netdev - Re: [PATCH v17 18/22] cxl: Allow region creation by type2 drivers

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Message-ID: <b4b5d1de-6682-4161-8c81-30cee0756a63@amd.com>
Date: Mon, 11 Aug 2025 15:45:54 +0100
From: Alejandro Lucero Palau <alucerop@....com>
To: dan.j.williams@...el.com, alejandro.lucero-palau@....com,
 linux-cxl@...r.kernel.org, netdev@...r.kernel.org, edward.cree@....com,
 davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
 edumazet@...gle.com, dave.jiang@...el.com
Cc: Jonathan Cameron <Jonathan.Cameron@...wei.com>
Subject: Re: [PATCH v17 18/22] cxl: Allow region creation by type2 drivers


On 8/5/25 17:33, dan.j.williams@...el.com wrote:
> alejandro.lucero-palau@ wrote:
>> From: Alejandro Lucero <alucerop@....com>
>>
>> Creating a CXL region requires userspace intervention through the cxl
>> sysfs files. Type2 support should allow accelerator drivers to create
>> such cxl region from kernel code.
>>
>> Adding that functionality and integrating it with current support for
>> memory expanders.
>>
>> Support an action by the type2 driver to be linked to the created region
>> for unwinding the resources allocated properly.
> The hardest part of CXL is the fact that typical straight-line driver
> expectations like "device present == MMIO available" are violated. An
> accelerator driver needs to worry about asynchronous region detach and
> CXL port detach.
>
> Ideally any event that takes down a CXL port or the region simply
> results in the accelerator driver being detached to clean everything up.
>
> The difficult part about that is that the remove path for regions and
> CXL ports hold locks that prevent the accelerator remove path from
> running.
>
> I do not think it is maintainable for every accelerator driver to invent
> its own cleanup scheme like this. The expectation should be that a
> region can go into a defunct state if someone triggers removal actions
> in the wrong order, but otherwise the accelerator driver should be able
> to rely on a detach event to clean everything up.
>
> So opting into CXL operation puts a driver into a situation where it can
> be unbound whenever the CXL link goes down logically or physically.
> Physical device removal of a CXL port expects that the operator has
> first shutdown all driver operations, and if they have not at least the
> driver should not crash while awaiting the remove event.
>
> Physical CXL port removal is the "easy" case since that will naturally
> result in the accelerator 'struct pci_dev' being removed. The more
> difficult cases are the logical removal / shutdown of a CXL port or
> region. Those should schedule accelerator detach and put the region into
> an error state until that cleanup runs.
>
> So, in summary, do not allow for custom region callbacks, arrange for
> accelerator detach and just solve the "fail in-flight operations while
> awaiting detach" problem.
>

This is similar to what I mentioned in patch 20: the idea is to handle 
cxl modules removal and not CXL errors.


But seizing on the error case, if the error is:


1) not fatal and related to CXL.mem: the sfc driver can keep working 
without the CXL PIO memory as a datapath.

2) not fatal and related to CXL.io: I see this like current pcie error 
handling.

3) fatal and CXL.mem related: probably nothing safe to do or too late

4) fatal and CXL.io: not sure if similar to 2 or to 3