW1118EXC.americas.hpqcorp.net> <20081106183630.GD11773@parisc-linux.org> <491371F0.7020805@codemonkey.ws> <87d4h7pnnm.fsf__4937.77150190926$1226071173$gmane$org@basil.nowhere.org> <491B5B97.2000407@codemonkey.ws> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Andi Kleen , "randy.dunlap@oracle.com" , "grundler@parisc-linux.org" , "Chiang, Alexander" , Matthew Wilcox , Greg KH , "rdreier@cisco.com" , "linux-kernel@vger.kernel.org" , "jbarnes@virtuousgeek.org" , "virtualization@lists.linux-foundation.org" , "kvm@vger.kernel.org" , "linux-pci@vger.kernel.org" , "mingo@elte.hu" To: Anthony Liguori Return-path: In-Reply-To: <491B5B97.2000407@codemonkey.ws> Sender: linux-pci-owner@vger.kernel.org List-Id: kvm.vger.kernel.org Anthony Liguori wrote: > I don't think it's established that PV/VF will have less latency than > using virtio-net. virtio-net requires a world switch to send a group > of packets. The cost of this (if it stays in kernel) is only a few > thousand cycles on the most modern processors. > > Using VT-d means that for every DMA fetch that misses in the IOTLB, > you potentially have to do four memory fetches to main memory. There > will be additional packet latency using VT-d compared to native, it's > just not known how much at this time. If the IOTLB has intermediate TLB entries like the processor, we're talking just one or two fetches. That's a lot less than the cacheline bouncing that virtio and kvm interrupt injection incur right now. -- error compiling committee.c: too many arguments to function