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

[Siwei Liu]

On Tue, Jun 5, 2018 at 2:32 PM, Michael S. Tsirkin <address@hidden> wrote:
> On Tue, Jun 05, 2018 at 02:16:44PM -0700, Siwei Liu wrote:
>> 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 <address@hidden> 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?
>
> I haven't thought about it but it's an option.
>
>> > 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.
>
> We can't be sure this will ever happen, can we? We might be asked to
> migrate at any time.

Right. For that to work, virtio driver needs a complicated state
tracking hand-in-hand with the host for VF's hot plugging activity. If
hot plugging is defered due to migration activity the failover
interface can be brought up immediately.

As said, this is just an option. I don't have strong preference here.

>
>> >
>> > 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
>
> I don't see a way to make it work since then you can't migrate, can you?

The thing is cloud service provider might prefer sticking to the same
level of service agreement (SLA) of keeping VF over migration,
particularly when guest OS or kernel does not have the support.The
footprint in guest OS support might NOT be prevailing in the early
days. There's no point the cloud vendor can switch back and forth, as
there's no way for hypervisor to detect what kernel version even OS
the guest will be running ahead of time, until the guest gets fully
loaded and boots up, right?

-Siwei

>
> The way I see it, if you don't need migration, just use PT without
> failover.  If you do, then you either use virtio or failover if guest
> supports it.
>
>> >
>> > 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 <address@hidden>
>> >> > ---
>> >> >   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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