[PATCH v6 2/7] x86,fs/resctrl: Create 'event_filter' files read only if they're not configurable
From: Ben Horgan
Date: Tue May 05 2026 - 12:02:41 EST
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, create 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@xxxxxxx>
Reviewed-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
---
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
Changes since v4:
Add documentation
make -> create in commit message
Changes since v5:
Commit message typo, In -> in
Add Reinette's Rb
---
Documentation/filesystems/resctrl.rst | 11 +++++------
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 +++++++++-------
6 files changed, 34 insertions(+), 14 deletions(-)
diff --git a/Documentation/filesystems/resctrl.rst b/Documentation/filesystems/resctrl.rst
index b003bed339fd..2898a51e6f4b 100644
--- a/Documentation/filesystems/resctrl.rst
+++ b/Documentation/filesystems/resctrl.rst
@@ -427,9 +427,9 @@ with the following files:
Two MBM events are supported by default: mbm_local_bytes and mbm_total_bytes.
Each MBM event's sub-directory contains a file named "event_filter" that is
- used to view and modify which memory transactions the MBM event is configured
- with. The file is accessible only when "mbm_event" counter assignment mode is
- enabled.
+ used to view and (if writable) modify which memory transactions the MBM event
+ is configured with. The file is accessible only when "mbm_event" counter
+ assignment mode is enabled.
List of memory transaction types supported:
@@ -454,9 +454,8 @@ with the following files:
# cat /sys/fs/resctrl/info/L3_MON/event_configs/mbm_local_bytes/event_filter
local_reads,local_non_temporal_writes,local_reads_slow_memory
- Modify the event configuration by writing to the "event_filter" file within
- the "event_configs" directory. The read/write "event_filter" file contains the
- configuration of the event that reflects which memory transactions are counted by it.
+ The memory transactions the MBM event is configured with can be changed
+ if "event_filter" is writable.
For example::
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 9fd901c78dc6..916f7a9d5658 100644
--- a/fs/resctrl/monitor.c
+++ b/fs/resctrl/monitor.c
@@ -1422,6 +1422,11 @@ 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) {
@@ -1886,6 +1891,8 @@ 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 fb6999178fe2..b382a348dd79 100644
--- a/fs/resctrl/rdtgroup.c
+++ b/fs/resctrl/rdtgroup.c
@@ -2022,7 +2022,7 @@ 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,
@@ -2215,6 +2215,15 @@ 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