[libvirt] [PATCH] qemu: Connect to guest agent iff needed

[Michal Privoznik]

On 21.12.2015 16:07, Michal Privoznik wrote:
> https://bugzilla.redhat.com/show_bug.cgi?id=1293351
> 
> I've came across a bit of a silly scenario, but long story short:
> after domain was resumed, the virDomainSetTime() API hung for 5
> seconds after which it failed with an error. Problem was, that
> there is no guest agent installed in the domain. But this got me
> thinking and digging into the code. So even though we do listen
> to VSERPORT_CHANGE events and connect and disconnect ourselves to
> guest agent, we still do connect to the guest agent at both
> domain startup and resume. This is a bit silly as it produces the
> delay - basically, because we connect to the guest agent,
> priv->agent is not NULL. Therefore qemuDomainAgentAvailable()
> will return true. And the place where we hang? Well,
> historically, when there was no VSERPORT_CHANGE event, we used a
> dummy ping command to the guest which has 5 seconds timeout. And
> it's still there and effective. So there's where the delay comes
> from.
> What's the resolution? Well, I'm introducing new capability
> QEMU_CAPS_VSERPORT_CHANGE that is enabled iff qemu is capable of
> the event. And, during domain startup, reconnect after resume and
> attach we do connect to the agent socket iff the capability is
> NOT set.
> 
> Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
> ---
>  src/qemu/qemu_capabilities.c                 |  2 +
>  src/qemu/qemu_capabilities.h                 |  1 +
>  src/qemu/qemu_process.c                      | 66 ++++++++++++++++++----------
>  tests/qemucapabilitiesdata/caps_2.1.1-1.caps |  1 +
>  tests/qemucapabilitiesdata/caps_2.4.0-1.caps |  1 +
>  tests/qemucapabilitiesdata/caps_2.5.0-1.caps |  1 +
>  6 files changed, 48 insertions(+), 24 deletions(-)
> 
> diff --git a/src/qemu/qemu_capabilities.c b/src/qemu/qemu_capabilities.c
> index 6e5d203..9e11af9 100644
> --- a/src/qemu/qemu_capabilities.c
> +++ b/src/qemu/qemu_capabilities.c
> @@ -308,6 +308,7 @@ VIR_ENUM_IMPL(virQEMUCaps, QEMU_CAPS_LAST,
>  
>                "virtio-tablet", /* 205 */
>                "virtio-input-host",
> +              "vserport-change-event",
>      );
>  
>  
> @@ -1479,6 +1480,7 @@ struct virQEMUCapsStringFlags virQEMUCapsEvents[] = {
>      { "SPICE_MIGRATE_COMPLETED", QEMU_CAPS_SEAMLESS_MIGRATION },
>      { "DEVICE_DELETED", QEMU_CAPS_DEVICE_DEL_EVENT },
>      { "MIGRATION", QEMU_CAPS_MIGRATION_EVENT },
> +    { "VSERPORT_CHANGE", QEMU_CAPS_VSERPORT_CHANGE },
>  };
>  
>  struct virQEMUCapsStringFlags virQEMUCapsObjectTypes[] = {
> diff --git a/src/qemu/qemu_capabilities.h b/src/qemu/qemu_capabilities.h
> index 61d6379..983faf6 100644
> --- a/src/qemu/qemu_capabilities.h
> +++ b/src/qemu/qemu_capabilities.h
> @@ -335,6 +335,7 @@ typedef enum {
>      /* 205 */
>      QEMU_CAPS_VIRTIO_TABLET, /* -device virtio-tablet-{device,pci} */
>      QEMU_CAPS_VIRTIO_INPUT_HOST, /* -device virtio-input-host-{device,pci} */
> +    QEMU_CAPS_VSERPORT_CHANGE, /* VSERPORT_CHANGE event */
>  
>      QEMU_CAPS_LAST /* this must always be the last item */
>  } virQEMUCapsFlags;
> diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
> index f274068..95a795c 100644
> --- a/src/qemu/qemu_process.c
> +++ b/src/qemu/qemu_process.c
> @@ -3524,15 +3524,21 @@ qemuProcessReconnect(void *opaque)
>      if (qemuConnectMonitor(driver, obj, QEMU_ASYNC_JOB_NONE, NULL) < 0)
>          goto error;
>  
> -    /* Failure to connect to agent shouldn't be fatal */
> -    if ((ret = qemuConnectAgent(driver, obj)) < 0) {
> -        if (ret == -2)
> -            goto error;
> +    /* With newer qemus capable of VSERPORT_CHANGE event, we are connecting and
> +     * disconnecting to the guest agent as it connects or disconnects to the
> +     * channel within the guest. So, it's important to connect here iff we are
> +     * running qemu not capable of the event. */
> +    if (!virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_VSERPORT_CHANGE)) {
> +        /* Failure to connect to agent shouldn't be fatal */
> +        if ((ret = qemuConnectAgent(driver, obj)) < 0) {
> +            if (ret == -2)
> +                goto error;
>  
> -        VIR_WARN("Cannot connect to QEMU guest agent for %s",
> -                 obj->def->name);
> -        virResetLastError();
> -        priv->agentError = true;
> +            VIR_WARN("Cannot connect to QEMU guest agent for %s",
> +                     obj->def->name);
> +            virResetLastError();
> +            priv->agentError = true;
> +        }
>      }
>  

Self NAK. We need slightly different approach. What can happen here is
that on domain save the guest agent was running and we were connected.
Then, on resume we will not get the event again. So we need to take the
'remembered' state of the channel into account. v2 on its way.

Michal