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

Reinette Chatre
2026-03-27T16:13:50Z

Hi Ben,

nit: Since there is some precedent for resctrl files changing permissions
after creation I think it would support this work to make clear that the
permissions are set before the files are created and as such they are
created with correct permissions.

This could just be a simple: "Make" -> "Create" in subject.

On 3/26/26 10:25 AM, Ben Horgan wrote:
> 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.

<split here into separata paragraphs>

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

"make" -> "create"

> 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().

I went back-and-forth a few times on whether we should add a permissions
check to rdtgroup_file_write() to not call rft->write() (and thus event_filter_write())
at all if the file is not writable. In the end I think this patch is ok since
there is the last_cmd_status help to give insight into why user is unable to
write to the file that may be writable under other circumstances. Any opinions
welcome here.

>
> 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;
> };
>
> /**

The above looks good to me. The documentation does still specifically state that "event_filter"
is read/write so it needs a change to match. For example, like something below but please feel
free to improve:

diff --git a/Documentation/filesystems/resctrl.rst b/Documentation/filesystems/resctrl.rst
index 3ec6b3b1b603..70a4ae89d8e1 100644
--- a/Documentation/filesystems/resctrl.rst
+++ b/Documentation/filesystems/resctrl.rst
@@ -419,9 +419,9 @@ 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:

@@ -446,9 +446,8 @@ 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::

Reinette