[PATCH hci-8.1 v3 3/4] vhost-blk: create kernel threads with the mainstream worker ioctls

[Alexander Ivanov <alexander.ivanov@virtuozzo.com>]

num-threads was passed to our own VHOST_SET_NWORKERS kernel ioctl. The
kernel drops it with the revert of "drivers/vhost: add ioctl to
increase the number of workers" in favour of the mainstream worker
ioctls, which vhost-blk now supports. Create num-threads - 1 workers
with VHOST_NEW_WORKER and attach them to the virtqueues round-robin
with VHOST_ATTACH_VRING_WORKER, the layout VHOST_SET_NWORKERS produced.

Our kernel, unlike mainline, defaults to kthread mode, where
VHOST_NEW_WORKER fails with -EFAULT rather than -ENOTTY. So switch the
device to vhost_task mode with VHOST_SET_FORK_FROM_OWNER before
VHOST_SET_OWNER. With num-threads=1 nothing changes: no worker ioctls,
kthread mode.

Released vz10 kernels have VHOST_SET_FORK_FROM_OWNER, but their
vhost-blk does not handle the worker ioctls and fails VHOST_NEW_WORKER
with -ENOTTY. On such a kernel, or one without
VHOST_SET_FORK_FROM_OWNER, warn and run a single thread rather than
fail to start vhost-blk.

https://virtuozzo.atlassian.net/browse/VSTOR-107537

Signed-off-by: Alexander Ivanov <alexander.ivanov@virtuozzo.com>
---
 hw/block/vhost-blk.c | 100 +++++++++++++++++++++++++++++++++++++------
 1 file changed, 86 insertions(+), 14 deletions(-)

diff --git a/hw/block/vhost-blk.c b/hw/block/vhost-blk.c
index 8a015d54fa7..cc05eb057df 100644
--- a/hw/block/vhost-blk.c
+++ b/hw/block/vhost-blk.c
@@ -33,9 +33,60 @@
 #include "migration/cpr.h"
 #include "system/runstate.h"
 
+/*
+ * Spread the virtqueues round-robin over nworkers kernel threads. Thread 0 is
+ * the default worker VHOST_SET_OWNER created; vq 0 and every nworkers-th vq
+ * after it stay on it.
+ */
+static int vhost_blk_set_workers(struct vhost_dev *hdev, int nworkers)
+{
+    uint32_t ids[VHOST_BLK_MAX_QUEUES];
+    int i, ret;
+
+    assert(nworkers <= ARRAY_SIZE(ids));
+
+    for (i = 1; i < hdev->nvqs; i++) {
+        int w = i % nworkers;
+        struct vhost_vring_worker vq_worker = { .index = i };
+
+        if (w == 0) {
+            continue;
+        }
+
+        /* vq i < nworkers is the first user of thread i: create it here */
+        if (i < nworkers) {
+            struct vhost_worker_state worker = { };
+
+            ret = hdev->vhost_ops->vhost_new_worker(hdev, &worker);
+            if (ret == -ENOTTY) {
+                /* vhost-blk predating worker ioctls, fails on vq 1 only */
+                warn_report_once("vhost-blk: kernel does not support worker "
+                                 "ioctls, using a single kernel thread");
+                return 0;
+            } else if (ret < 0) {
+                error_report("vhost-blk: unable to create kernel thread: %s",
+                             strerror(-ret));
+                return ret;
+            }
+            ids[w] = worker.worker_id;
+        }
+
+        vq_worker.worker_id = ids[w];
+        ret = hdev->vhost_ops->vhost_attach_vring_worker(hdev, &vq_worker);
+        if (ret < 0) {
+            error_report("vhost-blk: unable to attach kernel thread to "
+                         "queue %d: %s", i, strerror(-ret));
+            return ret;
+        }
+    }
+
+    return 0;
+}
+
 static int vhost_blk_start(VirtIODevice *vdev)
 {
     VHostBlk *s = VHOST_BLK(vdev);
+    struct vhost_dev *hdev = &s->dev;
     struct vhost_vring_file backend;
     int ret, i, nworkers;
     BusState *qbus = BUS(qdev_get_parent_bus(DEVICE(vdev)));
@@ -51,15 +102,36 @@ static int vhost_blk_start(VirtIODevice *vdev)
         return 0;
     }
 
+    nworkers = MIN(s->conf.num_threads, hdev->nvqs);
+    if (nworkers > 1) {
+        /*
+         * VHOST_NEW_WORKER needs vhost_task mode; our kernel defaults to
+         * kthreads and VHOST_RESET_OWNER brings that default back, so ask
+         * on every start. Must precede VHOST_SET_OWNER.
+         */
+        ret = hdev->vhost_ops->vhost_set_fork_from_owner(hdev,
+                                                         VHOST_FORK_OWNER_TASK);
+        if (ret == -ENOTTY) {
+            warn_report_once("vhost-blk: kernel does not support vhost tasks, "
+                             "using a single kernel thread");
+            nworkers = 1;
+        } else if (ret < 0) {
+            error_report("vhost-blk: unable to set vhost task mode: %s",
+                         strerror(-ret));
+            return ret;
+        }
+    }
+
     if (ioctl(s->vhostfd, VHOST_SET_OWNER, NULL)) {
         error_report("vhost-blk: unable to set owner");
         return -ENOSYS;
     }
 
-    nworkers = s->conf.num_threads;
-    if (nworkers != 1 && ioctl(s->vhostfd, VHOST_SET_NWORKERS, &nworkers)) {
-        error_report("vhost-blk: unable to set number of kernel threads");
-        return -EINVAL;
+    if (nworkers > 1) {
+        ret = vhost_blk_set_workers(hdev, nworkers);
+        if (ret < 0) {
+            return ret;
+        }
     }
 
     if (s->conf.serial) {
@@ -70,22 +142,22 @@ static int vhost_blk_start(VirtIODevice *vdev)
         }
     }
 
-    ret = k->set_guest_notifiers(qbus->parent, s->dev.nvqs, true);
+    ret = k->set_guest_notifiers(qbus->parent, hdev->nvqs, true);
     if (ret < 0) {
         error_report("vhost-blk: unable to bind guest notifiers");
         return ret;
     }
 
-    ret = vhost_dev_enable_notifiers(&s->dev, vdev);
+    ret = vhost_dev_enable_notifiers(hdev, vdev);
     if (ret < 0) {
         error_report("vhost-blk: unable to enable dev notifiers: %s",
                      strerror(errno));
         goto err_set;
     }
 
-    s->dev.acked_features = vdev->guest_features & s->dev.features;
+    hdev->acked_features = vdev->guest_features & hdev->features;
 
-    ret = vhost_dev_start(&s->dev, vdev, false);
+    ret = vhost_dev_start(hdev, vdev, false);
     if (ret < 0) {
         error_report("vhost-blk: unable to start vhost dev");
         goto err_disable;
@@ -100,11 +172,11 @@ static int vhost_blk_start(VirtIODevice *vdev)
         goto err_stop;
     }
 
-    for (i = 0; i < s->dev.nvqs; i++) {
-        vhost_virtqueue_mask(&s->dev, vdev, i, false);
+    for (i = 0; i < hdev->nvqs; i++) {
+        vhost_virtqueue_mask(hdev, vdev, i, false);
     }
 
-    for (i = 0; i < s->dev.nvqs; i++) {
+    for (i = 0; i < hdev->nvqs; i++) {
         event_notifier_set(virtio_queue_get_host_notifier(virtio_get_queue(vdev, i)));
     }
 
@@ -113,11 +185,11 @@ static int vhost_blk_start(VirtIODevice *vdev)
     return 0;
 
 err_stop:
-    vhost_dev_stop(&s->dev, vdev, false);
+    vhost_dev_stop(hdev, vdev, false);
 err_disable:
-    vhost_dev_disable_notifiers(&s->dev, vdev);
+    vhost_dev_disable_notifiers(hdev, vdev);
 err_set:
-    ret = k->set_guest_notifiers(qbus->parent, s->dev.nvqs, false);
+    ret = k->set_guest_notifiers(qbus->parent, hdev->nvqs, false);
     if (ret < 0) {
         error_report("vhost-blk: unable to unbind guest notifiers");
     }
-- 
2.53.0