[PATCH hci-8.1 v4 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. 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. Other errors are possible as well, so do not key on the
errno: if VHOST_SET_FORK_FROM_OWNER fails, keep all virtqueues on the
default thread; if VHOST_NEW_WORKER fails partway, spread the
remaining virtqueues over the threads created so far; if attaching a
virtqueue fails, it stays on the default thread. Either way warn rather
than fail to start vhost-blk. Unless VHOST_SET_FORK_FROM_OWNER failed,
the default thread stays a vhost_task, as the mode cannot be changed
after VHOST_SET_OWNER.

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

Signed-off-by: Alexander Ivanov <alexander.ivanov@virtuozzo.com>
---
 hw/block/vhost-blk.c | 64 +++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 60 insertions(+), 4 deletions(-)

diff --git a/hw/block/vhost-blk.c b/hw/block/vhost-blk.c
index b3eb181ceae..9416c3ac07e 100644
--- a/hw/block/vhost-blk.c
+++ b/hw/block/vhost-blk.c
@@ -33,6 +33,53 @@
 #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 void vhost_blk_set_workers(struct vhost_dev *hdev, int nworkers)
+{
+    uint32_t ids[VHOST_BLK_MAX_QUEUES];
+    int i, ret, err = 0, nfailed = 0;
+
+    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) {
+                warn_report("vhost-blk: unable to create kernel thread: %s",
+                            strerror(-ret));
+                nworkers = i;
+                continue;
+            }
+            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) {
+            err = ret;
+            nfailed++;
+        }
+    }
+
+    if (nfailed) {
+        warn_report("vhost-blk: unable to attach kernel threads to %d "
+                    "queues: %s", nfailed, strerror(-err));
+    }
+}
+
 static int vhost_blk_start(VirtIODevice *vdev)
 {
     VHostBlk *s = VHOST_BLK(vdev);
@@ -52,15 +99,24 @@ static int vhost_blk_start(VirtIODevice *vdev)
         return 0;
     }
 
+    nworkers = MIN(s->conf.num_threads, hdev->nvqs);
+    if (nworkers > 1) {
+        ret = hdev->vhost_ops->vhost_set_fork_from_owner(hdev,
+                                                         VHOST_FORK_OWNER_TASK);
+        if (ret < 0) {
+            warn_report("vhost-blk: unable to use multiple kernel threads: "
+                        "%s", strerror(-ret));
+            nworkers = 1;
+        }
+    }
+
     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) {
+        vhost_blk_set_workers(hdev, nworkers);
     }
 
     if (s->conf.serial) {
-- 
2.53.0