Re: [PATCH v7 2/5] riscv_cbqri: resctrl: Add cache allocation via capacity block mask

[Drew Fustini <fustini@kernel.org>]

On Thu, Sep 17, 2026 at 08:37:43AM -0700, Reinette Chatre wrote:
> Hi Drew,

Hi Reinette,

Thanks for looking at this series. 

> On 9/1/26 3:31 PM, Drew Fustini wrote:
> > resctrl exposes a single cap set per rid, so when more than one capacity
> > controller backs a cache level they must agree on those caps. A level
> > whose controllers disagree is left unexposed. Other levels are
> > unaffected.
> If I understand correctly, while attempting to translate to resctrl terms,
> this means that a cache resource with several domains may support different
> capacity masks in the domains?

Yes, each capacity controller reports its own number of capacity blocks,
so two controllers backing the same cache level can have different
maximum capacity masks.

> I believe the "single cap set per rid" you refer to is
> /sys/fs/resctrl/info/L3/cbm_mask and /sys/fs/resctrl/info/L2/cbm_mask that
> exposes a single capacity bitmask representing the maximum allocation.
>
> As you know, resctrl is in the process of changing the way in which schema
> formats are described. The most recent PoC [1] added new bitmap description
> files but it kept a single value "max" file to describe the highest allocation
> possible for a bitmap schema.
> 
> To support asymmetrical cache instances it looks like changing the interface
> to describe the maximum per domain (instead of one global value) could address
> the gap?
> 
> For example, if there is a hypothetical system with two L2 cache instances, one
> with a 0xFF max CBM and the other with a 0XF max CBM then resctrl could expose it
> as:
> 
> 	# cat /sys/fs/resctrl/info/L2/schemata/L2/max
> 	0=ff;1=f
> 
> The schemata file currently and will continue to display the allocations per domain.
> 
> resctrl will need more changes to support different CBM per domain internally and
> until then all these CBMs will be the same, but adding this change to user interface now
> may set resctrl up to support these systems in the future?

I think that would be a good change to the generic schema rfc as a max
per domain would be helpful for CBQRI. The hardware I am working with
right now (Tenstorrent Ascalon) does not have two controllers at one
level, but I think it is reasonable to assume other RISC-V platforms may
do that in the future.

BTW, I have been meaning to send v8 with some unrelated minor
improvements based on sashiko comments in v7. I will go ahead and send
right now in case you are looking at the rest of the series.

Thanks,
Drew