Re: [PATCH RFC 08/19] RISC-V: QoS: add resctrl interface for CBQRI controllers

[guo.wenjia23]

Hi Drew,

> On Thu, Mar 26, 2026 at 07:05:48PM +0800, guo.wenjia23@xxxxxxxxxx wrote:
> > Hi Drew,
> > 
> > On Thu, Jan 29, 2026 at 4:28 AM Drew Fustini <fustini@xxxxxxxxxx> wrote:>
> > > Add interface for CBQRI controller drivers to make use of the resctrl
> > > filesystem.
> > >
> > > Co-developed-by: Adrien Ricciardi <aricciardi@xxxxxxxxxxxx>
> > > Signed-off-by: Adrien Ricciardi <aricciardi@xxxxxxxxxxxx>
> > > Signed-off-by: Drew Fustini <fustini@xxxxxxxxxx>
> > > ---
> > >  arch/riscv/kernel/qos/qos_resctrl.c | 1192 +++++++++++++++++++++++++++++++++++
> > >  1 file changed, 1192 insertions(+)
> > >
> > > ...
> > >
> > > +
> > > +int resctrl_arch_update_one(struct rdt_resource *r, struct rdt_ctrl_domain *d,
> > > +                           u32 closid, enum resctrl_conf_type t, u32 cfg_val)
> > > +{
> > > +       struct cbqri_controller *ctrl;
> > > +       struct cbqri_resctrl_dom *dom;
> > > +       struct cbqri_config cfg;
> > > +       int err = 0;
> > > +
> > > +       dom = container_of(d, struct cbqri_resctrl_dom, resctrl_ctrl_dom);
> > > +       ctrl = dom->hw_ctrl;
> > > +
> > > +       if (!r->alloc_capable)
> > > +               return -EINVAL;
> > > +
> > > +       switch (r->rid) {
> > > +       case RDT_RESOURCE_L2:
> > > +       case RDT_RESOURCE_L3:
> > > +               cfg.cbm = cfg_val;
> > > +               err = cbqri_apply_cache_config(dom, closid, t, &cfg);
> > > +               break;
> > > +       case RDT_RESOURCE_MBA:
> > > +               /* covert from percentage to bandwidth blocks */
> > > +               cfg.rbwb = cfg_val * ctrl->bc.nbwblks / 100;
> > 
> > Should use bc.mrbwb to calculate rbwb? 
> > I think bc.nbwblks represent the available bw blks in the controller.
> It should should decrease as they are allocated.
> 
> The resctrl MBA resource is supposed to be the percentage of total
> memory bandwidth. nbwblks is the total number of bandwidth blocks, so I
> think the existing calucation is correct.
> 
> From the example SoC configuration, the number of bandwidth blocks is
> 1024, and mrbwb is meant to be 80% of nbwblks which is 819.
> 
> Setting MB to 80% in schemata would yield: 80 * 1024 / 100 = 819
> 

According to the specification
1. The MRBWB field reports the maximum number of bandwidth blocks that can be reserved.
2. The sum of Rbwb allocated across all rcids must not exceed MRBWB value.

Based on the specification and the example you mentioned, can this be understood as follows:

MRBWB is a fixed value determined by the SoC configuration, representing the effective bandwidth capacity of the bandwidth controller. 
And the MRBWB value itself should remain constant and must not vary after the bandwidth allocations assigned to RCIDs.


Thanks,
Wenjia