[PATCH hci-8.1 v5 4/5] 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. Do it for num-threads=1 as well, so the default
thread is of the same kind whatever the number of workers.

Fail to start vhost-blk if any of these ioctls fails rather than
silently run on fewer threads than requested. Released vz10 kernels
have VHOST_SET_FORK_FROM_OWNER, but their vhost-blk does not handle
the worker ioctls, so num-threads above 1 now requires a kernel that
does.

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

Signed-off-by: Alexander Ivanov <alexander.ivanov@virtuozzo.com>
---
 hw/block/vhost-blk.c | 62 +++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 58 insertions(+), 4 deletions(-)

diff --git a/hw/block/vhost-blk.c b/hw/block/vhost-blk.c
index b3eb181ceae..3cde12051fb 100644
--- a/hw/block/vhost-blk.c
+++ b/hw/block/vhost-blk.c
@@ -33,6 +33,50 @@
 #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;
+        }
+
+        if (i < nworkers) {
+            struct vhost_worker_state worker = { };
+
+            ret = hdev->vhost_ops->vhost_new_worker(hdev, &worker);
+            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);
@@ -52,15 +96,25 @@ static int vhost_blk_start(VirtIODevice *vdev)
         return 0;
     }
 
+    ret = hdev->vhost_ops->vhost_set_fork_from_owner(hdev,
+                                                     VHOST_FORK_OWNER_TASK);
+    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;
+    nworkers = MIN(s->conf.num_threads, hdev->nvqs);
+    if (nworkers > 1) {
+        ret = vhost_blk_set_workers(hdev, nworkers);
+        if (ret < 0) {
+            return ret;
+        }
     }
 
     if (s->conf.serial) {
-- 
2.53.0