netdev - Re: [RFC net-next 0/7] Provide an ism layer

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Message-ID: <17f0cbf9-71f7-4cce-93d2-522943d9d83b@linux.ibm.com>
Date: Mon, 20 Jan 2025 11:28:41 +0100
From: Alexandra Winter <wintera@...ux.ibm.com>
To: dust.li@...ux.alibaba.com, Julian Ruess <julianr@...ux.ibm.com>,
        Wenjia Zhang <wenjia@...ux.ibm.com>, Jan Karcher <jaka@...ux.ibm.com>,
        Gerd Bayer <gbayer@...ux.ibm.com>, Halil Pasic <pasic@...ux.ibm.com>,
        "D. Wythe" <alibuda@...ux.alibaba.com>,
        Tony Lu <tonylu@...ux.alibaba.com>, Wen Gu <guwen@...ux.alibaba.com>,
        Peter Oberparleiter
 <oberpar@...ux.ibm.com>,
        David Miller <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, Eric Dumazet <edumazet@...gle.com>,
        Andrew Lunn <andrew+netdev@...n.ch>
Cc: Niklas Schnelle <schnelle@...ux.ibm.com>,
        Thorsten Winkler <twinkler@...ux.ibm.com>, netdev@...r.kernel.org,
        linux-s390@...r.kernel.org, Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev
 <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>, Simon Horman <horms@...nel.org>
Subject: Re: [RFC net-next 0/7] Provide an ism layer



On 17.01.25 14:00, Alexandra Winter wrote:
> 
> 
> On 17.01.25 03:13, Dust Li wrote:
>>>>> Modular Approach: I've made the ism_loopback an independent kernel
>>>>> module since dynamic enable/disable functionality is not yet supported
>>>>> in SMC. Using insmod and rmmod for module management could provide the
>>>>> flexibility needed in practical scenarios.
>>>
>>> With this proposal ism_loopback is just another ism device and SMC-D will
>>> handle removal just like ism_client.remove(ism_dev) of other ism devices.
>>>
>>> But I understand that net/smc/ism_loopback.c today does not provide enable/disable,
>>> which is a big disadvantage, I agree. The ism layer is prepared for dynamic
>>> removal by ism_dev_unregister(). In case of this RFC that would only happen
>>> in case of rmmod ism. Which should be improved.
>>> One way to do that would be a separate ism_loopback kernel module, like you say.
>>> Today ism_loopback is only 10k LOC, so I'd be fine with leaving it in the ism module.
>>> I also think it is a great way for testing any ISM client, so it has benefit for
>>> anybody using the ism module.
>>> Another way would be e.g. an 'enable' entry in the sysfs of the loopback device.
>>> (Once we agree if and how to represent ism devices in genera in sysfs).
>> This works for me as well. I think it would be better to implement this
>> within the common ISM layer, rather than duplicating the code in each
>> device. Similar to how it's done in netdevice.
>>
>> Best regards,
>> Dust
> 
> 
> Is there a specific example for enable/disable in the netdevice code, you have in mind?
> Or do you mean in general how netdevice provides a common layer?
> Yes, everything that is common for all devices should be provided by the network layer.


Dust for some reason, you did not 'Reply-all':
Dust Li wrote:
> I think dev_close()/dev_open() are the high-level APIs, while
> ndo_stop()/ndo_open() are the underlying device operations that we
> can reference.


I hear you, it can be beneficial to have a way for upper layers to
enable/disable an ism device.
But all this is typically a tricky area. The device driver can also have
reasons to enable/disable a device, then hardware could do that or even
hotplug a device. Error recovery on different levels may want to run a
disable/enable sequence as a reset, etc. And all this has potential for
deadlocks.
All this is rather trivial for ism-loopback, as there is not much of a
lower layer.
ism-vpci already has 'HW' / device driver configure on/off and device
add/remove.
For a future ism-virtio, the Hipervisor may want to add/remove devices.

I wonder what could be the simplest definition of an enable/disable for
the ism layer, that we can start with? More sophisticated functionality
can always be added later.
Maybe support for add/remove ism-device by the device driver is
sufficient as  starting point?