Hi,
-----Original Message-----
https://lore.kernel.org/qemu-devel/CAMGffEn-DKpMZ4tA71MJYdyemg0Zda
15 > > wVAqk81vXtKzx-LfJQ@mail.gmail.com/ > > > > Appreciate a lot for everyone helping on the testings. > > > > > InfiniBand controller: Mellanox Technologies MT27800 > > > Family [ConnectX-5] > > > > > > which doesn't meet our purpose. I can choose RDMA or TCP > > > for VM migration. RDMA traffic is through InfiniBand and > > > TCP through Ethernet on these two hosts. One is standby > > > while the other is active. > > > > > > Now I'll try on a server with more recent Ethernet and > > > InfiniBand network adapters. One of them has: > > > BCM57414 NetXtreme-E 10Gb/25Gb RDMA Ethernet Controller > > > (rev > > > 01) > > > > > > The comparison between RDMA and TCP on the same NIC > > > could make more > > sense. > > > > It looks to me NICs are powerful now, but again as I > > mentioned I don't think it's a reason we need to deprecate > > rdma, especially if QEMU's rdma migration has the chance > > to be refactored using rsocket. > > > > Is there anyone who started looking into that direction? > > Would it make sense we start some PoC now? > > > > My team has finished the PoC refactoring which works well. > > Progress: > 1. Implement io/channel-rdma.c, 2. Add unit test > tests/unit/test-io-channel-rdma.c and verifying it is > successful, 3. Remove the original code from migration/rdma.c, 4. > Rewrite the rdma_start_outgoing_migration and > rdma_start_incoming_migration logic, 5. Remove all rdma_xxx > functions from migration/ram.c. (to prevent RDMA live > migration from polluting the core logic of live migration), 6. The soft-RoCE implemented by software is used to test the RDMA live migration. It's successful. > > We will be submit the patchset later.
That's great news, thank you!
-- Peter Xu
For rdma programming, the current mainstream implementation is to use rdma_cm to establish a connection, and then use verbs to transmit data.
rdma_cm and ibverbs create two FDs respectively. The two FDs have different responsibilities. rdma_cm fd is used to notify connection establishment events, and verbs fd is used to notify new CQEs. When poll/epoll monitoring is directly performed on the rdma_cm fd, only a pollin event can be monitored, which means that an rdma_cm event occurs. When the verbs fd is directly polled/epolled, only the pollin event can be listened, which indicates that a new CQE is generated.
Rsocket is a sub-module attached to the rdma_cm library and provides rdma calls that are completely similar to socket interfaces. However, this library returns only the rdma_cm fd for listening to link setup-related events and does not expose the verbs fd (readable and writable events for listening to data). Only the rpoll interface provided by the RSocket can be used to listen to related events. However, QEMU uses the ppoll interface to listen to the rdma_cm fd (gotten by raccept API). And cannot listen to the verbs fd event. I'm confused, the rs_poll_arm :https://github.com/linux-rdma/rdma-core/blob/master/librdmacm/rsocket.c# L3290 For STREAM, rpoll setup fd for both cq fd and cm fd.
Right. But the question is QEMU do not use rpoll but gilb's ppoll. :( Regards, -Gonglei