[PATCH 03/23] resctrl: Expose MBA resource_schemata mode sysfs

[Fenghua Yu]

Node-scoped MBA on MPAM needs a way to distinguish native memory-side
controls from legacy L3-shaped MB emulation. Track the selected emulate
mode on rdt_resource and expose it as
info/<resource>/resource_schemata/mode ("native" or "legacy") when the
architecture enables emulation.

The mode file is only created when rdt_resource::mode is non-zero
(RESCTRL_CTRL_LEGACY or RESCTRL_CTRL_NATIVE). It defaults to
RESCTRL_CTRL_MODE_NONE, so resources whose architecture does not support
control emulation get no mode file and are unaffected. Architecture
backends that support emulation set the initial mode when they create
their controls; on MPAM this is wired up together with the node-scoped
MB_NODE control in a later patch, so this commit only adds the (dormant)
generic mechanism.

The mode file is added read-only here: switching the mode at runtime
requires rebuilding the resource_schemata layout to match the new mode,
so the writable interface is added together with that rebuild logic in a
later patch. Keeping the file read-only until then avoids exposing a
writable-but-no-op interface.

Store rdt_resource_final in the resource_schemata directory priv so the
mode file can resolve the backing resource without dereferencing NULL.

Signed-off-by: Fenghua Yu <fenghuay@xxxxxxxxxx>
---
 fs/resctrl/rdtgroup.c   | 77 ++++++++++++++++++++++++++++++++++++++++-
 include/linux/resctrl.h | 17 +++++++++
 2 files changed, 93 insertions(+), 1 deletion(-)

diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
index 2abb7fda6091..6b1f24c6a1f2 100644
--- a/fs/resctrl/rdtgroup.c
+++ b/fs/resctrl/rdtgroup.c
@@ -2696,6 +2696,74 @@ static unsigned long fflags_from_resource(struct rdt_resource *r)
 	return WARN_ON_ONCE(1);
 }
 
+static int resctrl_ctrl_mb_mode_show(struct kernfs_open_file *of,
+				     struct seq_file *seq, void *v)
+{
+	struct rdt_resource_final *f = rdt_kn_parent_priv(of->kn);
+	struct rdt_resource *r = f->res;
+
+	guard(mutex)(&rdtgroup_mutex);
+
+	switch (r->mode) {
+	case RESCTRL_CTRL_LEGACY:
+		seq_puts(seq, "[legacy] native\n");
+		break;
+	case RESCTRL_CTRL_NATIVE:
+		seq_puts(seq, "legacy [native]\n");
+		break;
+	default:
+		WARN_ONCE(1, "%s: unexpected MB control mode %d\n",
+			  f->name, r->mode);
+		seq_puts(seq, "legacy native\n");
+		break;
+	}
+
+	return 0;
+}
+
+static struct rftype resctrl_ctrl_mb_files[] = {
+	{
+		.name		= "mode",
+		.mode		= 0444,
+		.kf_ops		= &rdtgroup_kf_single_ops,
+		.seq_show	= resctrl_ctrl_mb_mode_show,
+		/*
+		 * Directory-level file, not per-control: fflags is only a
+		 * presence flag here, not the BIT(ctrl->type) type filter used
+		 * by resctrl_add_ctrl_files().
+		 */
+		.fflags		= 1,
+	}
+};
+
+static int resctrl_ctrl_add_files(struct kernfs_node *kn)
+{
+	struct rftype *rfts, *rft;
+	int ret, len;
+
+	rfts = resctrl_ctrl_mb_files;
+	len = ARRAY_SIZE(resctrl_ctrl_mb_files);
+
+	lockdep_assert_held(&rdtgroup_mutex);
+
+	for (rft = rfts; rft < rfts + len; rft++) {
+		if (rft->fflags) {
+			ret = rdtgroup_add_file(kn, rft);
+			if (ret)
+				goto error;
+		}
+	}
+
+	return 0;
+error:
+	pr_warn("Failed to add %s, err=%d\n", rft->name, ret);
+	while (--rft >= rfts) {
+		if (rft->fflags)
+			kernfs_remove_by_name(kn, rft->name);
+	}
+	return ret;
+}
+
 /*
  * No need to cleanup on exit - caller calls the recursive kernfs_remove()
  * on failure.
@@ -2704,11 +2772,12 @@ static int resctrl_mkdir_schemata_dir(struct kernfs_node *kn,
 				      struct rdt_resource_final *f)
 {
 	struct kernfs_node *kn_subdir, *kn_ctrl;
+	struct rdt_resource *r = f->res;
 	struct resctrl_ctrl *ctrl;
 	char ctrl_full_name[20];
 	int ret;
 
-	kn_subdir = kernfs_create_dir(kn, "resource_schemata", kn->mode, NULL);
+	kn_subdir = kernfs_create_dir(kn, "resource_schemata", kn->mode, f);
 	if (IS_ERR(kn_subdir))
 		return PTR_ERR(kn_subdir);
 
@@ -2716,6 +2785,12 @@ static int resctrl_mkdir_schemata_dir(struct kernfs_node *kn,
 	if (ret)
 		return ret;
 
+	if (r->mode) {
+		ret = resctrl_ctrl_add_files(kn_subdir);
+		if (ret)
+			return ret;
+	}
+
 	for_each_resource_ctrl(ctrl, f->res) {
 		ret = snprintf(ctrl_full_name, sizeof(ctrl_full_name), "%s%s%s",
 			       f->name, resctrl_ctrl_is_default(ctrl) ? "" : "_",
diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
index 72fb7256270e..4fc41e269d0b 100644
--- a/include/linux/resctrl.h
+++ b/include/linux/resctrl.h
@@ -257,6 +257,16 @@ enum resctrl_ctrl_unit {
 	RESCTRL_CTRL_UNIT_GBPS,
 };
 
+enum resctrl_ctrl_mode {
+	/*
+	 * Default (zero) value: the resource does not support control
+	 * emulation, so no resource_schemata/mode file is created for it.
+	 */
+	RESCTRL_CTRL_MODE_NONE = 0,
+	RESCTRL_CTRL_LEGACY,
+	RESCTRL_CTRL_NATIVE,
+};
+
 /**
  * struct resctrl_membw - Memory bandwidth allocation related data
  * @min_bw:		Minimum memory bandwidth percentage user can request
@@ -399,6 +409,12 @@ struct resctrl_ctrl {
  *			different memory bandwidths
  * @cache_io_alloc_capable:True if portion of the cache can be configured
  *			   for I/O traffic.
+ * @mode:		Control emulation mode for this resource.
+ *			RESCTRL_CTRL_MODE_NONE if the resource does not support
+ *			emulation. "legacy": keep the legacy MB control,
+ *			emulating it with a native control when it has no MBW
+ *			hardware of its own. "native": expose native controls
+ *			directly with no emulation.
  * @controls:		List of controls of an alloc_capable resource
  */
 struct rdt_resource {
@@ -413,6 +429,7 @@ struct rdt_resource {
 	bool				bw_delay_linear;
 	enum membw_throttle_mode	bw_throttle_mode;
 	bool				cache_io_alloc_capable;
+	enum resctrl_ctrl_mode		mode;
 	struct list_head		controls;
 };
 
-- 
2.43.0