[dpdk-dev] dpdk/vhost-user and VM migration

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

Hello!
I am currently looking at how using dpdk on host, accessing VM memory
using the vhost-user interface, interacts with VM migration.

The issue is that any changes made to VM memory need to be tracked so
that updates can be sent from migration source to destination.

At the moment, there's a proposal of an interface extension to
vhost-user which adds ability to do this tracking through shared memory.
dpdk would then be responsible for tracking these updates using atomic
operations to set bits (per page written) in a memory bitmap.

This only needs to happen during migration, at other times there could
be a jump to skip this logging.

Is this a reasonable approach? Would performance degradation during
migration associated with atomics affect the performance to a level
where it's no longer useful?  Pls note these logs aren't latency
sensitive, so can be done on a separate core, and can be batched.


One alternative I'm considering is extending linux kernel so it can do
this tracking automatically, by marking pages read-only, detecting a
pagefault and logging the write, then making the pages writeable.  This
would mean higher worst-case overhead (pagefaults are expensive) but
lower average one (not extra code after the first fault).  Not sure how
feasible this is yet, this would be harder to implement and it will only
be apply to newer host kernels.

Any feedback would be appreciated.

-- 
MST