linux-kernel - RE: [PATCH v2 09/10] x86/resctrl: Add arch_has_sparse_bitmaps to explain AMD/Intel CAT difference

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Message-ID: <980a693a-b823-96e1-9b4f-d7bbf4f09714@amd.com>
Date:   Wed, 13 May 2020 15:03:57 -0500
From:   Babu Moger <babu.moger@....com>
To:     Reinette Chatre <reinette.chatre@...el.com>,
        James Morse <james.morse@....com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.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>
Subject: RE: [PATCH v2 09/10] x86/resctrl: Add arch_has_sparse_bitmaps to
 explain AMD/Intel CAT difference



> -----Original Message-----
> From: Reinette Chatre <reinette.chatre@...el.com>
> Sent: Monday, May 11, 2020 1:15 PM
> 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>; Moger, Babu
> <Babu.Moger@....com>
> Subject: Re: [PATCH v2 09/10] x86/resctrl: Add arch_has_sparse_bitmaps to
> explain AMD/Intel CAT difference
> 
> Hi James,
> 
> On 4/30/2020 10:03 AM, James Morse wrote:
> > Intel expects 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.
> >
> > To move resctrl out to /fs/ we need to explain platform differences
> > like this. Add a resource property arch_has_sparse_bitmaps. Test this
> > around the 'non-consecutive' test in cbm_validate().
> >
> > Merging the validate calls causes AMD top gain the min_cbm_bits test
> > needed for Haswell, but as it always sets this value to 1, it will
> > never match.
> >
> > CC: Babu Moger <Babu.Moger@....com>
> > Signed-off-by: James Morse <james.morse@....com>
> > Reviewed-by: Reinette Chatre <reinette.chatre@...el.com>
> 
> The Intel bits do indeed look good to me but we should check the AMD
> portion ... I peeked at the AMD spec [1] and found "If an L3_MASK_n
> register is programmed with all 0’s, that COS will be prevented from
> allocating any lines in the L3 cache" ... so AMD does allow bitmasks of
> all 0's (Intel does not).
> 
> Does MPAM also allow all 0's? Perhaps "arch_has_sparse_bitmaps" can be
> used to indicate that also?

That is right. AMD allows  L3 mask be all 0s.  I will be great if this
property can be indicated it here. Thanks