linux-kernel - Re: [RFC PATCH net-next v5 04/11] net/smc: implement some unsupported operations of loopback-ism

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Date: Thu, 04 Apr 2024 17:15:17 +0200
From: Niklas Schnelle <schnelle@...ux.ibm.com>
To: Wen Gu <guwen@...ux.alibaba.com>, Gerd Bayer <gbayer@...ux.ibm.com>,
        wintera@...ux.ibm.com, twinkler@...ux.ibm.com, hca@...ux.ibm.com,
        gor@...ux.ibm.com, agordeev@...ux.ibm.com, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
        wenjia@...ux.ibm.com, jaka@...ux.ibm.com
Cc: borntraeger@...ux.ibm.com, svens@...ux.ibm.com, alibuda@...ux.alibaba.com,
        tonylu@...ux.alibaba.com, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [RFC PATCH net-next v5 04/11] net/smc: implement some
 unsupported operations of loopback-ism

On Thu, 2024-04-04 at 21:12 +0800, Wen Gu wrote:
> 
> On 2024/4/4 19:42, Niklas Schnelle wrote:
> > On Thu, 2024-04-04 at 17:32 +0800, Wen Gu wrote:
> > > 
> > > On 2024/4/4 00:25, Gerd Bayer wrote:
> > > > On Sun, 2024-03-24 at 21:55 +0800, Wen Gu wrote:
> > > > > This implements some operations that loopback-ism does not support
> > > > > currently:
> > > > >    - vlan operations, since there is no strong use-case for it.
> > > > >    - signal_event operations, since there is no event to be processed
> > > > > by the loopback-ism device.
> > > > 
> > > > Hi Wen,
> > > > 
> > > > I wonder if the these operations that are not supported by loopback-ism
> > > > should rather be marked "optional" in the struct smcd_ops, and the
> > > > calling code should call these only when they are implemented.
> > > > 
> > > > Of course this would mean more changes to net/smc/smc_core.c - but
> > > > loopback-ism could omit these "boiler-plate" functions.
> > > > 
> > > 
> > > Hi Gerd.
> > > 
> > > Thank you for the thoughts! I agree that checks like 'if(smcd->ops->xxx)'
> > > can avoid the device driver from implementing unsupported operations. But I
> > > am afraid that which operations need to be defined as 'optional' may differ
> > > from different device perspectives (e.g. for loopback-ism they are vlan-related
> > > opts and signal_event). So I perfer to simply let the smc protocol assume
> > > that all operations have been implemented, and let drivers to decide which
> > > ones are unsupported in implementation. What do you think?
> > > 
> > > Thanks!
> > > 
> > 
> > I agree with Gerd, in my opinion it is better to document ops as
> > optional and then allow their function pointers to be NULL and check
> > for that. Acting like they are supported and then they turn out to be
> > nops to me seems to contradict the principle of least surprises. I also
> > think we can find a subset of mandatory ops without which SMC-D is
> > impossible and then everything else should be optional.
> 
> I see. If we all agree to classify smcd_ops into mandatory and optional ones,
> I'll add a patch to mark the optional ops and check if they are implemented.

Keep in mind I don't speak for the SMC maintainers but that does sound
reasonable to me.

> 
> > 
> > As a first guess I think the following options may be mandatory:
> > 
> > * query_remote_gid()
> > * register_dmb()/unregister_dmb()
> > * move_data()
> >    For this one could argue that either move_data() or
> >    attach_dmb()/detach_dmb() is required though personally I would
> >    prefer to always have move_data() as a fallback and simple API
> > * supports_v2()
> > * get_local_gid()
> > * get_chid()
> > * get_dev()
> I agree with this classification. Just one point, maybe we can take
> supports_v2() as an optional ops, like support_dmb_nocopy()? e.g. if
> it is not implemented, we treat it as an ISMv1.
> 
> Thanks!

Interpreting a NULL supports_v2() as not supporting v2 sounds
reasonable to me.