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Re: [Qemu-devel] [virtio-dev] Re: [PATCH] qemu: Introduce VIRTIO_NET_F_S

From: Samudrala, Sridhar
Subject: Re: [Qemu-devel] [virtio-dev] Re: [PATCH] qemu: Introduce VIRTIO_NET_F_STANDBY feature bit to virtio_net
Date: Tue, 12 Jun 2018 17:08:45 -0700
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On 6/12/2018 4:34 AM, Michael S. Tsirkin wrote:
On Mon, Jun 11, 2018 at 10:02:45PM -0700, Samudrala, Sridhar wrote:
On 6/11/2018 7:17 PM, Michael S. Tsirkin wrote:
On Tue, Jun 12, 2018 at 09:54:44AM +0800, Jason Wang wrote:
On 2018年06月12日 01:26, Michael S. Tsirkin wrote:
On Mon, May 07, 2018 at 04:09:54PM -0700, 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 
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>
So I do not think we can commit to this interface: we
really need to control visibility of the primary device.
The problem is legacy guest won't use primary device at all if we do this.
And that's by design - I think it's the only way to ensure the
legacy guest isn't confused.
Yes. I think so. But i am not sure if Qemu is the right place to control the 
of the primary device. The primary device may not be specified as an argument 
to Qemu. It
may be plugged in later.
The cloud service provider is providing a feature that enables low latency 
datapath and live
migration capability.
A tenant can use this feature only if he is running a VM that has virtio-net 
with failover support.
Well live migration is there already. The new feature is low latency
data path.

we get live migration with just virtio.  But I meant live migration with VF as
primary device.

And it's the guest that needs failover support not the VM.

Isn't guest and VM synonymous?

I think Qemu should check if guest virtio-net supports this feature and provide 
a mechanism for
an upper layer indicating if the STANDBY feature is successfully negotiated or 
The upper layer can then decide if it should hot plug a VF with the same MAC 
and manage the 2 links.
If VF is successfully hot plugged, virtio-net link should be disabled.
Did you even talk to upper layer management about it?
Just list the steps they need to do and you will see
that's a lot of machinery to manage by the upper layer.

What do we gain in flexibility? As far as I can see the
only gain is some resources saved for legacy VMs.

That's not a lot as tenant of the upper layer probably already has
at least a hunch that it's a new guest otherwise
why bother specifying the feature at all - you
save even more resources without it.

I am not all that familiar with how Qemu manages network devices. If we can do 
all the
required management of the primary/standby devices within Qemu, that is 
definitely a better
approach without upper layer involvement.

How about control the visibility of standby device?

standy the always there to guarantee no downtime.

However just for testing purposes, we could add a non-stable
interface "x-standby" with the understanding that as any
x- prefix it's unstable and will be changed down the road,
likely in the next release.

    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[] = {
        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, 
+                      false),
diff --git a/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 */
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