linux-kernel - Re: [PATCH v2 3/4] Documentation/x86: Document resctrl's new sparse

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Date:   Wed, 27 Sep 2023 17:47:42 -0500
From:   "Moger, Babu" <bmoger@....com>
To:     Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Jonathan Corbet <corbet@....net>
Cc:     linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v2 3/4] Documentation/x86: Document resctrl's new
 sparse_masks

Hi Maciej

On 9/22/2023 3:48 AM, Maciej Wieczor-Retman wrote:
> From: Fenghua Yu <fenghua.yu@...el.com>
>
> The documentation mentions that non-contiguous bit masks are not
> supported in Intel Cache Allocation Technology (CAT).
>
> Update the documentation on how to determine if sparse bit masks are
> allowed in L2 and L3 CAT.
>
> Mention the file with feature support information is located in
> the /sys/fs/resctrl/info/{resource}/ directories and enumerate what
> are the possible outputs on file read operation.
>
> Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
> ---
> Changelog v2:
> - Change bitmap naming convention to bit mask. (Reinette)
>
>   Documentation/arch/x86/resctrl.rst | 16 ++++++++++++----
>   1 file changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst
> index cb05d90111b4..4c6421e2aa31 100644
> --- a/Documentation/arch/x86/resctrl.rst
> +++ b/Documentation/arch/x86/resctrl.rst
> @@ -124,6 +124,13 @@ related to allocation:
>   			"P":
>   			      Corresponding region is pseudo-locked. No
>   			      sharing allowed.
> +"sparse_masks":
> +		Indicates if non-contiguous 1s value in CBM is supported.
> +
> +			"0":
> +			      Only contiguous 1s value in CBM is supported.

This is little confusing. How about?

Non-contiguous 1s value in CBM is not supported

Thanks
Babu

> +			"1":
> +			      Non-contiguous 1s value in CBM is supported.
>   
>   Memory bandwidth(MB) subdirectory contains the following files
>   with respect to allocation:
> @@ -445,12 +452,13 @@ For cache resources we describe the portion of the cache that is available
>   for allocation using a bitmask. The maximum value of the mask is defined
>   by each cpu model (and may be different for different cache levels). It
>   is found using CPUID, but is also provided in the "info" directory of
> -the resctrl file system in "info/{resource}/cbm_mask". Intel hardware
> +the resctrl file system in "info/{resource}/cbm_mask". Some Intel hardware
>   requires that these masks have all the '1' bits in a contiguous block. So
>   0x3, 0x6 and 0xC are legal 4-bit masks with two bits set, but 0x5, 0x9
> -and 0xA are not.  On a system with a 20-bit mask each bit represents 5%
> -of the capacity of the cache. You could partition the cache into four
> -equal parts with masks: 0x1f, 0x3e0, 0x7c00, 0xf8000.
> +and 0xA are not. Check /sys/fs/resctrl/info/{resource}/sparse_masks
> +if non-contiguous 1s value is supported. On a system with a 20-bit mask
> +each bit represents 5% of the capacity of the cache. You could partition
> +the cache into four equal parts with masks: 0x1f, 0x3e0, 0x7c00, 0xf8000.
>   
>   Memory bandwidth Allocation and monitoring
>   ==========================================