Hi Michael, Looks like the discussions tapered off, but do you have a plan to implement this if people are eventually fine with it? We want to extend this to support multiple VMs. On Mon, Aug 31, 2015 at 11:35 AM, Nakajima, Jun <jun.nakajima@xxxxxxxxx> wrote: > On Mon, Aug 31, 2015 at 7:11 AM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: >> Hello! >> During the KVM forum, we discussed supporting virtio on top >> of ivshmem. I have considered it, and came up with an alternative >> that has several advantages over that - please see below. >> Comments welcome. > > Hi Michael, > > I like this, and it should be able to achieve what I presented at KVM > Forum (vhost-user-shmem). > Comments below. > >> >> ----- >> >> Existing solutions to userspace switching between VMs on the >> same host are vhost-user and ivshmem. >> >> vhost-user works by mapping memory of all VMs being bridged into the >> switch memory space. >> >> By comparison, ivshmem works by exposing a shared region of memory to all VMs. >> VMs are required to use this region to store packets. The switch only >> needs access to this region. >> >> Another difference between vhost-user and ivshmem surfaces when polling >> is used. With vhost-user, the switch is required to handle >> data movement between VMs, if using polling, this means that 1 host CPU >> needs to be sacrificed for this task. >> >> This is easiest to understand when one of the VMs is >> used with VF pass-through. This can be schematically shown below: >> >> +-- VM1 --------------+ +---VM2-----------+ >> | virtio-pci +-vhost-user-+ virtio-pci -- VF | -- VFIO -- IOMMU -- NIC >> +---------------------+ +-----------------+ >> >> >> With ivshmem in theory communication can happen directly, with two VMs >> polling the shared memory region. >> >> >> I won't spend time listing advantages of vhost-user over ivshmem. >> Instead, having identified two advantages of ivshmem over vhost-user, >> below is a proposal to extend vhost-user to gain the advantages >> of ivshmem. >> >> >> 1: virtio in guest can be extended to allow support >> for IOMMUs. This provides guest with full flexibility >> about memory which is readable or write able by each device. > > I assume that you meant VFIO only for virtio by "use of VFIO". To get > VFIO working for general direct-I/O (including VFs) in guests, as you > know, we need to virtualize IOMMU (e.g. VT-d) and the interrupt > remapping table on x86 (i.e. nested VT-d). > >> By setting up a virtio device for each other VM we need to >> communicate to, guest gets full control of its security, from >> mapping all memory (like with current vhost-user) to only >> mapping buffers used for networking (like ivshmem) to >> transient mappings for the duration of data transfer only. > > And I think that we can use VMFUNC to have such transient mappings. > >> This also allows use of VFIO within guests, for improved >> security. >> >> vhost user would need to be extended to send the >> mappings programmed by guest IOMMU. > > Right. We need to think about cases where other VMs (VM3, etc.) join > the group or some existing VM leaves. > PCI hot-plug should work there (as you point out at "Advantages over > ivshmem" below). > >> >> 2. qemu can be extended to serve as a vhost-user client: >> remote VM mappings over the vhost-user protocol, and >> map them into another VM's memory. >> This mapping can take, for example, the form of >> a BAR of a pci device, which I'll call here vhost-pci - >> with bus address allowed >> by VM1's IOMMU mappings being translated into >> offsets within this BAR within VM2's physical >> memory space. > > I think it's sensible. > >> >> Since the translation can be a simple one, VM2 >> can perform it within its vhost-pci device driver. >> >> While this setup would be the most useful with polling, >> VM1's ioeventfd can also be mapped to >> another VM2's irqfd, and vice versa, such that VMs >> can trigger interrupts to each other without need >> for a helper thread on the host. >> >> >> The resulting channel might look something like the following: >> >> +-- VM1 --------------+ +---VM2-----------+ >> | virtio-pci -- iommu +--+ vhost-pci -- VF | -- VFIO -- IOMMU -- NIC >> +---------------------+ +-----------------+ >> >> comparing the two diagrams, a vhost-user thread on the host is >> no longer required, reducing the host CPU utilization when >> polling is active. At the same time, VM2 can not access all of VM1's >> memory - it is limited by the iommu configuration setup by VM1. >> >> >> Advantages over ivshmem: >> >> - more flexibility, endpoint VMs do not have to place data at any >> specific locations to use the device, in practice this likely >> means less data copies. >> - better standardization/code reuse >> virtio changes within guests would be fairly easy to implement >> and would also benefit other backends, besides vhost-user >> standard hotplug interfaces can be used to add and remove these >> channels as VMs are added or removed. >> - migration support >> It's easy to implement since ownership of memory is well defined. >> For example, during migration VM2 can notify hypervisor of VM1 >> by updating dirty bitmap each time is writes into VM1 memory. > > Also, the ivshmem functionality could be implemented by this proposal: > - vswitch (or some VM) allocates memory regions in its address space, and > - it sets up that IOMMU mappings on the VMs be translated into the regions > >> >> Thanks, >> >> -- >> MST -- Jun Intel Open Source Technology Center _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
