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

[Si-Wei Liu]

On 3/21/2022 8:47 PM, Jason Wang wrote:
On Sat, Mar 19, 2022 at 12:14 PM Si-Wei Liu <si-wei....@oracle.com> wrote:
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....@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.
See below, it looks like the code doesn't find the correct vhost_dev.

+     *
+     * 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?);
Relevant but not directly related, for the vhost_dev where
nc->is_datapath is false, it will assume it is backed by a single
queue not a queue pair.

+     *
+     * 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
+     */
Note that the vq_index of each vhost_dev is assigned during
vhost_net_start() according to whether or not the MQ or CVQ is
negotiated in vhost_net_start()

     for (i = 0; i < nvhosts; i++) {

         if (i < data_queue_pairs) {
             peer = qemu_get_peer(ncs, i);
         } else { /* Control Virtqueue */
             peer = qemu_get_peer(ncs, n->max_queue_pairs);
         }

         net = get_vhost_net(peer);
         vhost_net_set_vq_index(net, i * 2, index_end);

It means some of the peers won't be used when MQ is not negotiated. So
it looks to me the evil came from virtio_net_get_notifier_mask().
Yes, there it is. Where the control virtqueue first ever needs a 
guest_notifier for vhost_dev.
Where it doesn't mask the correct vhost dev when the guest doesn't
support MQ but the host does. So we had option 4:

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 2087516253..5e9ac019cd 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -3179,7 +3179,13 @@ static void
virtio_net_guest_notifier_mask(VirtIODevice *vdev, int idx,
                                             bool mask)
  {
      VirtIONet *n = VIRTIO_NET(vdev);
-    NetClientState *nc = qemu_get_subqueue(n->nic, vq2q(idx));
+    NetClientState *nc;
+
+    if (!virtio_vdev_has_feature(vdev, VIRTIO_NET_F_MQ) && idx == 2) {
Hmmm, I thought it would be more natural to align the layout of 
vhost_dev's with that of virtqueue's, not the other way around. Not sure 
how this vhost_dev selection scheme may work with additional queues 
discovered through transport specific mechanism, such as the admin 
virtqueue, but I can live with it for now:

--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -244,7 +244,8 @@ static void virtio_net_vhost_status(VirtIONet *n, 
uint8_t status)
     VirtIODevice *vdev = VIRTIO_DEVICE(n);
     NetClientState *nc = qemu_get_queue(n->nic);
     int queue_pairs = n->multiqueue ? n->max_queue_pairs : 1;
-    int cvq = n->max_ncs - n->max_queue_pairs;
+    int cvq = virtio_vdev_has_feature(vdev, VIRTIO_NET_F_CTRL_VQ) ?
+              n->max_ncs - n->max_queue_pairs : 0;

     if (!get_vhost_net(nc->peer)) {
         return;
@@ -3161,8 +3162,14 @@ static NetClientInfo net_virtio_info = {
 static bool virtio_net_guest_notifier_pending(VirtIODevice *vdev, int idx)
 {
     VirtIONet *n = VIRTIO_NET(vdev);
-    NetClientState *nc = qemu_get_subqueue(n->nic, vq2q(idx));
+    NetClientState *nc;
     assert(n->vhost_started);
+    if (!virtio_vdev_has_feature(vdev, VIRTIO_NET_F_MQ) && idx == 2) {
+        assert(virtio_vdev_has_feature(vdev, VIRTIO_NET_F_CTRL_VQ));
+        nc = qemu_get_subqueue(n->nic, n->max_queue_pairs);
+    } else {
+        nc = qemu_get_subqueue(n->nic, vq2q(idx));
+    }
     return vhost_net_virtqueue_pending(get_vhost_net(nc->peer), idx);
 }

@@ -3170,8 +3177,14 @@ static void 
virtio_net_guest_notifier_mask(VirtIODevice *vdev, int idx,
                                            bool mask)
 {
     VirtIONet *n = VIRTIO_NET(vdev);
-    NetClientState *nc = qemu_get_subqueue(n->nic, vq2q(idx));
+    NetClientState *nc;
     assert(n->vhost_started);
+    if (!virtio_vdev_has_feature(vdev, VIRTIO_NET_F_MQ) && idx == 2) {
+        assert(virtio_vdev_has_feature(vdev, VIRTIO_NET_F_CTRL_VQ));
+        nc = qemu_get_subqueue(n->nic, n->max_queue_pairs);
+    } else {
+        nc = qemu_get_subqueue(n->nic, vq2q(idx));
+    }
     vhost_net_virtqueue_mask(get_vhost_net(nc->peer),
                              vdev, idx, mask);
 }


Thanks,
-Siwei

+        nc = qemu_get_subqueue(n->nic, n->max_queue_pairs);
+    } else {
+        nc = qemu_get_subqueue(n->nic, vq2q(idx));
+    }
      assert(n->vhost_started);
      vhost_net_virtqueue_mask(get_vhost_net(nc->peer),
                               vdev, idx, mask);

Thanks

+    if (dev->vq_index + dev->nvqs < dev->vq_index_end) {
          return 0;
      }

--
1.8.3.1