Re: [Qemu-devel] [PATCH v2 0/5] Add feature to start QEMU without vhost-user backend

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jun 22, 2015 at 12:50:43PM +0900, Tetsuya Mukawa wrote:
> Hi guys,
> 
> Here are patches to add feature to start QEMU without vhost-user backend.
> Currently, if we want to use vhost-user backend, the backend must start before
> QEMU.

Aren't you looking for something like xinetd?

> Also, if QEMU or the backend is closed unexpectedly, there is no way to
> recover without restarting both applications.

This was previously discussed:
https://lists.gnu.org/archive/html/qemu-devel/2014-03/msg00585.html
It doesn't look like any of the issues have been addressed.

In my opinion, the right way to handle this requires
guest cooperation.  For example, we can extend virtio 1 to allow a
single queue to reset.

We then can do something like the following:
- hypervisor detects queue backend disconnect.
- hypervisor sets flag and notifies guest
- guest resets queue, optionally discarding queued packets
- hypervisor detects (or requests if it's a client), backend reconnect
- hypervisor detects guest queue reset
- hypervisor notifies guests about backend reconnect
- hypervisor sends hypervisor device state (e.g. queue addresses and
  indices) to backend

Reconnect isn't robust without such an extension.

> Practically, it's not useful.

One thing we do seem to lack is specifying an action
taken on disconnect. It could be an action similar to r/werror.
One extra useful option could be to exit qemu.

> This patch series adds following features.
>  - QEMU can start before the backend.

I don't see the point of this one.

>  - QEMU or the backend can restart anytime.
>    connectivity will be recovered automatically, when app starts again.
>    (if QEMU is server, QEMU just wait reconnection)
>    while lost connection, link status of virtio-net device is down,
>    so virtio-net driver on the guest can know it
> 
> To work like above, the patch introduces flags to specify features vhost-user
> backend will support.
> 
> Here are examples.
> ('backend_features' is the new flags. Each bit of the flag represents
> VIRTIO_NET_F_* in linux/virtio_net.h)
> 
>     * QEMU is configured as vhost-user client.
>      -chardev socket,id=chr0,path=/tmp/sock,reconnect=3 \
>      -netdev vhost-user,id=net0,chardev=chr0,vhostforce,backend-features=0x68000 \
>      -device virtio-net-pci,netdev=net0 \
> 
>     * QEMU is configured as vhost-user server.
>      -chardev socket,id=chr0,path=/tmp/sock,server,nowait \
>      -netdev vhost-user,id=net0,chardev=chr0,vhostforce,backend-features=0x68000 \
>      -device virtio-net-pci,netdev=net0 \

This relies on management knowing the feature bitmap encoding of the
backend, not sure how practical that is.

> 
> When virtio-net device is configured by virtio-net driver, QEMU should know
> vhost-user backend features. But if QEMU starts without the backend, QEMU cannot
> know it. So above the feature values specified by user will be used as features
> the backend will support.
> 
> When connection between QEMU and the backend is established, QEMU checkes feature
> values of the backend to make sure the expected features are provided.
> If it doesn't, the connection will be closed by QEMU.
> 
> Regards,
> Tetsuya
> 
> ----------
> Changes
> ----------
> - v2 changes from v1 patch
>    - Rebase to latest master.
>    - Change user interface to be able to specify each feature by UINT64.
>    - Replace backend_* to backend-* in qapi schema.
>    - Use close(2) interface for opended socket instead of using shutdown(2)
>      interface.
>    - Split 2nd patch of v1 into 2nd and 3rd patch of v2.
>    - Fix commit title and body.
>    - Add comment, and fix indent.
>    - Use {} even for single statement if bodies.
>    - Use PRIx64 instead of %lx.
> 
> 
> - v1 changes from RFC patch
>   The last patch of this series was changed like below.
>    - Rebase to latest master.
>    - Remove needless has_backend_feature variable.
>    - Change user interface to be able to specify each feature by name.
>    - Add (Since 2.4) to schema file.
>    - Fix commit title and body.
> 
> Tetsuya Mukawa (5):
>   vhost-user: Add ability to know vhost-user backend disconnection
>   qemu-char: Add qemu_chr_disconnect to close a fd accepted by listen fd
>   vhost-user: Shutdown vhost-user connection when wrong messages are
>     passed
>   vhost-user: Enable 'nowait' and 'reconnect' option
>   vhost-user: Add new option to specify vhost-user backend supports
> 
>  hw/net/vhost_net.c             |  6 +++++-
>  hw/net/virtio-net.c            | 15 +++++++++++++++
>  hw/scsi/vhost-scsi.c           |  2 +-
>  hw/virtio/vhost-user.c         | 24 ++++++++++++++++-------
>  hw/virtio/vhost.c              |  3 ++-
>  include/hw/virtio/vhost.h      |  3 ++-
>  include/hw/virtio/virtio-net.h |  1 +
>  include/net/net.h              |  3 +++
>  include/net/vhost_net.h        |  1 +
>  include/sysemu/char.h          |  7 +++++++
>  net/net.c                      |  9 +++++++++
>  net/tap.c                      |  1 +
>  net/vhost-user.c               | 43 ++++++++++++++++++++++++++++++++++++++++--
>  qapi-schema.json               | 12 ++++++++++--
>  qemu-char.c                    |  8 ++++++++
>  qemu-options.hx                |  3 ++-
>  16 files changed, 125 insertions(+), 16 deletions(-)
> 
> -- 
> 2.1.4