linux-kernel - Re: [PATCH v4 09/10] x86/resctrl: Add arch_has_{sparse,empty}

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Date:   Thu, 28 May 2020 13:43:30 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     James Morse <james.morse@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Babu Moger <Babu.Moger@....com>
Subject: Re: [PATCH v4 09/10] x86/resctrl: Add arch_has_{sparse,empty}_bitmaps
 to explain CAT differences

Hi James,

On 5/26/2020 6:40 AM, James Morse wrote:
> Intel CPUs expect the cache bitmap provided by user-space to have on
> a single span of 1s, whereas AMD can support bitmaps like 0xf00f.
> Arm's MPAM support also allows sparse bitmaps.
> 
> Similarly, Intel CPUs check at least one bit set, whereas AMD CPUs
> are quite happy with an empty bitmap. Arm's MPAM allows an empty
> bitmap.
> 
> To move resctrl out to /fs/ we need to explain platform differences
> like this. Add two resource properties arch_has_{empty,sparse}_bitmaps.
> Test these around the relevant parts of cbm_validate().
> 
> Merging the validate calls causes AMD to gain the min_cbm_bits test
> needed for Haswell, but as it always sets this value to 1, it will
> never match.
> 
> CC: Reinette Chatre <reinette.chatre@...el.com>
> Signed-off-by: James Morse <james.morse@....com>
> Reviewed-by: Babu Moger <babu.moger@....com>

Thank you for adding handling of empty bitmaps.

Reviewed-by: Reinette Chatre <reinette.chatre@...el.com>

Reinette