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Re: [PATCH 1/7] virtio-net: align ctrl_vq index for non-mq guest for vho

From: Si-Wei Liu
Subject: Re: [PATCH 1/7] virtio-net: align ctrl_vq index for non-mq guest for vhost_vdpa
Date: Fri, 1 Apr 2022 15:32:02 -0700
User-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.7.0 


On 3/31/2022 1:39 AM, Jason Wang wrote:

On Wed, Mar 30, 2022 at 11:48 PM Si-Wei Liu <si-wei.liu@oracle.com> wrote:



On 3/30/2022 2:00 AM, Jason Wang wrote:

On Wed, Mar 30, 2022 at 2:33 PM Si-Wei Liu <si-wei.liu@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, below assert
failure is seen:

../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 a single queue pair, consisting of 3 vqs in total (rx, tx and ctrl).
This results in ctrl_vq taking a different vhost_dev group index than
the default. We can map vq to the correct vhost_dev group by checking
if MQ is supported by guest and successfully negotiated. Since the
MQ feature is only present along with CTRL_VQ, we make sure the index
2 is only meant for the control vq while MQ is not supported by guest.

Be noted if QEMU or guest doesn't support control vq, there's no bother
exposing vhost_dev and guest notifier for the control vq. Since
vhost_net_start/stop implies DRIVER_OK is set in device status, feature
negotiation should be completed when reaching virtio_net_vhost_status().

Fixes: 22288fe ("virtio-net: vhost control virtqueue support")
Suggested-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
---
   hw/net/virtio-net.c | 19 ++++++++++++++++---
   1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 1067e72..484b215 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -245,7 +245,8 @@ 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;

Let's use a separate patch for this.

Yes, I can do that. Then the new patch will become a requisite for this
patch.


       if (!get_vhost_net(nc->peer)) {
           return;
@@ -3170,8 +3171,14 @@ 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));

This assert seems guest trigger-able. If yes, I would remove this or
replace it with log_guest_error.

This assert actually is relevant to the cvq change in
virtio_net_vhost_status(). Since the same check on VIRTIO_NET_F_CTRL_VQ
has been done earlier, it is assured that CTRL_VQ is negotiated when
getting here.
Noted the vhost_started is asserted in the same function, which in turn
implies DRIVER_OK is set meaning feature negotiation is complete. I
can't easily think of a scenario which guest may inadvertently or
purposely trigger the assert?

So the code can be triggered like e.g unmasking:

virtio_pci_vq_vector_unmask()
         k->guest_notifier_pending()
Hmmm, are you concerned more about idx being invalid, or VIRTIO_NET_F_CTRL_VQ getting cleared?
virtio_pci_vector_unmask() has validation through virtio_queue_get_num() that ensures the vq index is valid. While it doesn't seem possible for VIRTIO_NET_F_CTRL_VQ to be cleared without device reset first, during which the pending event left over on guest notifier eventfd should have been completed within virtio_pci_set_guest_notifiers(false) before vhost_net_stop() returns. If I am not missing something here, I guess we're probably fine? 

-Siwei



Thanks



-Siwei


+        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);
   }

@@ -3179,8 +3186,14 @@ 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));

And this.

Thanks



+        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);
   }
--
1.8.3.1
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