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

[Babu Moger]

> -----Original Message-----
> From: Reinette Chatre <reinette.chatre@xxxxxxxxx>
> Sent: Monday, May 11, 2020 1:15 PM
> To: James Morse <james.morse@xxxxxxx>; x86@xxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx
> Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>; Thomas Gleixner
> <tglx@xxxxxxxxxxxxx>; Ingo Molnar <mingo@xxxxxxxxxx>; Borislav Petkov
> <bp@xxxxxxxxx>; H Peter Anvin <hpa@xxxxxxxxx>; Moger, Babu
> <Babu.Moger@xxxxxxx>
> 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@xxxxxxx>
> > Signed-off-by: James Morse <james.morse@xxxxxxx>
> > Reviewed-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
> 
> 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 0âs, 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