[PATCH hci-8.1 v2 1/2] 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 is dropping it in favour of the mainstream worker ioctls,
which vhost-blk now supports, so use those: create num-threads - 1
workers with VHOST_NEW_WORKER and attach them to the virtqueues
round-robin with VHOST_ATTACH_VRING_WORKER, the default worker being
the first one. This is the layout VHOST_SET_NWORKERS produced.

VHOST_NEW_WORKER works only in vhost_task mode, while our kernel,
unlike mainline, defaults to kthreads, so switch the device to
vhost_task mode with VHOST_SET_FORK_FROM_OWNER before VHOST_SET_OWNER.
VHOST_RESET_OWNER restores the default, hence this is done on every
start. With num-threads=1 nothing changes: no worker ioctls, kthread
mode.

On a kernel without these ioctls 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 | 76 +++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 72 insertions(+), 4 deletions(-)

diff --git a/hw/block/vhost-blk.c b/hw/block/vhost-blk.c
index abddff02c6d..7b427083478 100644
--- a/hw/block/vhost-blk.c
+++ b/hw/block/vhost-blk.c
@@ -33,6 +33,52 @@
 #include "migration/cpr.h"
 #include "system/runstate.h"
 
+/*
+ * Spread the virtqueues round-robin over num-threads workers, the first one
+ * being the default worker that VHOST_SET_OWNER created.
+ */
+static int vhost_blk_set_workers(VHostBlk *s, int nworkers)
+{
+    uint32_t ids[VHOST_BLK_MAX_QUEUES];
+    struct vhost_worker_state worker;
+    struct vhost_vring_worker vq_worker;
+    int ret, i;
+
+    nworkers = MIN(nworkers, VHOST_BLK_MAX_QUEUES);
+    for (i = 1; i < nworkers; i++) {
+        memset(&worker, 0, sizeof(worker));
+        if (ioctl(s->vhostfd, VHOST_NEW_WORKER, &worker)) {
+            ret = -errno;
+            if (ret == -ENOTTY && i == 1) {
+                warn_report_once("vhost-blk: kernel does not support worker "
+                                 "ioctls, using a single kernel thread");
+                return 0;
+            }
+            error_report("vhost-blk: unable to create kernel thread: %s",
+                         strerror(-ret));
+            return ret;
+        }
+        ids[i] = worker.worker_id;
+    }
+
+    for (i = 0; i < s->dev.nvqs; i++) {
+        if (i % nworkers == 0) {
+            continue;
+        }
+        memset(&vq_worker, 0, sizeof(vq_worker));
+        vq_worker.index = i;
+        vq_worker.worker_id = ids[i % nworkers];
+        if (ioctl(s->vhostfd, VHOST_ATTACH_VRING_WORKER, &vq_worker)) {
+            ret = -errno;
+            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);
@@ -51,15 +97,37 @@ static int vhost_blk_start(VirtIODevice *vdev)
         return 0;
     }
 
+    nworkers = MIN(s->conf.num_threads, s->dev.nvqs);
+    if (nworkers > 1) {
+        /*
+         * VHOST_NEW_WORKER works only in vhost_task mode, and
+         * VHOST_RESET_OWNER brings the kthread default back.
+         */
+        uint8_t fork_owner = VHOST_FORK_OWNER_TASK;
+
+        if (ioctl(s->vhostfd, VHOST_SET_FORK_FROM_OWNER, &fork_owner)) {
+            ret = -errno;
+            if (ret != -ENOTTY) {
+                error_report("vhost-blk: unable to set vhost task mode: %s",
+                             strerror(-ret));
+                return ret;
+            }
+            warn_report_once("vhost-blk: kernel does not support vhost "
+                             "tasks, using a single kernel thread");
+            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) {
+        ret = vhost_blk_set_workers(s, nworkers);
+        if (ret < 0) {
+            return ret;
+        }
     }
 
     if (s->conf.serial) {
-- 
2.53.0