* Michael S. Tsirkin (mst@xxxxxxxxxx) wrote: > Here are some thoughts on bits that are still missing to get a working > virtio-rdma, with some suggestions. These are very preliminary but I > feel I kept these in my head (and discussed offline) for too long. All > of the below is just my personal humble opinion. > > Feature Requirements: > > The basic requirement is to be able to do RDMA to/from > VM memory, with support for VM migration and/or memory > overcommit and/or autonuma and/or THP. > Why are migration/overcommit/autonuma required? > Without these, you can do RDMA with device passthrough, > with likely better performance. Is this solution usable on a system without host-RDMA hardware? i.e. just to run RDMA between two VMs on the same host without using something like SoftROCE on the host? > Feature Non-requirements: > > It's not a requirement to support RDMA without VM exits, > e.g. like with device passthrough. While avoiding exits improves > performance, it would be handy to more than RDMA, > so there seems no reason to require it from RDMA when we > do not have it for e.g. network. > > Assumptions: > > It's OK to assume specific hardware capabilities at least initially. > > High level architecture: > > Follows the same lines as most other virtio devices: > > +----------------------------------- > + > + guest kernel > + ^ > +-------------|---------------------- > + v > + host kernel (kvm, vhost) > + > + ^ > +-------------|---------------------- > + v > + > + host userspace (QEMU, vhost-user) > + > +----------------------------------- > > Each request is forwarded by host kernel to QEMU, > that executes it using the ibverbs library. Should that be 'forwarded by guest kernel' ? Is there a guest-userspace here as well - most of the RDMA NICs seem to have a userspace component. > Most of this should be implementable host-side using existing > software. However, several issues remain and would need > infrastructure changes, as outlined below. > > Host-side todo list for virtio-rdma support: > > - Memory registration for guest userspace. > > Register memory region verb accepts a single virtual address, > which supplies both the on-wire key for access and the > range of memory to access. Guest kernel turns this into a > list of pages (e.g. by get_user_pages); when forwarded to host this > turns into a s/g list of virtual addresses in QEMU address space. > > Suggestion: add a new verb, along the lines of ibv_register_physical, > which splits these two parameters, accepting the on-wire VA key > and separately a list of userspace virtual address/size pairs. > > - Memory registration for guest kernels. > > Another ability used by some in-kernel users is registering all of memory. > Ranges not actually present are never accessed - this is OK as > kernel users are trusted. Memory hotplug changes which ranges > are present. > > Suggestion: add some throw-away memory and map all > non-present ranges there. Add ibv_reregister_physical_mr or similar > API to update mappings on guest memory hotplug/unplug. > > - Memory overcommit/autonuma/THP. > > This includes techniques such as swap,KSM,COW, page migration. > All these rely on ability to move pages around without > breaking hardware access. > > Suggestion: for hardware that supports it, > enabling on-demand paging for all registered memory seems > to address the issue more or less transparently to guests. > This isn't supported by all hardware but might be > at least a reasonable first step. > > - Migration: memory tracking. > > Migration requires detecting hardware access to pages > either on write (pre-copy) or any access (post-copy). > Post copy just requires ODP support to work with > userfaultfd properly. Can you explain what ODP support is? > Pre-copy would require a write-tracking API along > the lines of one exposed by KVM or vhost. > Each tracked page would be write-protected (causing faults on > hardware access) on hardware write fault is generated > and recorded, page is made writeable. Can you write-protect like that from the RDMA hardware? I'd be surprised if the hardware was happy with that. > - Migration: moving QP numbers. > > QP numbers are exposed on the wire and so must move together > with the VM. > > Suggestion: allow specifying QP number when creating a QP. > To avoid conflicts between multiple users, initial version can limit > library to a single user per device. Multiple VMs can simply > attach to distinct VFs. > > - Migration: moving QP state. > > When migrating the VM, a QP has to be torn down > on source and created on destination. > We have to migrate e.g. the current PSN - but what > should happen when a new packet arrives on source > after QP has been torn down? > > Suggestion 1: move QP to a special state "suspended" and ignore > packets, or cause source to retransmit with e.g. an out of > resources error. Retransmit counter might need to be > adjusted compared to what guest requested to account > for the extra need to retransmit. > Is there a good existing QP state that does this? > > Suggestion 2: forward packets to destination somehow. > Might overload the fabric as we are crossing e.g. > pci bus multiple times. > > - Migration: network update > > ROCE v1 and infiniband seem to tie connections to > hardware specific GIDs which can not be moved by software. > > Suggestion: limit migration to RoCE v2 initially. > > - Migration: packet loss recovery. > > As a RoCE address moves across the network, network has > to be updated which takes time, meanwhile packet loss seems > to be hard to avoid. > > Suggestion: limit initial support to hardware that is > able to recover from occasional packet drops, with > some slowdown. > > - Migration: suspend/resume API? > It might be easier to pack up state of all resources > such as all QP numbers and state of all QPs etc > in a single memory buffer, migrate then unpack on destination. > > Removes need for 2 separate APIs for suspended state and > for specifying QPN on creation. > > This creates a format for serialization that will have to > be maintained in a compatible way - it is not clear that > the maintainance overhead is worth the potential > simplification, if any. > > > That's it - I hope this helps, feel free to discuss, preferably copying > virtio-dev (subscription required for now, people are looking into > fixing this, sorry about that). Dave > Thanks! > > -- > MST > -- Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK -- To unsubscribe from this list: send the line "unsubscribe linux-rdma" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
