Re: [Qemu-devel] [virtio-dev] Re: [PATCH v3 0/5] Support for datapath switching during live migration

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

On Mon, Jan 07, 2019 at 05:45:22PM -0800, si-wei liu wrote:
> 
> 
> On 01/07/2019 03:32 PM, Michael S. Tsirkin wrote:
> > On Mon, Jan 07, 2019 at 05:29:39PM -0500, Venu Busireddy wrote:
> > > Implement the infrastructure to support datapath switching during live
> > > migration involving SR-IOV devices.
> > > 
> > > 1. This patch is based off on the current VIRTIO_NET_F_STANDBY feature
> > >     bit and MAC address device pairing.
> > > 
> > > 2. This set of events will be consumed by userspace management software
> > >     to orchestrate all the hot plug and datapath switching activities.
> > >     This scheme has the least QEMU modifications while allowing userspace
> > >     software to build its own intelligence to control the whole process
> > >     of SR-IOV live migration.
> > > 
> > > 3. While the hidden device model (viz. coupled device model) is still
> > >     being explored for automatic hot plugging (QEMU) and automatic datapath
> > >     switching (host-kernel), this series provides a supplemental set
> > >     of interfaces if management software wants to drive the SR-IOV live
> > >     migration on its own. It should not conflict with the hidden device
> > >     model but just offers simplicity of implementation.
> > > 
> > > 
> > > Si-Wei Liu (2):
> > >    vfio-pci: Add FAILOVER_PRIMARY_CHANGED event to shorten downtime during failover
> > >    pci: query command extension to check the bus master enabling status of the failover-primary device
> > > 
> > > Sridhar Samudrala (1):
> > >    virtio_net: Add VIRTIO_NET_F_STANDBY feature bit.
> > > 
> > > Venu Busireddy (2):
> > >    virtio_net: Add support for "Data Path Switching" during Live Migration.
> > >    virtio_net: Add a query command for FAILOVER_STANDBY_CHANGED event.
> > > 
> > > ---
> > > Changes in v3:
> > >    Fix issues with coding style in patch 3/5.
> > > 
> > > Changes in v2:
> > >    Added a query command for FAILOVER_STANDBY_CHANGED event.
> > >    Added a query command for FAILOVER_PRIMARY_CHANGED event.
> > Hmm it looks like all feedback I sent e.g. here:
> > https://patchwork.kernel.org/patch/10721571/
> > got ignored.
> > 
> > To summarize I suggest reworking the series adding a new command along
> > the lines of (naming is up to you):
> > 
> > query-pci-master - this returns status for a device
> > 		   and enables a *single* event after
> > 		   it changes
> > 
> > and then removing all status data from the event,
> > just notify about the change and *only once*.
> Why removing all status data from the event?

To make sure users do not forget to call query-pci-master to
re-enable more events.

> It does not hurt to keep them
> as the FAILOVER_PRIMARY_CHANGED event in general is of pretty low-frequency.

A malicious guest can make it as frequent as it wants to.
OTOH there is no way to limit.


> As can be seen other similar low-frequent QMP events do have data carried
> over.
> 
> As this event relates to datapath switching, there's implication to coalesce
> events as packets might not get a chance to send out as nothing would ever
> happen when  going through quick transitions like
> disabled->enabled->disabled. I would allow at least few packets to be sent
> over wire rather than nothing. Who knows how fast management can react and
> consume these events?
> 
> Thanks,
> -Siwei

OK if it's so important for latency let's include data in the event.
Please add comments explaining that you must always re-run query
afterwards to make sure it's stable and re-enable more events.



> > 	
> > 
> > upon event management does query-pci-master
> > and acts accordingly.
> > 
> > 
> > 
> > 
> > >   hmp.c                          |   5 +++
> > >   hw/acpi/pcihp.c                |  27 +++++++++++
> > >   hw/net/virtio-net.c            |  42 +++++++++++++++++
> > >   hw/pci/pci.c                   |   5 +++
> > >   hw/vfio/pci.c                  |  60 +++++++++++++++++++++++++
> > >   hw/vfio/pci.h                  |   1 +
> > >   include/hw/pci/pci.h           |   1 +
> > >   include/hw/virtio/virtio-net.h |   1 +
> > >   include/net/net.h              |   2 +
> > >   net/net.c                      |  61 +++++++++++++++++++++++++
> > >   qapi/misc.json                 |   5 ++-
> > >   qapi/net.json                  | 100 +++++++++++++++++++++++++++++++++++++++++
> > >   12 files changed, 309 insertions(+), 1 deletion(-)
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