[libvirt PATCH v2 6/9] qemu: prepare and stop the dbus daemon

Daniel P. Berrangé berrange at redhat.com
Tue Feb 25 11:49:25 UTC 2020


On Tue, Feb 25, 2020 at 10:55:10AM +0100, marcandre.lureau at redhat.com wrote:
> From: Marc-André Lureau <marcandre.lureau at redhat.com>
> 
> Signed-off-by: Marc-André Lureau <marcandre.lureau at redhat.com>
> ---
>  src/qemu/qemu_process.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
> index 8c1ed76677..3a6cb4b2b0 100644
> --- a/src/qemu/qemu_process.c
> +++ b/src/qemu/qemu_process.c
> @@ -58,6 +58,7 @@
>  #include "qemu_extdevice.h"
>  #include "qemu_firmware.h"
>  #include "qemu_backup.h"
> +#include "qemu_dbus.h"
>  
>  #include "cpu/cpu.h"
>  #include "cpu/cpu_x86.h"
> @@ -6480,6 +6481,9 @@ qemuProcessPrepareHost(virQEMUDriverPtr driver,
>      qemuDomainObjPrivatePtr priv = vm->privateData;
>      g_autoptr(virQEMUDriverConfig) cfg = virQEMUDriverGetConfig(driver);
>  
> +    if (qemuDBusPrepareHost(driver) < 0)
> +        return -1;

This launches dbus unconditionally for every VM regardless of whether
its actually going to be used.

There is certainly a nice conceptual simplicity in doing this, but
I'm pretty concerned that there are going to be mgmt apps which
do not like this extra overhead being added for every VM. I'm
thinking KubeVirt and Kata containers in particular. This is also
relevant for the libvirt embedded driver which is trying to eliminate
all libvirt added overhead on managing QEMU, so that our fastest
QEMU startup time can match that achieved by running QEMU directly.
Unconditionally starting dbus will make that much more challenging.

This is a long winded way of saying I think we need to do this
only when it is actually required. This will certainly add complexity
as we'll need to cope with dynamically launching DBus when we hotplug
certain types of device which require it. I think we can ignore the
hot-unplug case at least, as once you've taken the overhead for DBus
for the VM, I don't think there's much to complain about, even if the
device using it is unplugged.


Regards,
Daniel
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