Re: [Qemu-devel] [PATCH] virtio: introduce grab/release_ioeventfd to fix vhost

[Alex Williamson]

On Fri, 11 Nov 2016 20:28:55 +0100
Paolo Bonzini <address@hidden> wrote:

> Following the recent refactoring of virtio notifiers [1], more specifically
> the patch ed08a2a0b ("virtio: use virtio_bus_set_host_notifier to
> start/stop ioeventfd") that uses virtio_bus_set_host_notifier [2]
> by default, core virtio code requires 'ioeventfd_started' to be set
> to true/false when the host notifiers are configured.
> 
> When vhost is stopped and started, however, there is a stop followed by
> another start. Since ioeventfd_started was never set to true, the 'stop'
> operation triggered by virtio_bus_set_host_notifier() will not result
> in a call to virtio_pci_ioeventfd_assign(assign=false). This leaves
> the memory regions with stale notifiers and results on the next start
> triggering the following assertion:
> 
>   kvm_mem_ioeventfd_add: error adding ioeventfd: File exists
>   Aborted
> 
> This patch reintroduces (hopefully in a cleaner way) the concept
> that was present with ioeventfd_disabled before the refactoring.
> When ioeventfd_grabbed>0, ioeventfd_started tracks whether ioeventfd
> should be enabled or not, but ioeventfd is actually not started at
> all until vhost releases the host notifiers.
> 
> [1] http://lists.nongnu.org/archive/html/qemu-devel/2016-10/msg07748.html
> [2] http://lists.nongnu.org/archive/html/qemu-devel/2016-10/msg07760.html
> 
> Reported-by: Felipe Franciosi <address@hidden>
> Reported-by: Christian Borntraeger <address@hidden>
> Reported-by: Alex Williamson <address@hidden>
> Fixes: ed08a2a0b ("virtio: use virtio_bus_set_host_notifier to start/stop 
> ioeventfd")
> Signed-off-by: Paolo Bonzini <address@hidden>
> ---
>       You know you really screwed up when you get three Reported-bys...
>       The patch is largish, but I've added lots of comments.
>       It overrides Felipe's patch which did not for example fix vhost
>       multiqueue (tests/vhost-user-test).
> 
>  hw/virtio/vhost.c              | 11 ++++------
>  hw/virtio/virtio-bus.c         | 50 
> ++++++++++++++++++++++++++++++++++++------
>  hw/virtio/virtio.c             | 16 ++++++++++++++
>  include/hw/virtio/virtio-bus.h | 14 ++++++++++++
>  include/hw/virtio/virtio.h     |  2 ++
>  5 files changed, 79 insertions(+), 14 deletions(-)

I'm not entirely sure what to apply this to, it has conflicts with
MST's last pull request, so I applied it to mainline, which for me was:

6bbcb76 MAINTAINERS: Remove obsolete stable branches

But this just gives me a different failure, QEMU reports:

kvm_mem_ioeventfd_add: error adding ioeventfd: File exists

gdb backtrace:

Thread 4 "CPU 1/KVM" received signal SIGABRT, Aborted.
[Switching to Thread 0x7f60e4ce9700 (LWP 12239)]
0x00007f60f582b765 in __GI_raise (address@hidden) at 
../sysdeps/unix/sysv/linux/raise.c:54
54        return INLINE_SYSCALL (tgkill, 3, pid, selftid, sig);
(gdb) bt
#0  0x00007f60f582b765 in __GI_raise (address@hidden) at 
../sysdeps/unix/sysv/linux/raise.c:54
#1  0x00007f60f582d36a in __GI_abort () at abort.c:89
#2  0x000055af0a40a8f0 in kvm_mem_ioeventfd_add (listener=0x55af0b4d6e70, 
section=0x7f60e4ce5ef0, match_data=false, data=0, e=0x55af0c2e3690)
    at /net/gimli/home/alwillia/Work/qemu.git/kvm-all.c:898
#3  0x000055af0a4111d6 in address_space_add_del_ioeventfds (as=0x55af0aedef00 
<address_space_memory>, fds_new=0x7f60d8000c00, fds_new_nb=9, 
fds_old=0x7f60d4319e80, fds_old_nb=8)
    at /net/gimli/home/alwillia/Work/qemu.git/memory.c:757
#4  0x000055af0a4114e4 in address_space_update_ioeventfds (as=0x55af0aedef00 
<address_space_memory>) at /net/gimli/home/alwillia/Work/qemu.git/memory.c:803
#5  0x000055af0a411d1c in memory_region_transaction_commit () at 
/net/gimli/home/alwillia/Work/qemu.git/memory.c:932
#6  0x000055af0a414881 in memory_region_add_eventfd (mr=0x55af0bbcf740, addr=0, 
size=0, match_data=false, data=0, e=0x55af0c2e3690) at 
/net/gimli/home/alwillia/Work/qemu.git/memory.c:1984
#7  0x000055af0a71b0a8 in virtio_pci_ioeventfd_assign (d=0x55af0bbcea20, 
notifier=0x55af0c2e3690, n=0, assign=true) at hw/virtio/virtio-pci.c:300
#8  0x000055af0a7216f1 in virtio_bus_set_host_notifier (bus=0x55af0bbd6db8, 
n=0, assign=true) at hw/virtio/virtio-bus.c:257
#9  0x000055af0a48180b in vhost_dev_enable_notifiers (hdev=0x55af0b4d4f10, 
vdev=0x55af0bbd6e30) at 
/net/gimli/home/alwillia/Work/qemu.git/hw/virtio/vhost.c:1198
#10 0x000055af0a457941 in vhost_net_start_one (net=0x55af0b4d4f10, 
dev=0x55af0bbd6e30) at 
/net/gimli/home/alwillia/Work/qemu.git/hw/net/vhost_net.c:227
#11 0x000055af0a457e2d in vhost_net_start (dev=0x55af0bbd6e30, 
ncs=0x55af0c7e1890, total_queues=1) at 
/net/gimli/home/alwillia/Work/qemu.git/hw/net/vhost_net.c:324
#12 0x000055af0a4523f2 in virtio_net_vhost_status (n=0x55af0bbd6e30, status=7 
'\a') at /net/gimli/home/alwillia/Work/qemu.git/hw/net/virtio-net.c:156
#13 0x000055af0a45268e in virtio_net_set_status (vdev=0x55af0bbd6e30, status=7 
'\a') at /net/gimli/home/alwillia/Work/qemu.git/hw/net/virtio-net.c:229
#14 0x000055af0a478874 in virtio_set_status (vdev=0x55af0bbd6e30, val=7 '\a') 
at /net/gimli/home/alwillia/Work/qemu.git/hw/virtio/virtio.c:898
#15 0x000055af0a71b3c0 in virtio_ioport_write (opaque=0x55af0bbcea20, addr=18, 
val=7) at hw/virtio/virtio-pci.c:383
#16 0x000055af0a71b824 in virtio_pci_config_write (opaque=0x55af0bbcea20, 
addr=18, val=7, size=1) at hw/virtio/virtio-pci.c:508
#17 0x000055af0a4102fd in memory_region_write_accessor (mr=0x55af0bbcf310, 
addr=18, value=0x7f60e4ce64b8, size=1, shift=0, mask=255, attrs=...)
    at /net/gimli/home/alwillia/Work/qemu.git/memory.c:526
#18 0x000055af0a410515 in access_with_adjusted_size (addr=18, 
value=0x7f60e4ce64b8, size=1, access_size_min=1, access_size_max=4, access=
    0x55af0a410213 <memory_region_write_accessor>, mr=0x55af0bbcf310, 
attrs=...) at /net/gimli/home/alwillia/Work/qemu.git/memory.c:592
#19 0x000055af0a412c55 in memory_region_dispatch_write (mr=0x55af0bbcf310, 
addr=18, data=7, size=1, attrs=...) at 
/net/gimli/home/alwillia/Work/qemu.git/memory.c:1323
#20 0x000055af0a3be583 in address_space_write_continue (as=0x55af0aedede0 
<address_space_io>, addr=49458, attrs=..., buf=0x7f6110aa0000 "\a", len=1, 
addr1=18, l=1, mr=0x55af0bbcf310)
    at /net/gimli/home/alwillia/Work/qemu.git/exec.c:2621
#21 0x000055af0a3be6cb in address_space_write (as=0x55af0aedede0 
<address_space_io>, addr=49458, attrs=..., buf=0x7f6110aa0000 "\a", len=1)
    at /net/gimli/home/alwillia/Work/qemu.git/exec.c:2666
#22 0x000055af0a3bea57 in address_space_rw (as=0x55af0aedede0 
<address_space_io>, addr=49458, attrs=..., buf=0x7f6110aa0000 "\a", len=1, 
is_write=true)
    at /net/gimli/home/alwillia/Work/qemu.git/exec.c:2768
#23 0x000055af0a40c8d7 in kvm_handle_io (port=49458, attrs=..., 
data=0x7f6110aa0000, direction=1, size=1, count=1) at 
/net/gimli/home/alwillia/Work/qemu.git/kvm-all.c:1800
#24 0x000055af0a40cddd in kvm_cpu_exec (cpu=0x55af0b9c33f0) at 
/net/gimli/home/alwillia/Work/qemu.git/kvm-all.c:1958
#25 0x000055af0a3f3c58 in qemu_kvm_cpu_thread_fn (arg=0x55af0b9c33f0) at 
/net/gimli/home/alwillia/Work/qemu.git/cpus.c:998
#26 0x00007f60f5bc05ca in start_thread (arg=0x7f60e4ce9700) at 
pthread_create.c:333
#27 0x00007f60f58fa0ed in clone () at 
../sysdeps/unix/sysv/linux/x86_64/clone.S:109

If I'm applying it to the wrong place, let me know.  Thanks,

Alex

> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> index 131f164..a8b5ab8 100644
> --- a/hw/virtio/vhost.c
> +++ b/hw/virtio/vhost.c
> @@ -1186,17 +1186,14 @@ void vhost_dev_cleanup(struct vhost_dev *hdev)
>  int vhost_dev_enable_notifiers(struct vhost_dev *hdev, VirtIODevice *vdev)
>  {
>      BusState *qbus = BUS(qdev_get_parent_bus(DEVICE(vdev)));
> -    VirtioBusState *vbus = VIRTIO_BUS(qbus);
> -    VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(vbus);
>      int i, r, e;
>  
> -    if (!k->ioeventfd_assign) {
> +    r = virtio_device_grab_ioeventfd(vdev);
> +    if (r < 0) {
>          error_report("binding does not support host notifiers");
> -        r = -ENOSYS;
>          goto fail;
>      }
>  
> -    virtio_device_stop_ioeventfd(vdev);
>      for (i = 0; i < hdev->nvqs; ++i) {
>          r = virtio_bus_set_host_notifier(VIRTIO_BUS(qbus), hdev->vq_index + 
> i,
>                                           true);
> @@ -1216,7 +1213,7 @@ fail_vq:
>          }
>          assert (e >= 0);
>      }
> -    virtio_device_start_ioeventfd(vdev);
> +    virtio_device_release_ioeventfd(vdev);
>  fail:
>      return r;
>  }
> @@ -1239,7 +1236,7 @@ void vhost_dev_disable_notifiers(struct vhost_dev 
> *hdev, VirtIODevice *vdev)
>          }
>          assert (r >= 0);
>      }
> -    virtio_device_start_ioeventfd(vdev);
> +    virtio_device_release_ioeventfd(vdev);
>  }
>  
>  /* Test and clear event pending status.
> diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c
> index bf61f66..42467f1 100644
> --- a/hw/virtio/virtio-bus.c
> +++ b/hw/virtio/virtio-bus.c
> @@ -147,6 +147,38 @@ void virtio_bus_set_vdev_config(VirtioBusState *bus, 
> uint8_t *config)
>      }
>  }
>  
> +int virtio_bus_grab_ioeventfd(VirtioBusState *bus)
> +{
> +    VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(bus);
> +
> +    /* vhost can be used even if ioeventfd=off in the proxy device,
> +     * so do not check k->ioeventfd_enabled.
> +     */
> +    if (!k->ioeventfd_assign) {
> +        return -ENOSYS;
> +    }
> +
> +    if (bus->ioeventfd_grabbed == 0 && bus->ioeventfd_started) {
> +        virtio_bus_stop_ioeventfd(bus);
> +        /* Remember that we need to restart ioeventfd
> +         * when ioeventfd_grabbed becomes zero.
> +         */
> +        bus->ioeventfd_started = true;
> +    }
> +    bus->ioeventfd_grabbed++;
> +    return 0;
> +}
> +
> +void virtio_bus_release_ioeventfd(VirtioBusState *bus)
> +{
> +    assert(bus->ioeventfd_grabbed != 0);
> +    if (--bus->ioeventfd_grabbed == 0 && bus->ioeventfd_started) {
> +        /* Force virtio_bus_start_ioeventfd to act.  */
> +        bus->ioeventfd_started = false;
> +        virtio_bus_start_ioeventfd(bus);
> +    }
> +}
> +
>  int virtio_bus_start_ioeventfd(VirtioBusState *bus)
>  {
>      VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(bus);
> @@ -161,10 +193,12 @@ int virtio_bus_start_ioeventfd(VirtioBusState *bus)
>      if (bus->ioeventfd_started) {
>          return 0;
>      }
> -    r = vdc->start_ioeventfd(vdev);
> -    if (r < 0) {
> -        error_report("%s: failed. Fallback to userspace (slower).", 
> __func__);
> -        return r;
> +    if (!bus->ioeventfd_grabbed) {
> +        r = vdc->start_ioeventfd(vdev);
> +        if (r < 0) {
> +            error_report("%s: failed. Fallback to userspace (slower).", 
> __func__);
> +            return r;
> +        }
>      }
>      bus->ioeventfd_started = true;
>      return 0;
> @@ -179,9 +213,11 @@ void virtio_bus_stop_ioeventfd(VirtioBusState *bus)
>          return;
>      }
>  
> -    vdev = virtio_bus_get_device(bus);
> -    vdc = VIRTIO_DEVICE_GET_CLASS(vdev);
> -    vdc->stop_ioeventfd(vdev);
> +    if (!bus->ioeventfd_grabbed) {
> +        vdev = virtio_bus_get_device(bus);
> +        vdc = VIRTIO_DEVICE_GET_CLASS(vdev);
> +        vdc->stop_ioeventfd(vdev);
> +    }
>      bus->ioeventfd_started = false;
>  }
>  
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index bcbcfe0..89b0b80 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -2202,6 +2202,22 @@ void virtio_device_stop_ioeventfd(VirtIODevice *vdev)
>      virtio_bus_stop_ioeventfd(vbus);
>  }
>  
> +int virtio_device_grab_ioeventfd(VirtIODevice *vdev)
> +{
> +    BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
> +    VirtioBusState *vbus = VIRTIO_BUS(qbus);
> +
> +    return virtio_bus_grab_ioeventfd(vbus);
> +}
> +
> +void virtio_device_release_ioeventfd(VirtIODevice *vdev)
> +{
> +    BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
> +    VirtioBusState *vbus = VIRTIO_BUS(qbus);
> +
> +    virtio_bus_release_ioeventfd(vbus);
> +}
> +
>  static void virtio_device_class_init(ObjectClass *klass, void *data)
>  {
>      /* Set the default value here. */
> diff --git a/include/hw/virtio/virtio-bus.h b/include/hw/virtio/virtio-bus.h
> index fdf7fda..8a51e2c 100644
> --- a/include/hw/virtio/virtio-bus.h
> +++ b/include/hw/virtio/virtio-bus.h
> @@ -97,6 +97,16 @@ struct VirtioBusState {
>       * Set if ioeventfd has been started.
>       */
>      bool ioeventfd_started;
> +
> +    /*
> +     * Set if ioeventfd has been grabbed by vhost.  When ioeventfd
> +     * is grabbed by vhost, we track its started/stopped state (which
> +     * depends in turn on the virtio status register), but do not
> +     * register a handler for the ioeventfd.  When ioeventfd is
> +     * released, if ioeventfd_started is true we finally register
> +     * the handler so that QEMU's device model can use ioeventfd.
> +     */
> +    int ioeventfd_grabbed;
>  };
>  
>  void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp);
> @@ -131,6 +141,10 @@ bool virtio_bus_ioeventfd_enabled(VirtioBusState *bus);
>  int virtio_bus_start_ioeventfd(VirtioBusState *bus);
>  /* Stop the ioeventfd. */
>  void virtio_bus_stop_ioeventfd(VirtioBusState *bus);
> +/* Tell the bus that vhost is grabbing the ioeventfd. */
> +int virtio_bus_grab_ioeventfd(VirtioBusState *bus);
> +/* bus that vhost is not using the ioeventfd anymore. */
> +void virtio_bus_release_ioeventfd(VirtioBusState *bus);
>  /* Switch from/to the generic ioeventfd handler */
>  int virtio_bus_set_host_notifier(VirtioBusState *bus, int n, bool assign);
>  
> diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> index ac65d6a..35ede30 100644
> --- a/include/hw/virtio/virtio.h
> +++ b/include/hw/virtio/virtio.h
> @@ -270,6 +270,8 @@ void virtio_queue_set_guest_notifier_fd_handler(VirtQueue 
> *vq, bool assign,
>                                                  bool with_irqfd);
>  int virtio_device_start_ioeventfd(VirtIODevice *vdev);
>  void virtio_device_stop_ioeventfd(VirtIODevice *vdev);
> +int virtio_device_grab_ioeventfd(VirtIODevice *vdev);
> +void virtio_device_release_ioeventfd(VirtIODevice *vdev);
>  bool virtio_device_ioeventfd_enabled(VirtIODevice *vdev);
>  EventNotifier *virtio_queue_get_host_notifier(VirtQueue *vq);
>  void virtio_queue_host_notifier_read(EventNotifier *n);