[RFC PATCH] vhost_net: should not use max_queue_pairs for non-mq guest

[Si-Wei Liu <si-wei.liu@oracle.com>]

With MQ enabled vdpa device and non-MQ supporting guest e.g.
booting vdpa with mq=on over OVMF of single vqp, it's easy
to hit assert failure as the following:

../hw/virtio/vhost-vdpa.c:560: vhost_vdpa_get_vq_index: Assertion `idx >= dev->vq_index && idx < dev->vq_index + dev->nvqs' failed.

0  0x00007f8ce3ff3387 in raise () at /lib64/libc.so.6
1  0x00007f8ce3ff4a78 in abort () at /lib64/libc.so.6
2  0x00007f8ce3fec1a6 in __assert_fail_base () at /lib64/libc.so.6
3  0x00007f8ce3fec252 in  () at /lib64/libc.so.6
4  0x0000558f52d79421 in vhost_vdpa_get_vq_index (dev=<optimized out>, idx=<optimized out>) at ../hw/virtio/vhost-vdpa.c:563
5  0x0000558f52d79421 in vhost_vdpa_get_vq_index (dev=<optimized out>, idx=<optimized out>) at ../hw/virtio/vhost-vdpa.c:558
6  0x0000558f52d7329a in vhost_virtqueue_mask (hdev=0x558f55c01800, vdev=0x558f568f91f0, n=2, mask=<optimized out>) at ../hw/virtio/vhost.c:1557
7  0x0000558f52c6b89a in virtio_pci_set_guest_notifier (d=d@entry=0x558f568f0f60, n=n@entry=2, assign=assign@entry=true, with_irqfd=with_irqfd@entry=false)
   at ../hw/virtio/virtio-pci.c:974
8  0x0000558f52c6c0d8 in virtio_pci_set_guest_notifiers (d=0x558f568f0f60, nvqs=3, assign=true) at ../hw/virtio/virtio-pci.c:1019
9  0x0000558f52bf091d in vhost_net_start (dev=dev@entry=0x558f568f91f0, ncs=0x558f56937cd0, data_queue_pairs=data_queue_pairs@entry=1, cvq=cvq@entry=1)
   at ../hw/net/vhost_net.c:361
10 0x0000558f52d4e5e7 in virtio_net_set_status (status=<optimized out>, n=0x558f568f91f0) at ../hw/net/virtio-net.c:289
11 0x0000558f52d4e5e7 in virtio_net_set_status (vdev=0x558f568f91f0, status=15 '\017') at ../hw/net/virtio-net.c:370
12 0x0000558f52d6c4b2 in virtio_set_status (vdev=vdev@entry=0x558f568f91f0, val=val@entry=15 '\017') at ../hw/virtio/virtio.c:1945
13 0x0000558f52c69eff in virtio_pci_common_write (opaque=0x558f568f0f60, addr=<optimized out>, val=<optimized out>, size=<optimized out>) at ../hw/virtio/virtio-pci.c:1292
14 0x0000558f52d15d6e in memory_region_write_accessor (mr=0x558f568f19d0, addr=20, value=<optimized out>, size=1, shift=<optimized out>, mask=<optimized out>, attrs=...)
   at ../softmmu/memory.c:492
15 0x0000558f52d127de in access_with_adjusted_size (addr=addr@entry=20, value=value@entry=0x7f8cdbffe748, size=size@entry=1, access_size_min=<optimized out>, access_size_max=<optimized out>, access_fn=0x558f52d15cf0 <memory_region_write_accessor>, mr=0x558f568f19d0, attrs=...) at ../softmmu/memory.c:554
16 0x0000558f52d157ef in memory_region_dispatch_write (mr=mr@entry=0x558f568f19d0, addr=20, data=<optimized out>, op=<optimized out>, attrs=attrs@entry=...)
   at ../softmmu/memory.c:1504
17 0x0000558f52d078e7 in flatview_write_continue (fv=fv@entry=0x7f8accbc3b90, addr=addr@entry=103079215124, attrs=..., ptr=ptr@entry=0x7f8ce6300028, len=len@entry=1, addr1=<optimized out>, l=<optimized out>, mr=0x558f568f19d0) at /home/opc/qemu-upstream/include/qemu/host-utils.h:165
18 0x0000558f52d07b06 in flatview_write (fv=0x7f8accbc3b90, addr=103079215124, attrs=..., buf=0x7f8ce6300028, len=1) at ../softmmu/physmem.c:2822
19 0x0000558f52d0b36b in address_space_write (as=<optimized out>, addr=<optimized out>, attrs=..., buf=buf@entry=0x7f8ce6300028, len=<optimized out>)
   at ../softmmu/physmem.c:2914
20 0x0000558f52d0b3da in address_space_rw (as=<optimized out>, addr=<optimized out>, attrs=...,
   attrs@entry=..., buf=buf@entry=0x7f8ce6300028, len=<optimized out>, is_write=<optimized out>) at ../softmmu/physmem.c:2924
21 0x0000558f52dced09 in kvm_cpu_exec (cpu=cpu@entry=0x558f55c2da60) at ../accel/kvm/kvm-all.c:2903
22 0x0000558f52dcfabd in kvm_vcpu_thread_fn (arg=arg@entry=0x558f55c2da60) at ../accel/kvm/kvm-accel-ops.c:49
23 0x0000558f52f9f04a in qemu_thread_start (args=<optimized out>) at ../util/qemu-thread-posix.c:556
24 0x00007f8ce4392ea5 in start_thread () at /lib64/libpthread.so.0
25 0x00007f8ce40bb9fd in clone () at /lib64/libc.so.6

The cause for the assert failure is due to that the vhost_dev index
for the ctrl vq was not aligned with actual one in use by the guest.
Upon multiqueue feature negotiation in virtio_net_set_multiqueue(),
if guest doesn't support multiqueue, the guest vq layout would shrink
to single queue pair of 3 vqs in total (rx, tx and ctrl). This results
in ctrl_vq taking a different vhost_dev group index than the default
n->max_queue_pairs, the latter of which is only valid for multiqueue
guest. While on those additional vqs not exposed to the guest,
vhost_net_set_vq_index() never populated vq_index properly, hence
getting the assert failure.

A possible fix is to pick the correct vhost_dev group for the control
vq according to this table [*]:

vdpa tool / QEMU arg / guest config    / ctrl_vq group index
----------------------------------------------------------------
max_vqp 8 / mq=on    / mq=off  (UEFI) => data_queue_pairs
max_vqp 8 / mq=on    / mq=on  (Linux) => n->max_queue_pairs(>1)
max_vqp 8 / mq=off   / mq=on  (Linux) => n->max_queue_pairs(=1)

[*] Please see FIXME in the code for open question and discussion

Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
---
 hw/net/vhost_net.c     | 13 +++++++++----
 hw/virtio/vhost-vdpa.c | 25 ++++++++++++++++++++++++-
 2 files changed, 33 insertions(+), 5 deletions(-)

diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
index 30379d2..9a4479b 100644
--- a/hw/net/vhost_net.c
+++ b/hw/net/vhost_net.c
@@ -322,6 +322,7 @@ int vhost_net_start(VirtIODevice *dev, NetClientState *ncs,
     BusState *qbus = BUS(qdev_get_parent_bus(DEVICE(dev)));
     VirtioBusState *vbus = VIRTIO_BUS(qbus);
     VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(vbus);
+    bool mq = virtio_host_has_feature(dev, VIRTIO_NET_F_MQ);
     int total_notifiers = data_queue_pairs * 2 + cvq;
     VirtIONet *n = VIRTIO_NET(dev);
     int nvhosts = data_queue_pairs + cvq;
@@ -343,7 +344,7 @@ int vhost_net_start(VirtIODevice *dev, NetClientState *ncs,
         if (i < data_queue_pairs) {
             peer = qemu_get_peer(ncs, i);
         } else { /* Control Virtqueue */
-            peer = qemu_get_peer(ncs, n->max_queue_pairs);
+            peer = qemu_get_peer(ncs, mq ? data_queue_pairs : n->max_queue_pairs);
         }
 
         net = get_vhost_net(peer);
@@ -368,7 +369,7 @@ int vhost_net_start(VirtIODevice *dev, NetClientState *ncs,
         if (i < data_queue_pairs) {
             peer = qemu_get_peer(ncs, i);
         } else {
-            peer = qemu_get_peer(ncs, n->max_queue_pairs);
+            peer = qemu_get_peer(ncs, mq ? data_queue_pairs : n->max_queue_pairs);
         }
         r = vhost_net_start_one(get_vhost_net(peer), dev);
 
@@ -390,7 +391,10 @@ int vhost_net_start(VirtIODevice *dev, NetClientState *ncs,
 
 err_start:
     while (--i >= 0) {
-        peer = qemu_get_peer(ncs , i);
+        if (mq)
+            peer = qemu_get_peer(ncs, i < data_queue_pairs ? i : data_queue_pairs);
+        else
+            peer = qemu_get_peer(ncs, i < data_queue_pairs ? i : n->max_queue_pairs);
         vhost_net_stop_one(get_vhost_net(peer), dev);
     }
     e = k->set_guest_notifiers(qbus->parent, total_notifiers, false);
@@ -409,6 +413,7 @@ void vhost_net_stop(VirtIODevice *dev, NetClientState *ncs,
     VirtioBusState *vbus = VIRTIO_BUS(qbus);
     VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(vbus);
     VirtIONet *n = VIRTIO_NET(dev);
+    bool mq = virtio_host_has_feature(dev, VIRTIO_NET_F_MQ);
     NetClientState *peer;
     int total_notifiers = data_queue_pairs * 2 + cvq;
     int nvhosts = data_queue_pairs + cvq;
@@ -418,7 +423,7 @@ void vhost_net_stop(VirtIODevice *dev, NetClientState *ncs,
         if (i < data_queue_pairs) {
             peer = qemu_get_peer(ncs, i);
         } else {
-            peer = qemu_get_peer(ncs, n->max_queue_pairs);
+            peer = qemu_get_peer(ncs, mq ? data_queue_pairs : n->max_queue_pairs);
         }
         vhost_net_stop_one(get_vhost_net(peer), dev);
     }
diff --git a/hw/virtio/vhost-vdpa.c b/hw/virtio/vhost-vdpa.c
index 27ea706..623476e 100644
--- a/hw/virtio/vhost-vdpa.c
+++ b/hw/virtio/vhost-vdpa.c
@@ -1097,7 +1097,30 @@ static int vhost_vdpa_dev_start(struct vhost_dev *dev, bool started)
         vhost_vdpa_host_notifiers_uninit(dev, dev->nvqs);
     }
 
-    if (dev->vq_index + dev->nvqs != dev->vq_index_end) {
+    /* FIXME the vhost_dev group for the control vq may have bogus nvqs=2
+     * value rather than nvqs=1. This can happen in case the guest doesn't
+     * support multiqueue, as a result of virtio_net_change_num_queue_pairs()
+     * destroying and rebuilding all the vqs, the guest index for control vq
+     * will no longer align with the host's. Currently net_init_vhost_vdpa()
+     * only initializes all vhost_dev's and net_clients once during
+     * net_client_init1() time, way earlier before multiqueue feature
+     * negotiation can kick in.
+     *
+     * Discussion - some possible fixes so far I can think of:
+     *
+     * option 1: fix vhost_net->dev.nvqs and nc->is_datapath in place for
+     * vdpa's ctrl vq, or rebuild all vdpa's vhost_dev groups and the
+     * net_client array, in the virtio_net_set_multiqueue() path;
+     *
+     * option 2: fix vhost_dev->nvqs in place at vhost_vdpa_set_features()
+     * before coming down to vhost_vdpa_dev_start() (Q: nc->is_datapath
+     * seems only used in virtio_net_device_realize, is it relevant?);
+     *
+     * option 3: use host queue index all along in vhost-vdpa ioctls instead
+     * of using guest vq index, so that vhost_net_start/stop() can remain
+     * as-is today
+     */
+    if (dev->vq_index + dev->nvqs < dev->vq_index_end) {
         return 0;
     }
 
-- 
1.8.3.1