[virtio-dev] Re: [PATCH] qemu: Introduce VIRTIO_NET_F_STANDBY feature bit to virtio_net

Siwei Liu
Tue, 05 Jun 2018 21:16:44 -0000

Good to see this discussion going. I share the same feeling that the
decision of plugging the primary (passthrough) should only be made
until guest driver acknowledges DRIVER_OK and _F_STANDBY.
Architecturally this intelligence should be baken to QEMU itself
rather than moving up to management stack, such as libvirt.

A few questions in line below.

On Tue, Jun 5, 2018 at 5:33 AM, Michael S. Tsirkin <mst at redhat.com>
wrote:
> I don't think this is sufficient.
>
> If both primary and standby devices are present, a legacy guest without
> support for the feature might see two devices with same mac and get
> confused.
>
> I think that we should only make primary visible after guest acked the
> backup feature bit.
>
> And on reset or when backup is cleared in some other way, unplug the
> primary.
>
> Something like the below will do the job:
>
> Primary device is added with a special "primary-failover" flag.

I wonder if you envision this flag as a user interface or some
internal attribute/property to QEMU device?

Since QEMU needs to associate this particular primary-failover device
with a virtio standby instance for checking DRIVER_OK as you indicate
below, I wonder if the group ID thing will be helpful to set
"primary-failover" flag internally without having to add another
option in CLI?

> A virtual machine is then initialized with just a standby virtio
> device. Primary is not yet added.
>
> Later QEMU detects that guest driver device set DRIVER_OK.
> It then exposes the primary device to the guest, and triggers
> a device addition event (hot-plug event) for it.

Sounds OK. While there might be a small window the guest already
starts to use virtio interface before the passthrough gets plugged in,
I think that should be fine.

Another option is to wait until the primary appears and gets
registered in the guest, so it can bring up the upper failover
interface.

>
> If QEMU detects guest driver removal, it initiates a hot-unplug sequence
> to remove the primary driver.  In particular, if QEMU detects guest
> re-initialization (e.g. by detecting guest reset) it immediately removes
> the primary device.

Agreed.

For legacy guest, user might prefer seeing a single passthrough device
rather than a virtio device. I would hope there's an option to allow
it to happen, instead of removing all passthrough devices.

Regards,
-Siwei

>
> We can move some of this code to management as well, architecturally it
> does not make too much sense but it might be easier implementation-wise.
>
> HTH
>
> On Mon, Jun 04, 2018 at 06:41:48PM -0700, Samudrala, Sridhar wrote:
>> Ping on this patch now that the kernel patches are accepted into
davem's net-next tree.
>> https://patchwork.ozlabs.org/cover/920005/
>>
>>
>> On 5/7/2018 4:09 PM, Sridhar Samudrala wrote:
>> > This feature bit can be used by hypervisor to indicate virtio_net
device to
>> > act as a standby for another device with the same MAC address.
>> >
>> > I tested this with a small change to the patch to mark the STANDBY
feature 'true'
>> > by default as i am using libvirt to start the VMs.
>> > Is there a way to pass the newly added feature bit
'standby' to qemu via libvirt
>> > XML file?
>> >
>> > Signed-off-by: Sridhar Samudrala <sridhar.samudrala at
intel.com>
>> > ---
>> >   hw/net/virtio-net.c                         | 2 ++
>> >   include/standard-headers/linux/virtio_net.h | 3 +++
>> >   2 files changed, 5 insertions(+)
>> >
>> > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
>> > index 90502fca7c..38b3140670 100644
>> > --- a/hw/net/virtio-net.c
>> > +++ b/hw/net/virtio-net.c
>> > @@ -2198,6 +2198,8 @@ static Property virtio_net_properties[] = {
>> >                        true),
>> >       DEFINE_PROP_INT32("speed", VirtIONet,
net_conf.speed, SPEED_UNKNOWN),
>> >       DEFINE_PROP_STRING("duplex", VirtIONet,
net_conf.duplex_str),
>> > +    DEFINE_PROP_BIT64("standby", VirtIONet,
host_features, VIRTIO_NET_F_STANDBY,
>> > +                      false),
>> >       DEFINE_PROP_END_OF_LIST(),
>> >   };
>> > diff --git a/include/standard-headers/linux/virtio_net.h
b/include/standard-headers/linux/virtio_net.h
>> > index e9f255ea3f..01ec09684c 100644
>> > --- a/include/standard-headers/linux/virtio_net.h
>> > +++ b/include/standard-headers/linux/virtio_net.h
>> > @@ -57,6 +57,9 @@
>> >                                      * Steering */
>> >   #define VIRTIO_NET_F_CTRL_MAC_ADDR 23     /* Set MAC address */
>> > +#define VIRTIO_NET_F_STANDBY      62    /* Act as standby for
another device
>> > +                                         * with the same MAC.
>> > +                                         */
>> >   #define VIRTIO_NET_F_SPEED_DUPLEX 63      /* Device set
linkspeed and duplex */
>> >   #ifndef VIRTIO_NET_NO_LEGACY
>
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