[PATCH v4 2/7] x86,fs/resctrl: Make 'event_filter' files read only if they're not configurable

Ben Horgan
2026-03-26T17:26:04Z

When the counter assignment mode is mbm_event resctrl assumes the MBM
events are configurable and exposes the 'event_filter' files. These files
live at info/L3_MON/event_configs/<event>/event_filter and are used to
display and set the event configuration. The MPAM architecture has support
for configuring the memory bandwidth utilization (MBWU) counters to only
count reads or only count writes. However, In MPAM, this event filtering
support is optional in the hardware (and not yet implemented in the MPAM
driver) but MBM counter assignment is always possible for MPAM MBWU
counters.

In order to support mbm_event mode with MPAM, make the 'event_filter' files
read only if the event configuration can't be changed. A user can still
chmod the file and so also return early with an error from
event_filter_write().

Introduce a new monitor property, mbm_cntr_configurable, to indicate
whether or not assignable MBM counters are configurable. On x86, set this
to true whenever mbm_cntr_assignable is true to keep existing behaviour.

Signed-off-by: Ben Horgan <ben.horgan@arm.com>
---
Changes since v2:
Use property, mbm_cntr_configurable, rather than arch hook
Change the event_filter mode to read only in res_common_files[]
Add resctrl_file_mode_init() and use in resctrl_l3_mon_resource_init()
set mbm_cntr_configurable for x86 ABMC and mention in commit message
---
arch/x86/kernel/cpu/resctrl/monitor.c | 1 +
fs/resctrl/internal.h | 2 ++
fs/resctrl/monitor.c | 7 +++++++
fs/resctrl/rdtgroup.c | 11 ++++++++++-
include/linux/resctrl.h | 16 +++++++++-------
5 files changed, 29 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
index 9bd87bae4983..794a6fb175e4 100644
--- a/arch/x86/kernel/cpu/resctrl/monitor.c
+++ b/arch/x86/kernel/cpu/resctrl/monitor.c
@@ -454,6 +454,7 @@ int __init rdt_get_l3_mon_config(struct rdt_resource *r)
(rdt_cpu_has(X86_FEATURE_CQM_MBM_TOTAL) ||
rdt_cpu_has(X86_FEATURE_CQM_MBM_LOCAL))) {
r->mon.mbm_cntr_assignable = true;
+ r->mon.mbm_cntr_configurable = true;
cpuid_count(0x80000020, 5, &eax, &ebx, &ecx, &edx);
r->mon.num_mbm_cntrs = (ebx & GENMASK(15, 0)) + 1;
hw_res->mbm_cntr_assign_enabled = true;
diff --git a/fs/resctrl/internal.h b/fs/resctrl/internal.h
index 1a9b29119f88..48af75b9dc85 100644
--- a/fs/resctrl/internal.h
+++ b/fs/resctrl/internal.h
@@ -408,6 +408,8 @@ void __check_limbo(struct rdt_l3_mon_domain *d, bool force_free);

void resctrl_file_fflags_init(const char *config, unsigned long fflags);

+void resctrl_file_mode_init(const char *config, umode_t mode);
+
void rdt_staged_configs_clear(void);

bool closid_allocated(unsigned int closid);
diff --git a/fs/resctrl/monitor.c b/fs/resctrl/monitor.c
index 49f3f6b846b2..8fec3dea33c3 100644
--- a/fs/resctrl/monitor.c
+++ b/fs/resctrl/monitor.c
@@ -1420,6 +1420,11 @@ ssize_t event_filter_write(struct kernfs_open_file *of, char *buf, size_t nbytes
ret = -EINVAL;
goto out_unlock;
}
+ if (!r->mon.mbm_cntr_configurable) {
+ rdt_last_cmd_puts("event_filter is not configurable\n");
+ ret = -EPERM;
+ goto out_unlock;
+ }

ret = resctrl_parse_mem_transactions(buf, &evt_cfg);
if (!ret && mevt->evt_cfg != evt_cfg) {
@@ -1884,6 +1889,8 @@ int resctrl_l3_mon_resource_init(void)
resctrl_file_fflags_init("available_mbm_cntrs",
RFTYPE_MON_INFO | RFTYPE_RES_CACHE);
resctrl_file_fflags_init("event_filter", RFTYPE_ASSIGN_CONFIG);
+ if (r->mon.mbm_cntr_configurable)
+ resctrl_file_mode_init("event_filter", 0644);
resctrl_file_fflags_init("mbm_assign_on_mkdir", RFTYPE_MON_INFO |
RFTYPE_RES_CACHE);
resctrl_file_fflags_init("mbm_L3_assignments", RFTYPE_MON_BASE);
diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
index 4753841c2ca3..fa5712db3778 100644
--- a/fs/resctrl/rdtgroup.c
+++ b/fs/resctrl/rdtgroup.c
@@ -2020,7 +2020,7 @@ static struct rftype res_common_files[] = {
},
{
.name = "event_filter",
- .mode = 0644,
+ .mode = 0444,
.kf_ops = &rdtgroup_kf_single_ops,
.seq_show = event_filter_show,
.write = event_filter_write,
@@ -2213,6 +2213,15 @@ void resctrl_file_fflags_init(const char *config, unsigned long fflags)
rft->fflags = fflags;
}

+void resctrl_file_mode_init(const char *config, umode_t mode)
+{
+ struct rftype *rft;
+
+ rft = rdtgroup_get_rftype_by_name(config);
+ if (rft)
+ rft->mode = mode;
+}
+
/**
* rdtgroup_kn_mode_restrict - Restrict user access to named resctrl file
* @r: The resource group with which the file is associated.
diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
index 006e57fd7ca5..06e8c72e8660 100644
--- a/include/linux/resctrl.h
+++ b/include/linux/resctrl.h
@@ -286,13 +286,14 @@ enum resctrl_schema_fmt {

/**
* struct resctrl_mon - Monitoring related data of a resctrl resource.
- * @num_rmid: Number of RMIDs available.
- * @mbm_cfg_mask: Memory transactions that can be tracked when bandwidth
- * monitoring events can be configured.
- * @num_mbm_cntrs: Number of assignable counters.
- * @mbm_cntr_assignable:Is system capable of supporting counter assignment?
- * @mbm_assign_on_mkdir:True if counters should automatically be assigned to MBM
- * events of monitor groups created via mkdir.
+ * @num_rmid: Number of RMIDs available.
+ * @mbm_cfg_mask: Memory transactions that can be tracked when
+ * bandwidth monitoring events can be configured.
+ * @num_mbm_cntrs: Number of assignable counters.
+ * @mbm_cntr_assignable: Is system capable of supporting counter assignment?
+ * @mbm_assign_on_mkdir: True if counters should automatically be assigned to MBM
+ * events of monitor groups created via mkdir.
+ * @mbm_cntr_configurable: True if assignable counters are configurable.
*/
struct resctrl_mon {
u32 num_rmid;
@@ -300,6 +301,7 @@ struct resctrl_mon {
int num_mbm_cntrs;
bool mbm_cntr_assignable;
bool mbm_assign_on_mkdir;
+ bool mbm_cntr_configurable;
};

/**
--
2.43.0