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Merge "Add release note for vDPA"

Zuul 1 year ago committed by Gerrit Code Review
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The libvirt driver has added support for hardware-offloaded OVS
with vDPA (vhost Data Path Acceleration) type interfaces.
vDPA allows virtio net interfaces to be presented to the guest while
the datapath can be offloaded to a software or hardware implementation.
This enables high performance networking with the portablity of standard
virtio interfaces.
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Nova currenly does not support the following livecycle operations when
combined with a instance using vDPA ports: shelve, resize, cold migration,
live migration, evacuate, suspend or interface attach/detach.
Attempting to use one of the above operations will result in a HTTP 409
(Conflict) error. While some operations like "resize to same host",
shelve or attach interface technically work, they have been blocked since
unshelve and detach interface currently do not. Resize to a different
host has been blocked since its untested, evacuate has also been blocked
for the same reason. These limitation may be removed in the future as
testing is improved. Live migration is currently not supported with vDPA
interfaces by QEMU and therefore cannot be enabled in openstack at this
Like SR-IOV, vDPA leverages DMA transfer between the guest and hardware.
This requires the DMA buffers to be locked in memory. As the DMA buffers
are allocated by the guest and can be allocated anywhere in the guest RAM,
QEMU locks **all** guest RAM. By default the ``RLIMIT_MEMLOCK`` for a
normal QEMU intance is set to 0 and qemu is not allowed to lock guest
memory. In the case of SR-IOV, libvirt automatically set the limit to guest
RAM + 1G which enables QEMU to lock the memory. This does not happen today
with vDPA ports. As a result if you use VDPA ports without enabling locking
of the guest memory you will get DMA errors. To workaround this issues
until libvirt is updated, you must set ``hw:cpu_realtime=yes`` and define a
valid ``CPU-REALTIME-MASK`` e.g ``hw:cpu_realtime_mask=^0`` or define
``hw:emulator_threads_policy=share|isolate``. Note that since we are just
using ``hw:cpu_realtime`` for its side-effect of locking the guest memory,
this usage does not require the guest or host to use realtime kernels.
However, all other requirements of ``hw:cpu_realtime`` such as requiring
hw:cpu_policy=dedicated still apply. It is also stongly recommended that
hugpages be enabled for all instnace with locked memory. This can be done
by setting ``hw:mem_page_size``. This will enable nova to correctly account
for the fact that the memory is unswapable.