linux-kernel - RE: [PATCH v5 1/2] x86/resctrl: Fix min_cbm

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Message-ID: <IA1PR11MB60972C5A48D0003D3E6705259B559@IA1PR11MB6097.namprd11.prod.outlook.com>
Date:   Tue, 27 Sep 2022 23:11:46 +0000
From:   "Yu, Fenghua" <fenghua.yu@...el.com>
To:     Babu Moger <babu.moger@....com>, "corbet@....net" <corbet@....net>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>
CC:     "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
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        <pawan.kumar.gupta@...ux.intel.com>,
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        "Eranian, Stephane" <eranian@...gle.com>
Subject: RE: [PATCH v5 1/2] x86/resctrl: Fix min_cbm_bits for AMD

> AMD systems support zero CBM (capacity bit mask) for cache allocation.
> That is reflected in rdt_init_res_defs_amd() by:
> 
>         r->cache.arch_has_empty_bitmaps = true;
> 
> However given the unified code in cbm_validate(), checking for:
>         val == 0 && !arch_has_empty_bitmaps
> 
> is not enough because of another check in cbm_validate():
> 
>         if ((zero_bit - first_bit) < r->cache.min_cbm_bits)
> 
> The default value of r->cache.min_cbm_bits = 1.
> 
> Leading to:
> 
>         $ cd /sys/fs/resctrl
>         $ mkdir foo
>         $ cd foo
>         $ echo L3:0=0 > schemata
>           -bash: echo: write error: Invalid argument
>         $ cat /sys/fs/resctrl/info/last_cmd_status
>           Need at least 1 bits in the mask
> 
> Fix the issue by initializing the min_cbm_bits to 0 for AMD.
> Also, remove the default setting of min_cbm_bits and initialize it separately.
> 
> After the fix
>         $ cd /sys/fs/resctrl
>         $ mkdir foo
>         $ cd foo
>         $ echo L3:0=0 > schemata
>         $ cat /sys/fs/resctrl/info/last_cmd_status
>           ok
> 
> Link: https://lore.kernel.org/lkml/20220517001234.3137157-1-
> eranian@...gle.com/
> Fixes: 316e7f901f5a ("x86/resctrl: Add struct rdt_cache::arch_has_{sparse,
> empty}_bitmaps")
> Co-developed-by: Stephane Eranian <eranian@...gle.com>
> Signed-off-by: Stephane Eranian <eranian@...gle.com>
> Signed-off-by: Babu Moger <babu.moger@....com>
> Reviewed-by: Ingo Molnar <mingo@...nel.org>
> Reviewed-by: James Morse <james.morse@....com>
> Reviewed-by: Reinette Chatre <reinette.chatre@...el.com>

Reviewed-by: Fenghua Yu <fenghua.yu@...el.com>