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Re: [PATCH 4/7] virtio: don't read pending event on host notifier if dis

From: Si-Wei Liu
Subject: Re: [PATCH 4/7] virtio: don't read pending event on host notifier if disabled
Date: Fri, 1 Apr 2022 13:37:23 -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 1:36 AM, Jason Wang wrote:

On Thu, Mar 31, 2022 at 12:41 AM Si-Wei Liu <si-wei.liu@oracle.com> wrote:



On 3/30/2022 2:14 AM, Jason Wang wrote:

On Wed, Mar 30, 2022 at 2:33 PM Si-Wei Liu <si-wei.liu@oracle.com> wrote:

Previous commit prevents vhost-user and vhost-vdpa from using
userland vq handler via disable_ioeventfd_handler. The same
needs to be done for host notifier cleanup too, as the
virtio_queue_host_notifier_read handler still tends to read
pending event left behind on ioeventfd and attempts to handle
outstanding kicks from QEMU userland vq.

If vq handler is not disabled on cleanup, it may lead to sigsegv
with recursive virtio_net_set_status call on the control vq:

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

I feel it's probably a bug elsewhere e.g when we fail to start
vhost-vDPA, it's the charge of the Qemu to poll host notifier and we
will fallback to the userspace vq handler.

Apologies, an incorrect stack trace was pasted which actually came from
patch #1. I will post a v2 with the corresponding one as below:

0  0x000055f800df1780 in qdev_get_parent_bus (dev=0x0) at
../hw/core/qdev.c:376
1  0x000055f800c68ad8 in virtio_bus_device_iommu_enabled
(vdev=vdev@entry=0x0) at ../hw/virtio/virtio-bus.c:331
2  0x000055f800d70d7f in vhost_memory_unmap (dev=<optimized out>) at
../hw/virtio/vhost.c:318
3  0x000055f800d70d7f in vhost_memory_unmap (dev=<optimized out>,
buffer=0x7fc19bec5240, len=2052, is_write=1, access_len=2052) at
../hw/virtio/vhost.c:336
4  0x000055f800d71867 in vhost_virtqueue_stop
(dev=dev@entry=0x55f8037ccc30, vdev=vdev@entry=0x55f8044ec590,
vq=0x55f8037cceb0, idx=0) at ../hw/virtio/vhost.c:1241
5  0x000055f800d7406c in vhost_dev_stop (hdev=hdev@entry=0x55f8037ccc30,
vdev=vdev@entry=0x55f8044ec590) at ../hw/virtio/vhost.c:1839
6  0x000055f800bf00a7 in vhost_net_stop_one (net=0x55f8037ccc30,
dev=0x55f8044ec590) at ../hw/net/vhost_net.c:315
7  0x000055f800bf0678 in vhost_net_stop (dev=dev@entry=0x55f8044ec590,
ncs=0x55f80452bae0, data_queue_pairs=data_queue_pairs@entry=7,
cvq=cvq@entry=1)
     at ../hw/net/vhost_net.c:423
8  0x000055f800d4e628 in virtio_net_set_status (status=<optimized out>,
n=0x55f8044ec590) at ../hw/net/virtio-net.c:296
9  0x000055f800d4e628 in virtio_net_set_status
(vdev=vdev@entry=0x55f8044ec590, status=15 '\017') at
../hw/net/virtio-net.c:370

I don't understand why virtio_net_handle_ctrl() call virtio_net_set_stauts()...
The pending request left over on the ctrl vq was a VIRTIO_NET_CTRL_MQ command, i.e. in virtio_net_handle_mq(): 

1413     n->curr_queue_pairs = queue_pairs;
1414     /* stop the backend before changing the number of queue_pairs to avoid handling a 
1415      * disabled queue */
1416     virtio_net_set_status(vdev, vdev->status);
1417     virtio_net_set_queue_pairs(n);
Noted before the vdpa multiqueue support, there was never a vhost_dev for ctrl_vq exposed, i.e. there's no host notifier set up for the ctrl_vq on vhost_kernel as it is emulated in QEMU software. 




10 0x000055f800d534d8 in virtio_net_handle_ctrl (iov_cnt=<optimized
out>, iov=<optimized out>, cmd=0 '\000', n=0x55f8044ec590) at
../hw/net/virtio-net.c:1408
11 0x000055f800d534d8 in virtio_net_handle_ctrl (vdev=0x55f8044ec590,
vq=0x7fc1a7e888d0) at ../hw/net/virtio-net.c:1452
12 0x000055f800d69f37 in virtio_queue_host_notifier_read
(vq=0x7fc1a7e888d0) at ../hw/virtio/virtio.c:2331
13 0x000055f800d69f37 in virtio_queue_host_notifier_read
(n=n@entry=0x7fc1a7e8894c) at ../hw/virtio/virtio.c:3575
14 0x000055f800c688e6 in virtio_bus_cleanup_host_notifier
(bus=<optimized out>, n=n@entry=14) at ../hw/virtio/virtio-bus.c:312
15 0x000055f800d73106 in vhost_dev_disable_notifiers
(hdev=hdev@entry=0x55f8035b51b0, vdev=vdev@entry=0x55f8044ec590)
     at ../../../include/hw/virtio/virtio-bus.h:35
16 0x000055f800bf00b2 in vhost_net_stop_one (net=0x55f8035b51b0,
dev=0x55f8044ec590) at ../hw/net/vhost_net.c:316
17 0x000055f800bf0678 in vhost_net_stop (dev=dev@entry=0x55f8044ec590,
ncs=0x55f80452bae0, data_queue_pairs=data_queue_pairs@entry=7,
cvq=cvq@entry=1)
     at ../hw/net/vhost_net.c:423
18 0x000055f800d4e628 in virtio_net_set_status (status=<optimized out>,
n=0x55f8044ec590) at ../hw/net/virtio-net.c:296
19 0x000055f800d4e628 in virtio_net_set_status (vdev=0x55f8044ec590,
status=15 '\017') at ../hw/net/virtio-net.c:370
20 0x000055f800d6c4b2 in virtio_set_status (vdev=0x55f8044ec590,
val=<optimized out>) at ../hw/virtio/virtio.c:1945
21 0x000055f800d11d9d in vm_state_notify (running=running@entry=false,
state=state@entry=RUN_STATE_SHUTDOWN) at ../softmmu/runstate.c:333
22 0x000055f800d04e7a in do_vm_stop
(state=state@entry=RUN_STATE_SHUTDOWN, send_stop=send_stop@entry=false)
at ../softmmu/cpus.c:262
23 0x000055f800d04e99 in vm_shutdown () at ../softmmu/cpus.c:280
24 0x000055f800d126af in qemu_cleanup () at ../softmmu/runstate.c:812
25 0x000055f800ad5b13 in main (argc=<optimized out>, argv=<optimized
out>, envp=<optimized out>) at ../softmmu/main.c:51

  From the trace pending read only occurs in stop path. The recursive
virtio_net_set_status from virtio_net_handle_ctrl doesn't make sense to me.

Yes, we need to figure this out to know the root cause.
I think it has something to do with the virtqueue unready issue that the vhost_reset_device() refactoring series attempt to fix. If that is fixed we should not see this sigsegv with mlx5_vdpa. However I guess we both agreed that the vq_unready support would need new uAPI (some flag) to define, hence this fix applies to the situation where uAPI doesn't exist on the kernel or the vq_unready is not supported by vdpa vendor driver. 



The code should work for the case when vhost-vdp fails to start.
Unlike the other datapath queues for net vdpa, the events left behind in the control queue can't be processed by the userspace, as unlike vhost-kernel we don't have a fallback path in the userspace. To ignore the pending event and let vhost vdpa process it on resume/start is perhaps the best thing to do. This is even true for datapath queues for other vdpa devices than of network. 




Not sure I got the reason why we need to handle pending host
notification in userland vq, can you elaborate?

Because vhost-vDPA fails to start, we will "fallback" to a dummy userspace.
Is the dummy userspace working or operational? What's the use case of this "fallback" dummy if what guest user can get is a busted NIC? I think this is very different from the vhost-kernel case in that once vhost fails, we can fallback to userspace to emulate the network through the tap fd in a good way. However, there's no equivalent yet for vhost-vdpa... 

Thanks,
-Siwei



Thanks


Thanks,
-Siwei


Thanks


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 
../../../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

Fixes: 4023784 ("vhost-vdpa: multiqueue support")
Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
---
   hw/virtio/virtio-bus.c | 3 ++-
   1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c
index 0f69d1c..3159b58 100644
--- a/hw/virtio/virtio-bus.c
+++ b/hw/virtio/virtio-bus.c
@@ -311,7 +311,8 @@ void virtio_bus_cleanup_host_notifier(VirtioBusState *bus, 
int n)
       /* Test and clear notifier after disabling event,
        * in case poll callback didn't have time to run.
        */
-    virtio_queue_host_notifier_read(notifier);
+    if (!vdev->disable_ioeventfd_handler)
+        virtio_queue_host_notifier_read(notifier);
       event_notifier_cleanup(notifier);
   }

--
1.8.3.1
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