On 05/11/2010 02:59 AM, Avi Kivity wrote:
(Replying again to list)
What data structure would you use? For a lockless ring queue, you
can only support a single producer and consumer. To achieve
bidirectional communication in virtio, we always use two queues.
You don't have to use a lockless ring queue. You can use locks
(spinlocks without interrupt support, full mutexes with interrupts)
and any data structure you like. Say a hash table + LRU for a shared
cache.
Yeah, the mailslot enables this.
I think the question boils down to whether we can support transparent
peer connections and disconnections. I think that's important in order
to support transparent live migration.
If you have two peers that are disconnected and then connect to each
other, there's simply no way to choose who's content gets preserved.
It's necessary to designate one peer as a master in order to break the tie.
So this could simply involve an additional option to the shared memory
driver: role=master|peer. If role=master, when a new shared memory
segment is mapped, the contents of the BAR ram is memcpy()'d to the
shared memory segment. In either case, the contents of the shared
memory segment should be memcpy()'d to the BAR ram whenever the shared
memory segment is disconnected.
I believe role=master should be default because I think a relationship
of master/slave is going to be much more common than peering.
If you're adding additional queues to support other levels of
communication, you can always use different areas of shared memory.
You'll need O(n^2) shared memory areas (n=peer count), and it is a lot
less flexible that real shared memory. Consider using threading where
the only communication among threads is a pipe (erlang?)
I can't think of a use of multiple peers via shared memory today with
virtualization. I know lots of master/slave uses of shared memory
though. I agree that it's useful to support from an academic
perspective but I don't believe it's going to be the common use.
Regards,
Anthony Liguori
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