[PATCH 1/4] resctrl: Add kmode arch stubs for ARM MPAM and hide kernel_mode on non-PLZA platforms

[Qinyun Tan]

fs/resctrl/rdtgroup.c calls resctrl_arch_get_kmode_support() and
resctrl_arch_configure_kmode() which are only implemented under
arch/x86/.  ARM MPAM enables CONFIG_RESCTRL_FS but has no PLZA
hardware, causing link failures on aarch64 allyesconfig:

  undefined reference to `resctrl_arch_configure_kmode'
  undefined reference to `resctrl_arch_get_kmode_support'

Add empty stubs in drivers/resctrl/mpam_resctrl.c, following the
same pattern used for resctrl_arch_io_alloc_enable().

Additionally, the kernel_mode file under info/ was unconditionally
visible (fflags = RFTYPE_TOP_INFO).  On platforms without PLZA
(ARM MPAM, older AMD/Intel), the file shows only inherit_ctrl_and_mon
with no other options, which is unnecessary.  Set the initial fflags
to 0 and dynamically enable the file in resctrl_kmode_init() only
when resctrl_arch_get_kmode_support() registers additional modes,
matching the io_alloc pattern.

Signed-off-by: Qinyun Tan <qinyuntan@xxxxxxxxxxxxxxxxx>
Reviewed-by: Xunlei Pang <xlpang@xxxxxxxxxxxxxxxxx>
---
 drivers/resctrl/mpam_resctrl.c | 9 +++++++++
 fs/resctrl/rdtgroup.c          | 5 ++++-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
index 1dd3bbd0772cd..fc2b3f508c039 100644
--- a/drivers/resctrl/mpam_resctrl.c
+++ b/drivers/resctrl/mpam_resctrl.c
@@ -161,6 +161,15 @@ bool resctrl_arch_get_io_alloc_enabled(struct rdt_resource *r)
 	return false;
 }
 
+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)
+{
+}
+
 void resctrl_arch_pre_mount(void)
 {
 }
diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
index dd9c4da836983..0b2606f10e2e9 100644
--- a/fs/resctrl/rdtgroup.c
+++ b/fs/resctrl/rdtgroup.c
@@ -2602,7 +2602,7 @@ static struct rftype res_common_files[] = {
 		.kf_ops		= &rdtgroup_kf_single_ops,
 		.seq_show	= resctrl_kernel_mode_show,
 		.write		= resctrl_kernel_mode_write,
-		.fflags		= RFTYPE_TOP_INFO,
+		.fflags		= 0,
 	},
 	{
 		.name		= "mbm_assign_on_mkdir",
@@ -2968,6 +2968,9 @@ static void resctrl_kmode_init(void)
 	resctrl_kcfg.k_rdtgrp = &rdtgroup_default;
 
 	resctrl_arch_get_kmode_support(&resctrl_kcfg);
+
+	if (resctrl_kcfg.kmode != BIT(INHERIT_CTRL_AND_MON))
+		resctrl_file_fflags_init("kernel_mode", RFTYPE_TOP_INFO);
 }
 
 void resctrl_file_fflags_init(const char *config, unsigned long fflags)
-- 
2.43.7