linux-kernel - Re: [RFC PATCH net-next v4 0/9] net/smc: Introduce SMC-D-based OS internal communication acceleration

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Date:   Thu, 6 Apr 2023 13:14:28 +0200
From:   Alexandra Winter <wintera@...ux.ibm.com>
To:     Niklas Schnelle <schnelle@...ux.ibm.com>,
        Wen Gu <guwen@...ux.alibaba.com>, kgraul@...ux.ibm.com,
        wenjia@...ux.ibm.com, jaka@...ux.ibm.com, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com
Cc:     linux-s390@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH net-next v4 0/9] net/smc: Introduce SMC-D-based OS
 internal communication acceleration

On 05.04.23 19:04, Niklas Schnelle wrote:
> One more question though, what about the SEID why does that have to be
> fixed and at least partially match what ISM devices use? I think I'm
> missing some SMC protocol/design detail here. I'm guessing this would
> require a protocol change?
> 
> Thanks,
> Niklas

Niklas,
in the initial SMC CLC handshake the client and server exchange the SEID (one per peer system)
and up to 8 proposals for SMC-D interfaces.
Wen's current proposal assumes that smc-d loopback can be one of these 8 proposed interfaces,
iiuc. So on s390 the proposal can contain ISM devices and a smc-d loopback device at the same time.
If one of the peers is e.g. an older Linux version, it will just ignore the loopback-device
in the list (Don't find a match for CHID 0xFFFF) and use an ISM interface for SMC-D if possible.
Therefor it is important that the SEID is used in the same way as it is today in the handshake.

If we decide for some reason (virtio-ism open issues?) that a protocol change/extension is
required/wanted, then it is a new game and we can come up with new identifiers, but we may
lose compatibility to backlevel systems.

Alexandra