linux-kernel - Re: [PATCH v9 05/10] fs/resctrl: Introduce interface to display "io

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Message-ID: <7b1452b2-38d7-49e1-bd34-ea61eca01419@intel.com>
Date: Wed, 17 Sep 2025 22:28:00 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Babu Moger <babu.moger@....com>, <corbet@....net>, <tony.luck@...el.com>,
	<Dave.Martin@....com>, <james.morse@....com>, <tglx@...utronix.de>,
	<mingo@...hat.com>, <bp@...en8.de>, <dave.hansen@...ux.intel.com>
CC: <x86@...nel.org>, <hpa@...or.com>, <kas@...nel.org>,
	<rick.p.edgecombe@...el.com>, <akpm@...ux-foundation.org>,
	<paulmck@...nel.org>, <pmladek@...e.com>,
	<pawan.kumar.gupta@...ux.intel.com>, <rostedt@...dmis.org>,
	<kees@...nel.org>, <arnd@...db.de>, <fvdl@...gle.com>, <seanjc@...gle.com>,
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	<linux-coco@...ts.linux.dev>, <kvm@...r.kernel.org>
Subject: Re: [PATCH v9 05/10] fs/resctrl: Introduce interface to display
 "io_alloc" support

Hi Babu,

On 9/2/25 3:41 PM, Babu Moger wrote:
> "io_alloc" feature in resctrl allows direct insertion of data from I/O
> devices into the cache.
> 
> Introduce the 'io_alloc' resctrl file to indicate the support for the
> feature.

Changelog that aims to address feeback received in ABMC series (avoid repetition
and document any non-obvious things), please feel free to improve:

	Introduce the "io_alloc" resctrl file to the "info" area of a cache
	resource, for example /sys/fs/resctrl/info/L3/io_alloc. "io_alloc"
	indicates support for the "io_alloc" feature that allows direct
	insertion of data from I/O devices into the cache.                                                         
                                                                                
	Restrict exposing support for "io_alloc" to the L3 resource that is the        
	only resource where this feature can be backed by AMD's L3 Smart Data Cache
	Injection Allocation Enforcement (SDCIAE). With that, the "io_alloc" file is only
	visible to user space if the L3 resource supports "io_alloc". Doing     
	so makes the file visible for all cache resources though, for example also L2   
	cache (if it supports cache allocation). As a consequence, add capability for
	file to report expected "enabled" and "disabled", as well as "not supported".                  


> 
> Signed-off-by: Babu Moger <babu.moger@....com>
> ---

...

> ---
>  Documentation/filesystems/resctrl.rst | 30 +++++++++++++++++++++++++++
>  fs/resctrl/ctrlmondata.c              | 21 +++++++++++++++++++
>  fs/resctrl/internal.h                 |  5 +++++
>  fs/resctrl/rdtgroup.c                 | 24 ++++++++++++++++++++-
>  4 files changed, 79 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/filesystems/resctrl.rst b/Documentation/filesystems/resctrl.rst
> index 4866a8a4189f..89aab17b00cb 100644
> --- a/Documentation/filesystems/resctrl.rst
> +++ b/Documentation/filesystems/resctrl.rst
> @@ -136,6 +136,36 @@ related to allocation:
>  			"1":
>  			      Non-contiguous 1s value in CBM is supported.
>  
> +"io_alloc":
> +		"io_alloc" enables system software to configure the portion of
> +		the cache allocated for I/O traffic. File may only exist if the
> +		system supports this feature on some of its cache resources.
> +
> +			"disabled":
> +			      Resource supports "io_alloc" but the feature is disabled.
> +			      Portions of cache used for allocation of I/O traffic cannot
> +			      be configured.
> +			"enabled":
> +			      Portions of cache used for allocation of I/O traffic
> +			      can be configured using "io_alloc_cbm".
> +			"not supported":
> +			      Support not available for this resource.
> +

After trying to rework the changelogs I believe the portion of doc below is better suited for
the next patch that adds support for enable/disable where CLOSIDs are relevant.

> +		The underlying implementation may reduce resources available to
> +		general (CPU) cache allocation. See architecture specific notes
> +		below. Depending on usage requirements the feature can be enabled
> +		or disabled.
> +
> +		On AMD systems, io_alloc feature is supported by the L3 Smart
> +		Data Cache Injection Allocation Enforcement (SDCIAE). The CLOSID for
> +		io_alloc is the highest CLOSID supported by the resource. When
> +		io_alloc is enabled, the highest CLOSID is dedicated to io_alloc and
> +		no longer available for general (CPU) cache allocation. When CDP is
> +		enabled, io_alloc routes I/O traffic using the highest CLOSID allocated
> +		for the instruction cache (L3CODE), making this CLOSID no longer
> +		available for general (CPU) cache allocation for both the L3CODE and
> +		L3DATA resources.
> +
>  Memory bandwidth(MB) subdirectory contains the following files
>  with respect to allocation:
>  

Reinette