Re: [PATCH 2/9] IB: add a proper completion queue abstraction

From: Caitlin Bestler
Date: Mon Nov 23 2015 - 21:42:41 EST

On 11/23/2015 4:00 PM, Jason Gunthorpe wrote:
On Mon, Nov 23, 2015 at 03:30:42PM -0800, Caitlin Bestler wrote:
The receive completion can be safely assumed to indicate transmit
completion over a reliable connection unless your peer has gone
completely bonkers and is replying to a command that it did not
Perhaps iWarp is different and does specify this ordering but IB does

The issue with IB is how the ACK protocol is designed. There is not
strong ordering between ACKs and data transfers. A HCA can send
ACK,DATA and the network could drop the ACK. The recevier side does
not know the ACK was lost and goes ahead to process DATA.

Since only ACK advances the sendq and DATA advances the recvq it is
trivial to get a case where the recvq is advanced with a reply while
the sendq continues to wait for the ACK to be resent.

Further IB allows ACK coalescing and has no rules for how an ACK is
placed. It is entirely valid for a HCA to RECV,REPLY,ACK - for

Is it possible for an IB HCA to transmit a response on a QP and not in that packet
or a previous packet acknowledge something that it has delivered to the user?

My recollection of the IB verbs is that they were unlikely to have overlooked something
like that. If it did slip through then there should be an errata.

But regardless of specification lawyering, is this an implementation issue. Are there
actual HCAs that make this mistake?

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