sched_ext: Maintain an online cid mask in the scheduler arenagrafted

Schedulers on the default cid mapping treat [0, nr_online_cids) as the
online set and restart on hotplug. Schedulers that install their own mapping
with scx_bpf_cid_override() have no way to learn which cids are online: the
count no longer identifies members and the CPU-form cpumask is unusable from
cid programs. This is an obvious hole in the cid API.

Add scx_bpf_online_cmask(), a kernel-maintained cmask in the scheduler's
arena, allocated alongside the per-CPU scratch masks and populated after the
cid mapping is finalized and before ops.init(), for child schedulers too.
The pointer stays valid through ops.exit() with no reference to take. It is
the arena offset as a void pointer, the same form struct_ops arena arguments
arrive in. The verifier types the void return as a scalar for the program's
arena cast.

The mask follows the SCX hotplug notifications: seeded from cpu_active_mask
and updated before ops.cid_online/offline() runs, so it lags cpu_online_mask
only inside a hotplug transition. Updates walk the scheduler list under the
lock that also serializes unlinking. Reads are live, not atomic snapshots.
Root initialization excludes hotplug.

v2: Reworded the getter kerneldoc (Andrea Righi).

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

1 files changed, 379 insertions, 0 deletions

diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
new file mode 100644
index 000000000..24514c8fd
--- /dev/null
+++ b/drivers/vhost/test.c
@@ -0,0 +1,379 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Copyright (C) 2009 Red Hat, Inc.
+ * Author: Michael S. Tsirkin <mst@redhat.com>
+ *
+ * test virtio server in host kernel.
+ */
+
+#include <linux/compat.h>
+#include <linux/eventfd.h>
+#include <linux/vhost.h>
+#include <linux/miscdevice.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/workqueue.h>
+#include <linux/file.h>
+#include <linux/slab.h>
+
+#include "test.h"
+#include "vhost.h"
+
+/* Max number of bytes transferred before requeueing the job.
+ * Using this limit prevents one virtqueue from starving others. */
+#define VHOST_TEST_WEIGHT 0x80000
+
+/* Max number of packets transferred before requeueing the job.
+ * Using this limit prevents one virtqueue from starving others with
+ * pkts.
+ */
+#define VHOST_TEST_PKT_WEIGHT 256
+
+static const int vhost_test_bits[] = {
+	VHOST_FEATURES
+};
+
+#define VHOST_TEST_FEATURES VHOST_FEATURES_U64(vhost_test_bits, 0)
+
+enum {
+	VHOST_TEST_VQ = 0,
+	VHOST_TEST_VQ_MAX = 1,
+};
+
+struct vhost_test {
+	struct vhost_dev dev;
+	struct vhost_virtqueue vqs[VHOST_TEST_VQ_MAX];
+};
+
+/* Expects to be always run from workqueue - which acts as
+ * read-size critical section for our kind of RCU. */
+static void handle_vq(struct vhost_test *n)
+{
+	struct vhost_virtqueue *vq = &n->vqs[VHOST_TEST_VQ];
+	unsigned out, in;
+	int head;
+	size_t len, total_len = 0;
+	void *private;
+
+	mutex_lock(&vq->mutex);
+	private = vhost_vq_get_backend(vq);
+	if (!private) {
+		mutex_unlock(&vq->mutex);
+		return;
+	}
+
+	vhost_disable_notify(&n->dev, vq);
+
+	for (;;) {
+		head = vhost_get_vq_desc(vq, vq->iov,
+					 ARRAY_SIZE(vq->iov),
+					 &out, &in,
+					 NULL, NULL);
+		/* On error, stop handling until the next kick. */
+		if (unlikely(head < 0))
+			break;
+		/* Nothing new?  Wait for eventfd to tell us they refilled. */
+		if (head == vq->num) {
+			if (unlikely(vhost_enable_notify(&n->dev, vq))) {
+				vhost_disable_notify(&n->dev, vq);
+				continue;
+			}
+			break;
+		}
+		if (in) {
+			vq_err(vq, "Unexpected descriptor format for TX: "
+			       "out %d, int %d\n", out, in);
+			break;
+		}
+		len = iov_length(vq->iov, out);
+		/* Sanity check */
+		if (!len) {
+			vq_err(vq, "Unexpected 0 len for TX\n");
+			break;
+		}
+		vhost_add_used_and_signal(&n->dev, vq, head, 0);
+		total_len += len;
+		if (unlikely(vhost_exceeds_weight(vq, 0, total_len)))
+			break;
+	}
+
+	mutex_unlock(&vq->mutex);
+}
+
+static void handle_vq_kick(struct vhost_work *work)
+{
+	struct vhost_virtqueue *vq = container_of(work, struct vhost_virtqueue,
+						  poll.work);
+	struct vhost_test *n = container_of(vq->dev, struct vhost_test, dev);
+
+	handle_vq(n);
+}
+
+static int vhost_test_open(struct inode *inode, struct file *f)
+{
+	struct vhost_test *n = kmalloc_obj(*n);
+	struct vhost_dev *dev;
+	struct vhost_virtqueue **vqs;
+
+	if (!n)
+		return -ENOMEM;
+	vqs = kmalloc_objs(*vqs, VHOST_TEST_VQ_MAX);
+	if (!vqs) {
+		kfree(n);
+		return -ENOMEM;
+	}
+
+	dev = &n->dev;
+	vqs[VHOST_TEST_VQ] = &n->vqs[VHOST_TEST_VQ];
+	n->vqs[VHOST_TEST_VQ].handle_kick = handle_vq_kick;
+	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV,
+		       VHOST_TEST_PKT_WEIGHT, VHOST_TEST_WEIGHT, true, NULL);
+
+	f->private_data = n;
+
+	return 0;
+}
+
+static void *vhost_test_stop_vq(struct vhost_test *n,
+				struct vhost_virtqueue *vq)
+{
+	void *private;
+
+	mutex_lock(&vq->mutex);
+	private = vhost_vq_get_backend(vq);
+	vhost_vq_set_backend(vq, NULL);
+	mutex_unlock(&vq->mutex);
+	return private;
+}
+
+static void vhost_test_stop(struct vhost_test *n, void **privatep)
+{
+	*privatep = vhost_test_stop_vq(n, n->vqs + VHOST_TEST_VQ);
+}
+
+static void vhost_test_flush(struct vhost_test *n)
+{
+	vhost_dev_flush(&n->dev);
+}
+
+static int vhost_test_release(struct inode *inode, struct file *f)
+{
+	struct vhost_test *n = f->private_data;
+	void  *private;
+
+	vhost_test_stop(n, &private);
+	vhost_test_flush(n);
+	vhost_dev_stop(&n->dev);
+	vhost_dev_cleanup(&n->dev);
+	kfree(n->dev.vqs);
+	kfree(n);
+	return 0;
+}
+
+static long vhost_test_run(struct vhost_test *n, int test)
+{
+	void *priv, *oldpriv;
+	struct vhost_virtqueue *vq;
+	int r, index;
+
+	if (test < 0 || test > 1)
+		return -EINVAL;
+
+	mutex_lock(&n->dev.mutex);
+	r = vhost_dev_check_owner(&n->dev);
+	if (r)
+		goto err;
+
+	for (index = 0; index < n->dev.nvqs; ++index) {
+		/* Verify that ring has been setup correctly. */
+		if (!vhost_vq_access_ok(&n->vqs[index])) {
+			r = -EFAULT;
+			goto err;
+		}
+	}
+
+	for (index = 0; index < n->dev.nvqs; ++index) {
+		vq = n->vqs + index;
+		mutex_lock(&vq->mutex);
+		priv = test ? n : NULL;
+
+		/* start polling new socket */
+		oldpriv = vhost_vq_get_backend(vq);
+		vhost_vq_set_backend(vq, priv);
+
+		r = vhost_vq_init_access(&n->vqs[index]);
+
+		mutex_unlock(&vq->mutex);
+
+		if (r)
+			goto err;
+
+		if (oldpriv) {
+			vhost_test_flush(n);
+		}
+	}
+
+	mutex_unlock(&n->dev.mutex);
+	return 0;
+
+err:
+	mutex_unlock(&n->dev.mutex);
+	return r;
+}
+
+static long vhost_test_reset_owner(struct vhost_test *n)
+{
+	void *priv = NULL;
+	long err;
+	struct vhost_iotlb *umem;
+
+	mutex_lock(&n->dev.mutex);
+	err = vhost_dev_check_owner(&n->dev);
+	if (err)
+		goto done;
+	umem = vhost_dev_reset_owner_prepare();
+	if (!umem) {
+		err = -ENOMEM;
+		goto done;
+	}
+	vhost_test_stop(n, &priv);
+	vhost_test_flush(n);
+	vhost_dev_stop(&n->dev);
+	vhost_dev_reset_owner(&n->dev, umem);
+done:
+	mutex_unlock(&n->dev.mutex);
+	return err;
+}
+
+static int vhost_test_set_features(struct vhost_test *n, u64 features)
+{
+	struct vhost_virtqueue *vq;
+
+	mutex_lock(&n->dev.mutex);
+	if ((features & (1 << VHOST_F_LOG_ALL)) &&
+	    !vhost_log_access_ok(&n->dev)) {
+		mutex_unlock(&n->dev.mutex);
+		return -EFAULT;
+	}
+	vq = &n->vqs[VHOST_TEST_VQ];
+	mutex_lock(&vq->mutex);
+	vq->acked_features = features;
+	mutex_unlock(&vq->mutex);
+	mutex_unlock(&n->dev.mutex);
+	return 0;
+}
+
+static long vhost_test_set_backend(struct vhost_test *n, unsigned index, int fd)
+{
+	static void *backend;
+
+	const bool enable = fd != -1;
+	struct vhost_virtqueue *vq;
+	int r;
+
+	mutex_lock(&n->dev.mutex);
+	r = vhost_dev_check_owner(&n->dev);
+	if (r)
+		goto err;
+
+	if (index >= VHOST_TEST_VQ_MAX) {
+		r = -ENOBUFS;
+		goto err;
+	}
+	vq = &n->vqs[index];
+	mutex_lock(&vq->mutex);
+
+	/* Verify that ring has been setup correctly. */
+	if (!vhost_vq_access_ok(vq)) {
+		r = -EFAULT;
+		goto err_vq;
+	}
+	if (!enable) {
+		vhost_poll_stop(&vq->poll);
+		backend = vhost_vq_get_backend(vq);
+		vhost_vq_set_backend(vq, NULL);
+	} else {
+		vhost_vq_set_backend(vq, backend);
+		r = vhost_vq_init_access(vq);
+		if (r == 0)
+			r = vhost_poll_start(&vq->poll, vq->kick);
+	}
+
+	mutex_unlock(&vq->mutex);
+
+	if (enable) {
+		vhost_test_flush(n);
+	}
+
+	mutex_unlock(&n->dev.mutex);
+	return 0;
+
+err_vq:
+	mutex_unlock(&vq->mutex);
+err:
+	mutex_unlock(&n->dev.mutex);
+	return r;
+}
+
+static long vhost_test_ioctl(struct file *f, unsigned int ioctl,
+			     unsigned long arg)
+{
+	struct vhost_vring_file backend;
+	struct vhost_test *n = f->private_data;
+	void __user *argp = (void __user *)arg;
+	u64 __user *featurep = argp;
+	int test;
+	u64 features;
+	int r;
+	switch (ioctl) {
+	case VHOST_TEST_RUN:
+		if (copy_from_user(&test, argp, sizeof test))
+			return -EFAULT;
+		return vhost_test_run(n, test);
+	case VHOST_TEST_SET_BACKEND:
+		if (copy_from_user(&backend, argp, sizeof backend))
+			return -EFAULT;
+		return vhost_test_set_backend(n, backend.index, backend.fd);
+	case VHOST_GET_FEATURES:
+		features = VHOST_TEST_FEATURES;
+		if (copy_to_user(featurep, &features, sizeof features))
+			return -EFAULT;
+		return 0;
+	case VHOST_SET_FEATURES:
+		if (copy_from_user(&features, featurep, sizeof features))
+			return -EFAULT;
+		if (features & ~VHOST_TEST_FEATURES)
+			return -EOPNOTSUPP;
+		return vhost_test_set_features(n, features);
+	case VHOST_RESET_OWNER:
+		return vhost_test_reset_owner(n);
+	default:
+		mutex_lock(&n->dev.mutex);
+		r = vhost_dev_ioctl(&n->dev, ioctl, argp);
+                if (r == -ENOIOCTLCMD)
+                        r = vhost_vring_ioctl(&n->dev, ioctl, argp);
+		vhost_test_flush(n);
+		mutex_unlock(&n->dev.mutex);
+		return r;
+	}
+}
+
+static const struct file_operations vhost_test_fops = {
+	.owner          = THIS_MODULE,
+	.release        = vhost_test_release,
+	.unlocked_ioctl = vhost_test_ioctl,
+	.compat_ioctl   = compat_ptr_ioctl,
+	.open           = vhost_test_open,
+	.llseek		= noop_llseek,
+};
+
+static struct miscdevice vhost_test_misc = {
+	MISC_DYNAMIC_MINOR,
+	"vhost-test",
+	&vhost_test_fops,
+};
+module_misc_device(vhost_test_misc);
+
+MODULE_VERSION("0.0.1");
+MODULE_LICENSE("GPL v2");
+MODULE_AUTHOR("Michael S. Tsirkin");
+MODULE_DESCRIPTION("Host kernel side for virtio simulator");