Re: [PATCH v3 00/12] x86/resctrl: Add kernel-mode (e.g., PLZA) support to the resctrl subsystem

[Qinyun Tan]

Hi Babu,

While reviewing this series I noticed two cross-architecture issues.

1) aarch64 allyesconfig link failure

   resctrl_arch_configure_kmode() and resctrl_arch_get_kmode_support()
   are declared in include/linux/resctrl.h:

     void resctrl_arch_get_kmode_support(struct resctrl_kmode_cfg *kcfg);
     void resctrl_arch_configure_kmode(const struct cpumask *cpu_mask,
                                       u32 closid, u32 rmid, bool enable);

   but only implemented under arch/x86/.  ARM MPAM selects
   CONFIG_RESCTRL_FS, so fs/resctrl/rdtgroup.c is compiled on aarch64
   and the linker fails:

     ld: fs/resctrl/rdtgroup.o: in function `rdtgroup_config_kmode_clear':
     rdtgroup.c: undefined reference to `resctrl_arch_configure_kmode'
     ld: fs/resctrl/rdtgroup.o: in function `resctrl_init':
     rdtgroup.c: undefined reference to `resctrl_arch_get_kmode_support'

   Other arch-specific functions already have empty stubs in
   drivers/resctrl/mpam_resctrl.c, e.g.:

     int resctrl_arch_io_alloc_enable(struct rdt_resource *r, bool enable)
     {
             return -EOPNOTSUPP;
     }

   Adding the same for the two kmode functions would fix the build.

2) info/kernel_mode visible on non-PLZA platforms

   The "kernel_mode" entry is registered with fflags = RFTYPE_TOP_INFO:

     {
             .name           = "kernel_mode",
             .mode           = 0644,
             ...
             .fflags         = RFTYPE_TOP_INFO,
     },

   This makes the file appear unconditionally under info/ -- even on
   platforms without PLZA (ARM MPAM, older AMD/Intel without
   X86_FEATURE_PLZA).  On those platforms the file only shows the
   default inherit_ctrl_and_mon mode, which is confusing since there
   are no other modes to switch to.

   The io_alloc file handles this by starting with fflags = 0 and
   enabling it conditionally:

     /* io_alloc: fflags defaults to 0 (hidden) */

     static void io_alloc_init(void)
     {
             if (r->cache.io_alloc_capable) {
                     resctrl_file_fflags_init("io_alloc", ...);
             }
     }

   The same pattern could work for kernel_mode: set fflags = 0 and
   call resctrl_file_fflags_init("kernel_mode", RFTYPE_TOP_INFO)
   in resctrl_kmode_init() only when resctrl_arch_get_kmode_support()
   registers modes beyond INHERIT_CTRL_AND_MON.

Thanks,
Qinyun