linux-kernel - Re: [PATCH 3/4] cxl, acpi/hmat: Update CXL access coordinates directly instead of through HMAT

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Message-ID: <689e64229859f_50ce10082@dwillia2-xfh.jf.intel.com.notmuch>
Date: Thu, 14 Aug 2025 15:33:06 -0700
From: <dan.j.williams@...el.com>
To: Dave Jiang <dave.jiang@...el.com>, <linux-cxl@...r.kernel.org>,
	<linux-acpi@...r.kernel.org>, <linux-kernel@...r.kernel.org>
CC: <gregkh@...uxfoundation.org>, <rafael@...nel.org>, <dakr@...nel.org>,
	<dave@...olabs.net>, <jonathan.cameron@...wei.com>,
	<alison.schofield@...el.com>, <vishal.l.verma@...el.com>,
	<ira.weiny@...el.com>, <dan.j.williams@...el.com>,
	<marc.herbert@...ux.intel.com>, <akpm@...ux-foundation.org>,
	<david@...hat.com>
Subject: Re: [PATCH 3/4] cxl, acpi/hmat: Update CXL access coordinates
 directly instead of through HMAT

Dave Jiang wrote:
> The current implementation of CXL memory hotplug notifier gets called
> before the HMAT memory hotplug notifier. The CXL driver calculates the
> access coordinates (bandwidth and latency values) for the CXL end to
> end path (i.e. CPU to endpoint). When the CXL region is onlined, the CXL
> memory hotplug notifier writes the access coordinates to the HMAT target
> structs. Then the HMAT memory hotplug notifier is called and it creates
> the access coordinates for the node sysfs attributes.

Perhaps summarize quickly here the before and after of sysfs, so people
know if they are impacted by this bug, and backporters can verify they
fixed it?

> The original intent of the 'ext_updated' flag in HMAT handling code was to
> stop HMAT memory hotplug callback from clobbering the access coordinates
> after CXL has injected its calculated coordinates and replaced the generic
> target access coordinates provided by the HMAT table in the HMAT target
> structs. However the flag is hacky at best and blocks the updates from
> other CXL regions that are onlined in the same node later on. Remove the
> 'ext_updated' flag usage and just update the access coordinates for the
> nodes directly without touching HMAT target data.
> 
> The hotplug memory callback ordering is changed. Instead of changing CXL,
> move HMAT back so there's room for the levels rather than have CXL share
> the same level as SLAB_CALLBACK_PRI. The change will resulting in the CXL
> callback to be executed after the HMAT callback.
> 
> With the change, the CXL hotplug memory notifier runs after the HMAT
> callback. The HMAT callback will create the node sysfs attributes for
> access coordinates. The CXL callback will write the access coordinates to
> the now created node sysfs attributes directly and will not pollute the
> HMAT target values.
> 
> Fixes: debdce20c4f2 ("cxl/region: Deal with numa nodes not enumerated by SRAT")

Why that one and not?

067353a46d8c cxl/region: Add memory hotplug notifier for cxl region

It is the ext_updated machinery that is the main problem that messes up
sysfs, right?

...and per the backport concern this should be cc: stable as well.

Other than that you can add:

Reviewed-by: Dan Williams <dan.j.williams@...el.com>