linux-kernel - Re: [PATCH v4 5/9] cxl/region, dax/hmem: Arbitrate Soft Reserved ownership with cxl_regions_fully

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Message-ID: <c1ddae30-688f-425e-abb0-b0fa55b5f37c@amd.com>
Date: Thu, 11 Dec 2025 15:42:12 -0800
From: "Koralahalli Channabasappa, Smita" <skoralah@....com>
To: dan.j.williams@...el.com,
 Smita Koralahalli <Smita.KoralahalliChannabasappa@....com>,
 linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org,
 nvdimm@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
 linux-pm@...r.kernel.org
Cc: Alison Schofield <alison.schofield@...el.com>,
 Vishal Verma <vishal.l.verma@...el.com>, Ira Weiny <ira.weiny@...el.com>,
 Jonathan Cameron <jonathan.cameron@...wei.com>,
 Yazen Ghannam <yazen.ghannam@....com>, Dave Jiang <dave.jiang@...el.com>,
 Davidlohr Bueso <dave@...olabs.net>, Matthew Wilcox <willy@...radead.org>,
 Jan Kara <jack@...e.cz>, "Rafael J . Wysocki" <rafael@...nel.org>,
 Len Brown <len.brown@...el.com>, Pavel Machek <pavel@...nel.org>,
 Li Ming <ming.li@...omail.com>, Jeff Johnson
 <jeff.johnson@....qualcomm.com>, Ying Huang <huang.ying.caritas@...il.com>,
 Yao Xingtao <yaoxt.fnst@...itsu.com>, Peter Zijlstra <peterz@...radead.org>,
 Greg KH <gregkh@...uxfoundation.org>,
 Nathan Fontenot <nathan.fontenot@....com>,
 Terry Bowman <terry.bowman@....com>, Robert Richter <rrichter@....com>,
 Benjamin Cheatham <benjamin.cheatham@....com>,
 Zhijian Li <lizhijian@...itsu.com>, Borislav Petkov <bp@...en8.de>,
 Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH v4 5/9] cxl/region, dax/hmem: Arbitrate Soft Reserved
 ownership with cxl_regions_fully_map()

On 12/2/2025 7:50 PM, dan.j.williams@...el.com wrote:
> Smita Koralahalli wrote:
>> Introduce cxl_regions_fully_map() to check whether CXL regions form a
>> single contiguous, non-overlapping cover of a given Soft Reserved range.
>>
>> Use this helper to decide whether Soft Reserved memory overlapping CXL
>> regions should be owned by CXL or registered by HMEM.
>>
>> If the span is fully covered by CXL regions, treat the Soft Reserved
>> range as owned by CXL and have HMEM skip registration. Else, let HMEM
>> claim the range and register the corresponding devdax for it.
> 
> This all feels a bit too custom when helpers like resource_contains()
> exist.
> 
> Also remember that the default list of soft-reserved ranges that dax
> grabs is filtered by the ACPI HMAT. So while there is a chance that one
> EFI memory map entry spans multiple CXL regions, there is a lower chance
> that a single ACPI HMAT range spans multiple CXL regions.
> 
> I think it is fair for Linux to be simple and require that an algorithm
> of:
> 
> cxl_contains_soft_reserve()
>      for_each_cxl_intersecting_hmem_resource()
>          found = false
>          for_each_region()
>             if (resource_contains(cxl_region_resource, hmem_resource))
>                 found = true
>          if (!found)
>              return false
>      return true
> 
> ...should be good enough, otherwise fallback to pure hmem operation, and
> do not worry about the corner cases.
> 
> If Linux really needs to understand that ACPI HMAT ranges may span
> multiple CXL regions then I would want to understand more what is
> driving that configuration.

I was trying to handle a case like Tomasz's setup in [2], where a single 
Soft Reserved span and CFMWS cover two CXL regions:

kernel: [    0.000000][    T0] BIOS-e820: [mem 
0x0000000a90000000-0x0000000c8fffffff] soft reserved

a90000000-c8fffffff : CXL Window 0
   a90000000-b8fffffff : region1
   b90000000-c8fffffff : region0

…so I ended up with the more generic cxl_regions_fully_map() walker. I 
missed the detail that the HMAT filtered Soft reserved ranges we 
actually act on are much less likely to span multiple regions, and on 
AMD platforms we effectively have a 1:1 mapping. Im fine with 
simplifying this per your suggestion.

> 
> Btw, I do not see a:
> 
>      guard(rwsem_read)(&cxl_rwsem.region)
> 
> ...anywhere in the proposed patch. That needs to be held be sure the
> region's resource settings are not changed out from underneath you. This
> should probably also be checking that the region is in the commit state
> because it may still be racing regions under creation post
> wait_for_device_probe().

Sure, I will add this.

> 
>>   void cxl_endpoint_parse_cdat(struct cxl_port *port);
>> diff --git a/drivers/dax/hmem/hmem.c b/drivers/dax/hmem/hmem.c
>> index f70a0688bd11..db4c46337ac3 100644
>> --- a/drivers/dax/hmem/hmem.c
>> +++ b/drivers/dax/hmem/hmem.c
>> @@ -3,6 +3,8 @@
>>   #include <linux/memregion.h>
>>   #include <linux/module.h>
>>   #include <linux/dax.h>
>> +
>> +#include "../../cxl/cxl.h"
>>   #include "../bus.h"
>>   
>>   static bool region_idle;
>> @@ -150,7 +152,17 @@ static int hmem_register_device(struct device *host, int target_nid,
>>   static int handle_deferred_cxl(struct device *host, int target_nid,
>>   			       const struct resource *res)
>>   {
>> -	/* TODO: Handle region assembly failures */
>> +	if (region_intersects(res->start, resource_size(res), IORESOURCE_MEM,
>> +			      IORES_DESC_CXL) != REGION_DISJOINT) {
>> +
>> +		if (cxl_regions_fully_map(res->start, res->end))
>> +			dax_cxl_mode = DAX_CXL_MODE_DROP;
>> +		else
>> +			dax_cxl_mode = DAX_CXL_MODE_REGISTER;
>> +
>> +		hmem_register_device(host, target_nid, res);
>> +	}
>> +
> 
> I think there is enough content to just create the new
> cxl_contains_soft_reserve() ABI, and then hookup handle_deferred_cxl in
> a follow-on patch.

Okay.

Thanks
Smita