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

[Babu Moger <babu.moger@amd.com>]

> -----Original Message-----
> From: James Morse <james.morse@arm.com>
> Sent: Friday, May 15, 2020 1:22 PM
> To: Moger, Babu <Babu.Moger@amd.com>; Reinette Chatre
> <reinette.chatre@intel.com>
> Cc: x86@kernel.org; linux-kernel@vger.kernel.org; Fenghua Yu
> <fenghua.yu@intel.com>; Thomas Gleixner <tglx@linutronix.de>; Ingo Molnar
> <mingo@redhat.com>; Borislav Petkov <bp@alien8.de>; H Peter Anvin
> <hpa@zytor.com>
> Subject: Re: [PATCH v2 09/10] x86/resctrl: Add arch_has_sparse_bitmaps to
> explain AMD/Intel CAT difference
> 
> Hi guys,
> 
> On 13/05/2020 21:03, Babu Moger wrote:
> >> From: Reinette Chatre <reinette.chatre@intel.com>
> >> 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
> 
> top -> to
> 
> >>> needed for Haswell, but as it always sets this value to 1, it will
> >>> never match.
> >>>
> >>> CC: Babu Moger <Babu.Moger@amd.com>
> >>> Signed-off-by: James Morse <james.morse@arm.com>
> >>> Reviewed-by: Reinette Chatre <reinette.chatre@intel.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?
> 
> It does...
> 
> 
> > That is right. AMD allows  L3 mask be all 0s.  I will be great if this
> > property can be indicated it here. Thanks
> 
> Ah, this is a bug I didn't intend to introduce!
> 
> Intel has:
> |       if (val == 0 || val > r->default_ctrl) {
> 
> Whereas AMD:
> |       if (val > r->default_ctrl) {
> 
> 
> So this empty bitmap is permitted today with resctrl. (and, its another try-it-and-
> see).
> 
> I'll add an 'arch_has_empty_bitmaps', I don't think overloading 'sparse' to mean
> 'sparse
> and empty' is a good idea.

Sounds good to me. Thanks