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

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

On Tue, Jan 08, 2019 at 08:55:35PM -0800, si-wei liu wrote:
> 
> 
> On 1/7/2019 6:25 PM, Michael S. Tsirkin wrote:
> 
>     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.
> 
> IMO the FAILOVER_PRIMARY_CHANGED event is on the performance path, it's an
> overkill to enforce round trip query for each event in normal situations.
> 
>         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.
> 
> Will throttle the event rate (say, limiting to no more than 1 event per second)

And if guest *does* need the switch because e.g. attaching xdp wants to
resent the card?

> a way to limit (as opposed to control guest behavior) ? The other similar
> events that apply rate limiting don't suppress event emission until the next
> query at all.

We have some problematic interfaces already, that's true.

> Doing so would just cause more events missing. As stated in the
> earlier example, we should give guest NIC a chance to flush queued packets even
> if ending state is same between two events.

I haven't seen that requirement. I guess a reset just stops processing
buffers rather than flush. Care to repeat?

> 
>         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.
> 
> I can add comments describing why we need to carry data in the event, and apply
> rate limiting to events. But I don't follow why it must suppress event until
> next query.

Rate limiting is fundamentally broken.
Try a stress of resets and there goes your promise of low downtime.

Let me try to re-state: state query is fundamentally required
because otherwise e.g. management restarts do not work.
And it is much better to force management to run query every event
and every restart than just hope it handles racy corner cases correctly.

If we do include data in the event then there is no real latency cost to
that, since management can take action in response to the event then do
the query asynchronously at leasure.
So let me turn it around and say I don't follow why you have objections
to blocking following events until query.


> 
> Thanks,
> -Siwei
> 
> 
> 
> 
> 
> 
> 
>             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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