linux-kernel - [RFC PATCH v2 10/11] rseq: extend struct rseq with per memory space vcpu id

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Message-Id: <20220218210633.23345-11-mathieu.desnoyers@efficios.com>
Date:   Fri, 18 Feb 2022 16:06:32 -0500
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        "H . Peter Anvin" <hpa@...or.com>, Paul Turner <pjt@...gle.com>,
        linux-api@...r.kernel.org,
        Christian Brauner <christian.brauner@...ntu.com>,
        Florian Weimer <fw@...eb.enyo.de>, David.Laight@...LAB.COM,
        carlos@...hat.com, Peter Oskolkov <posk@...k.io>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: [RFC PATCH v2 10/11] rseq: extend struct rseq with per memory space vcpu id

If a memory space has fewer threads than cores, or is limited to run on
few cores concurrently through sched affinity or cgroup cpusets, the
virtual cpu ids will be values close to 0, thus allowing efficient use
of user-space memory for per-cpu data structures.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
---
 include/uapi/linux/rseq.h |  9 +++++++++
 kernel/rseq.c             | 10 +++++++++-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h
index 1cb90a435c5c..77a136586ac6 100644
--- a/include/uapi/linux/rseq.h
+++ b/include/uapi/linux/rseq.h
@@ -139,6 +139,15 @@ struct rseq {
 	 */
 	__u32 node_id;
 
+	/*
+	 * Restartable sequences vm_vcpu_id field. Updated by the kernel. Read by
+	 * user-space with single-copy atomicity semantics. This field should
+	 * only be read by the thread which registered this data structure.
+	 * Aligned on 32-bit. Contains the current thread's virtual CPU ID
+	 * (allocated uniquely within a memory space).
+	 */
+	__u32 vm_vcpu_id;
+
 	/*
 	 * Flexible array member at end of structure, after last feature field.
 	 */
diff --git a/kernel/rseq.c b/kernel/rseq.c
index cb7d8a5afc82..1b00339c341b 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -89,12 +89,14 @@ static int rseq_update_cpu_node_id(struct task_struct *t)
 	struct rseq __user *rseq = t->rseq;
 	u32 cpu_id = raw_smp_processor_id();
 	u32 node_id = cpu_to_node(cpu_id);
+	u32 vm_vcpu_id = task_mm_vcpu_id(t);
 
 	if (!user_write_access_begin(rseq, t->rseq_len))
 		goto efault;
 	unsafe_put_user(cpu_id, &rseq->cpu_id_start, efault_end);
 	unsafe_put_user(cpu_id, &rseq->cpu_id, efault_end);
 	unsafe_put_user(node_id, &rseq->node_id, efault_end);
+	unsafe_put_user(vm_vcpu_id, &rseq->vm_vcpu_id, efault_end);
 	/*
 	 * Additional feature fields added after ORIG_RSEQ_SIZE
 	 * need to be conditionally updated only if
@@ -112,7 +114,8 @@ static int rseq_update_cpu_node_id(struct task_struct *t)
 
 static int rseq_reset_rseq_cpu_node_id(struct task_struct *t)
 {
-	u32 cpu_id_start = 0, cpu_id = RSEQ_CPU_ID_UNINITIALIZED, node_id = 0;
+	u32 cpu_id_start = 0, cpu_id = RSEQ_CPU_ID_UNINITIALIZED, node_id = 0,
+	    vm_vcpu_id = 0;
 
 	/*
 	 * Reset cpu_id_start to its initial state (0).
@@ -131,6 +134,11 @@ static int rseq_reset_rseq_cpu_node_id(struct task_struct *t)
 	 */
 	if (put_user(node_id, &t->rseq->node_id))
 		return -EFAULT;
+	/*
+	 * Reset vm_vcpu_id to its initial state (0).
+	 */
+	if (put_user(vm_vcpu_id, &t->rseq->vm_vcpu_id))
+		return -EFAULT;
 	/*
 	 * Additional feature fields added after ORIG_RSEQ_SIZE
 	 * need to be conditionally reset only if
-- 
2.17.1