From 06cd3ac67f9e039219df21d4bf63bf53b666355a Mon Sep 17 00:00:00 2001
From: Kimplul <kimi.h.kuparinen@gmail.com>
Date: Sun, 14 Jul 2024 13:19:10 +0300
Subject: expose max number of concurrent threads to users

+ Can be used to build fast hashmaps to speed up ipc
---
 include/kmi/syscalls.h |  9 +++++++++
 include/kmi/tcb.h      |  5 +++++
 src/tcb.c              | 19 ++++++++++++-------
 src/uapi/conf.c        |  4 ++++
 4 files changed, 30 insertions(+), 7 deletions(-)

diff --git a/include/kmi/syscalls.h b/include/kmi/syscalls.h
index 5079d74..4319812 100644
--- a/include/kmi/syscalls.h
+++ b/include/kmi/syscalls.h
@@ -258,6 +258,15 @@ enum conf_param {
 	 * \c R
 	 */
 	CONF_PAGE_SIZE,
+
+	/**
+	 * Maximum number of threads active at the same time.
+	 * Can be used by userspace to build a pretty fast hashmaps of thread
+	 * IDs to whatever, see \ref get_tcb().
+	 * Guaranteed to be some power of 2.
+	 * \c R
+	 */
+	CONF_MAX_THREADS,
 };
 
 /** Capabilities of process. */
diff --git a/include/kmi/tcb.h b/include/kmi/tcb.h
index a6522f8..4214cb0 100644
--- a/include/kmi/tcb.h
+++ b/include/kmi/tcb.h
@@ -198,6 +198,11 @@ struct tcb {
  */
 void init_tcbs();
 
+/**
+ * @return Maximum number of threads active at the same time.
+ */
+size_t max_tcbs();
+
 /**
  * Destroy thread control subsystem.
  */
diff --git a/src/tcb.c b/src/tcb.c
index 506b756..238d8e3 100644
--- a/src/tcb.c
+++ b/src/tcb.c
@@ -26,8 +26,8 @@
 /** Thread ID to start looking from when allocating new ID. */
 static id_t start_tid = 0;
 
-/** Total number of possible thread IDs. */
-static id_t num_tids;
+/** Maximum number of concurrently active threads. */
+static id_t num_tcbs;
 
 /** Pointer to array of \ref tcb structures. Length of the array is \c num_tids.*/
 static struct tcb **tcbs;
@@ -40,14 +40,19 @@ static struct tcb **tcbs;
  */
 static struct tcb *__cpu_tcb[MAX_CPUS] = { 0 };
 
+size_t max_tcbs()
+{
+	return num_tcbs;
+}
+
 void init_tcbs()
 {
 	/* MM_O1 is 2MiB on riscv64, so 262144 different possible thread ids.
 	 * Should be enough, if we're really strapped for memory I might try
 	 * something smaller but this is fine for now. */
 	tcbs = (struct tcb **)alloc_page(MM_O1);
-	num_tids = order_size(MM_O1) / sizeof(struct tcb *);
-	assert(is_powerof2(num_tids));
+	num_tcbs = order_size(MM_O1) / sizeof(struct tcb *);
+	assert(is_powerof2(num_tcbs));
 	memset(tcbs, 0, order_size(MM_O1));
 }
 
@@ -74,10 +79,10 @@ static id_t __alloc_tid(struct tcb *t)
 		if (i == stop_tid)
 			return ERR_NF;
 
-		if (tcbs[i & (num_tids - 1)] || i == 0)
+		if (tcbs[i & (num_tcbs - 1)] || i == 0)
 			continue;
 
-		tcbs[i & (num_tids - 1)] = t;
+		tcbs[i & (num_tcbs - 1)] = t;
 		start_tid = i + 1;
 		return i;
 	}
@@ -369,7 +374,7 @@ struct tcb *get_tcb(id_t tid)
 	if (tid <= 0)
 		return NULL;
 
-	struct tcb *t = tcbs[tid & (num_tids - 1)];
+	struct tcb *t = tcbs[tid & (num_tcbs - 1)];
 	if (!t)
 		return NULL;
 
diff --git a/src/uapi/conf.c b/src/uapi/conf.c
index 6a801f3..24a4706 100644
--- a/src/uapi/conf.c
+++ b/src/uapi/conf.c
@@ -76,6 +76,10 @@ SYSCALL_DEFINE2(get_conf)(struct tcb *t, sys_arg_t param, sys_arg_t d0)
 		val = order_size(d0);
 		break;
 
+	case CONF_MAX_THREADS:
+		val = max_tcbs();
+		break;
+
 	default:
 		return_args1(t, ERR_NF);
 	}
-- 
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