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Message-ID: <1f05ccef-5b45-4eac-b3ca-588b1e5ec6f5@intel.com>
Date: Thu, 7 Nov 2024 08:23:28 -0700
From: Dave Jiang <dave.jiang@...el.com>
To: Dan Williams <dan.j.williams@...el.com>,
"Zhijian Li (Fujitsu)" <lizhijian@...itsu.com>,
Huaisheng Ye <huaisheng.ye@...el.com>,
"linux-cxl@...r.kernel.org" <linux-cxl@...r.kernel.org>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"pei.p.jia@...el.com" <pei.p.jia@...el.com>
Subject: Re: [PATCH] [RFC] cxl/region: Fix region creation for greater than x2
switches
On 11/6/24 6:11 PM, Dan Williams wrote:
> [ add Dave so you
>
> Zhijian Li (Fujitsu) wrote:
>>
>>
>> On 27/10/2024 15:57, Huaisheng Ye wrote:
>>> The cxl_port_setup_targets() algorithm fails to identify valid target list
>>> ordering in the presence of 4-way and above switches resulting in
>>> 'cxl create-region' failures of the form:
>>>
>>> # cxl create-region -d decoder0.0 -g 1024 -s 2G -t ram -w 8 -m mem4 mem1 mem6 mem3 mem2 mem5 mem7 mem0
>>> cxl region: create_region: region0: failed to set target7 to mem0
>>> cxl region: cmd_create_region: created 0 regions
>>>
>>> [kernel debug message]
>>> check_last_peer:1213: cxl region0: pci0000:0c:port1: cannot host mem6:decoder7.0 at 2
>>> bus_remove_device:574: bus: 'cxl': remove device region0
>>>
>>> QEMU can create this failing topology:
>>>
>>> ACPI0017:00 [root0]
>>> |
>>> HB_0 [port1]
>>> / \
>>> RP_0 RP_1
>>> | |
>>> USP [port2] USP [port3]
>>> / / \ \ / / \ \
>>> DSP DSP DSP DSP DSP DSP DSP DSP
>>> | | | | | | | |
>>> mem4 mem6 mem2 mem7 mem1 mem3 mem5 mem0
>>> Pos: 0 2 4 6 1 3 5 7
>>
>> Yeah, I tried this topology long long ago, it didn't work. At that time, I thought it
>> might be just like that. Until recently that I saw this [1] in section
>> 2.13.15.1 Region Spanning 2 HB Root Ports Example Configuration Checks
>>
>> I once tried to understand why the code used "distance" to determine the order of the target,
>> but in the end, I still couldn't figure it out (and I still don't understand it now).
>> IIRC, neither the CXL spec nor this document mentioned the keyword "distance" at all.
>
> Oh, that means this needs a comment or a better variable name.
>
> In this patch discussion [1] Jim came up with the term "ancestral_ways"
> to describe the same concept of what is the offset ("distance") between
> consecutive indices in the target list.
>
> [1]: http://lore.kernel.org/ZUHeTLZb+od8q4EE@ubuntu
>
> Does "ancestral_ways" more clearly convey the math that is being
> performed at each level of the hierarchy?
>
> Now, "ancestral_ways" also does not show up in the CXL specification,
> but that is because the CXL specification leaves at as an exercise for
> software to figure out an algorithm to validate that a proposed ordering
> of memory-device-decoders in a region can be supported by the given CXL
> topology.
>
> In the meantime I have flagged this patch to Dave for consideration in
> the next cxl-fixes pull request, but I suspect it will need to be
> updated with a comment and/or rename to resovle the "distance"
> confusion.
>
> https://patchwork.kernel.org/bundle/cxllinux/cxl-fixes/
If we can get it respin and tagged by next week, there's time to get it into the 6.13 merge window. Otherwise it can wait until 6.13-rc fixes.
>
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