Am 2015-04-27 um 15:01 schrieb Stefan Hajnoczi: > On Mon, Apr 27, 2015 at 1:55 PM, Jan Kiszka <jan.kiszka@xxxxxxxxxxx> wrote: >> Am 2015-04-27 um 14:35 schrieb Jan Kiszka: >>> Am 2015-04-27 um 12:17 schrieb Stefan Hajnoczi: >>>> On Sun, Apr 26, 2015 at 2:24 PM, Luke Gorrie <luke@xxxxxxxx> wrote: >>>>> On 24 April 2015 at 15:22, Stefan Hajnoczi <stefanha@xxxxxxxxx> wrote: >>>>>> >>>>>> The motivation for making VM-to-VM fast is that while software >>>>>> switches on the host are efficient today (thanks to vhost-user), there >>>>>> is no efficient solution if the software switch is a VM. >>>>> >>>>> >>>>> I see. This sounds like a noble goal indeed. I would love to run the >>>>> software switch as just another VM in the long term. It would make it much >>>>> easier for the various software switches to coexist in the world. >>>>> >>>>> The main technical risk I see in this proposal is that eliminating the >>>>> memory copies might not have the desired effect. I might be tempted to keep >>>>> the copies but prevent the kernel from having to inspect the vrings (more >>>>> like vhost-user). But that is just a hunch and I suppose the first step >>>>> would be a prototype to check the performance anyway. >>>>> >>>>> For what it is worth here is my view of networking performance on x86 in the >>>>> Haswell+ era: >>>>> https://groups.google.com/forum/#!topic/snabb-devel/aez4pEnd4ow >>>> >>>> Thanks. >>>> >>>> I've been thinking about how to eliminate the VM <-> host <-> VM >>>> switching and instead achieve just VM <-> VM. >>>> >>>> The holy grail of VM-to-VM networking is an exitless I/O path. In >>>> other words, packets can be transferred between VMs without any >>>> vmexits (this requires a polling driver). >>>> >>>> Here is how it works. QEMU gets "-device vhost-user" so that a VM can >>>> act as the vhost-user server: >>>> >>>> VM1 (virtio-net guest driver) <-> VM2 (vhost-user device) >>>> >>>> VM1 has a regular virtio-net PCI device. VM2 has a vhost-user device >>>> and plays the host role instead of the normal virtio-net guest driver >>>> role. >>>> >>>> The ugly thing about this is that VM2 needs to map all of VM1's guest >>>> RAM so it can access the vrings and packet data. The solution to this >>>> is something like the Shared Buffers BAR but this time it contains not >>>> just the packet data but also the vring, let's call it the Shared >>>> Virtqueues BAR. >>>> >>>> The Shared Virtqueues BAR eliminates the need for vhost-net on the >>>> host because VM1 and VM2 communicate directly using virtqueue notify >>>> or polling vring memory. Virtqueue notify works by connecting an >>>> eventfd as ioeventfd in VM1 and irqfd in VM2. And VM2 would also have >>>> an ioeventfd that is irqfd for VM1 to signal completions. >>> >>> We had such a discussion before: >>> http://thread.gmane.org/gmane.comp.emulators.kvm.devel/123014/focus=279658 >>> >>> Would be great to get this ball rolling again. >>> >>> Jan >>> >> >> But one challenge would remain even then (unless both sides only poll): >> exit-free inter-VM signaling, no? But that's a hardware issue first of all. > > To start with ioeventfd<->irqfd can be used. It incurs a light-weight > exit in VM1 and interrupt injection in VM2. > > For networking the cost is mitigated by NAPI drivers which switch > between interrupts and polling. During notification-heavy periods the > guests would use polling anyway. > > A hardware solution would be some kind of inter-guest interrupt > injection. I don't know VMX well enough to know whether that is > possible on Intel CPUs. Today, we have posted interrupts to avoid the vm-exit on the target CPU, but there is nothing yet (to my best knowledge) to avoid the exit on the sender side (unless we ignore security). That's the same problem with intra-guest IPIs, BTW. For throughput and given NAPI patterns, that's probably not an issue as you noted. It may be for latency, though, when almost every cycle counts. Jan -- Siemens AG, Corporate Technology, CT RTC ITP SES-DE Corporate Competence Center Embedded Linux _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
