On Tue, Sep 08, 2009 at 10:20:35AM -0700, Ira W. Snyder wrote: > On Mon, Sep 07, 2009 at 01:15:37PM +0300, Michael S. Tsirkin wrote: > > On Thu, Sep 03, 2009 at 11:39:45AM -0700, Ira W. Snyder wrote: > > > On Thu, Aug 27, 2009 at 07:07:50PM +0300, Michael S. Tsirkin wrote: > > > > What it is: vhost net is a character device that can be used to reduce > > > > the number of system calls involved in virtio networking. > > > > Existing virtio net code is used in the guest without modification. > > > > > > > > There's similarity with vringfd, with some differences and reduced scope > > > > - uses eventfd for signalling > > > > - structures can be moved around in memory at any time (good for migration) > > > > - support memory table and not just an offset (needed for kvm) > > > > > > > > common virtio related code has been put in a separate file vhost.c and > > > > can be made into a separate module if/when more backends appear. I used > > > > Rusty's lguest.c as the source for developing this part : this supplied > > > > me with witty comments I wouldn't be able to write myself. > > > > > > > > What it is not: vhost net is not a bus, and not a generic new system > > > > call. No assumptions are made on how guest performs hypercalls. > > > > Userspace hypervisors are supported as well as kvm. > > > > > > > > How it works: Basically, we connect virtio frontend (configured by > > > > userspace) to a backend. The backend could be a network device, or a > > > > tun-like device. In this version I only support raw socket as a backend, > > > > which can be bound to e.g. SR IOV, or to macvlan device. Backend is > > > > also configured by userspace, including vlan/mac etc. > > > > > > > > Status: > > > > This works for me, and I haven't see any crashes. > > > > I have done some light benchmarking (with v4), compared to userspace, I > > > > see improved latency (as I save up to 4 system calls per packet) but not > > > > bandwidth/CPU (as TSO and interrupt mitigation are not supported). For > > > > ping benchmark (where there's no TSO) troughput is also improved. > > > > > > > > Features that I plan to look at in the future: > > > > - tap support > > > > - TSO > > > > - interrupt mitigation > > > > - zero copy > > > > > > > > > > Hello Michael, > > > > > > I've started looking at vhost with the intention of using it over PCI to > > > connect physical machines together. > > > > > > The part that I am struggling with the most is figuring out which parts > > > of the rings are in the host's memory, and which parts are in the > > > guest's memory. > > > > All rings are in guest's memory, to match existing virtio code. > > Ok, this makes sense. > > > vhost > > assumes that the memory space of the hypervisor userspace process covers > > the whole of guest memory. > > Is this necessary? Why? Because with virtio ring can give us arbitrary guest addresses. If guest was limited to using a subset of addresses, hypervisor would only have to map these. > The assumption seems very wrong when you're > doing data transport between two physical systems via PCI. > I know vhost has not been designed for this specific situation, but it > is good to be looking toward other possible uses. > > > And there's a translation table. > > Ring addresses are userspace addresses, they do not undergo translation. > > > > > If I understand everything correctly, the rings are all userspace > > > addresses, which means that they can be moved around in physical memory, > > > and get pushed out to swap. > > > > Unless they are locked, yes. > > > > > AFAIK, this is impossible to handle when > > > connecting two physical systems, you'd need the rings available in IO > > > memory (PCI memory), so you can ioreadXX() them instead. To the best of > > > my knowledge, I shouldn't be using copy_to_user() on an __iomem address. > > > Also, having them migrate around in memory would be a bad thing. > > > > > > Also, I'm having trouble figuring out how the packet contents are > > > actually copied from one system to the other. Could you point this out > > > for me? > > > > The code in net/packet/af_packet.c does it when vhost calls sendmsg. > > > > Ok. The sendmsg() implementation uses memcpy_fromiovec(). Is it possible > to make this use a DMA engine instead? Maybe. > I know this was suggested in an earlier thread. Yes, it might even give some performance benefit with e.g. I/O AT. > > > Is there somewhere I can find the userspace code (kvm, qemu, lguest, > > > etc.) code needed for interacting with the vhost misc device so I can > > > get a better idea of how userspace is supposed to work? > > > > Look in archives for kvm@xxxxxxxxxxxxxxxx the subject is qemu-kvm: vhost net. > > > > > (Features > > > negotiation, etc.) > > > > > > > That's not yet implemented as there are no features yet. I'm working on > > tap support, which will add a feature bit. Overall, qemu does an ioctl > > to query supported features, and then acks them with another ioctl. I'm > > also trying to avoid duplicating functionality available elsewhere. So > > that to check e.g. TSO support, you'd just look at the underlying > > hardware device you are binding to. > > > > Ok. Do you have plans to support the VIRTIO_NET_F_MRG_RXBUF feature in > the future? I found that this made an enormous improvement in throughput > on my virtio-net <-> virtio-net system. Perhaps it isn't needed with > vhost-net. Yes, I'm working on it. > Thanks for replying, > Ira -- 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
