Re: [Qemu-devel] host side todo list for virtio rdma

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jul 19, 2017 at 11:55:50AM +0100, Dr. David Alan Gilbert wrote:
> * Michael S. Tsirkin (mst@redhat.com) 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?

Hacks could be implemented to enable this. But IMHO this
is yes another thing that should be a follow-up.
Just like e.g. KVM, let's focus on capable hardware
as the 1st step.


> > 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' ?

No I really mean the host: we get requests from guest, they land in host
kernel same as any exit.

> Is there a guest-userspace here as well - most of the
> RDMA NICs seem to have a userspace component.

Good point, I think you are right, there is. Bypassing
guest kernel for data path requests seems like a reasonable
requirement to add.


> > 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?

On demand paging. grep for odp and ODP in libibverbs sources.

> >   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.

With ODP capable hardware I think you should be able to.

> > - 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@redhat.com / Manchester, UK