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Date:   Wed, 20 Dec 2017 19:52:53 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Baron <jbaron@...mai.com>
Cc:     qemu-devel@...gnu.org, linux-kernel@...r.kernel.org,
        jasowang@...hat.com
Subject: Re: [PATCH net-next 1/2] virtio_net: allow hypervisor to indicate
 linkspeed and duplex setting

On Wed, Dec 20, 2017 at 12:07:55PM -0500, Jason Baron wrote:
> 
> 
> On 12/20/2017 09:57 AM, Michael S. Tsirkin wrote:
> > On Thu, Dec 14, 2017 at 02:33:53PM -0500, Jason Baron wrote:
> >> If the hypervisor exports the link and duplex speed, let's use that instead
> >> of the default unknown speed. The user can still overwrite it later if
> >> desired via: 'ethtool -s'. This allows the hypervisor to set the default
> >> link speed and duplex setting without requiring guest changes and is
> >> consistent with how other network drivers operate. We ran into some cases
> >> where the guest software was failing due to a lack of linkspeed and had to
> >> fall back to a fully emulated network device that does export a linkspeed
> >> and duplex setting.
> >>
> >> Implement by adding a new VIRTIO_NET_F_SPEED_DUPLEX feature flag, to
> >> indicate that a linkspeed and duplex setting are present.
> >>
> >> Signed-off-by: Jason Baron <jbaron@...mai.com>
> >> Cc: "Michael S. Tsirkin" <mst@...hat.com>
> >> Cc: Jason Wang <jasowang@...hat.com>
> >> ---
> >>  drivers/net/virtio_net.c        | 11 ++++++++++-
> >>  include/uapi/linux/virtio_net.h |  4 ++++
> >>  2 files changed, 14 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> >> index 6fb7b65..e7a2ad6 100644
> >> --- a/drivers/net/virtio_net.c
> >> +++ b/drivers/net/virtio_net.c
> >> @@ -2671,6 +2671,14 @@ static int virtnet_probe(struct virtio_device *vdev)
> >>  	netif_set_real_num_rx_queues(dev, vi->curr_queue_pairs);
> >>  
> >>  	virtnet_init_settings(dev);
> >> +	if (virtio_has_feature(vdev, VIRTIO_NET_F_SPEED_DUPLEX)) {
> >> +		vi->speed = virtio_cread32(vdev,
> >> +					offsetof(struct virtio_net_config,
> >> +					speed));
> >> +		vi->duplex = virtio_cread8(vdev,
> >> +					offsetof(struct virtio_net_config,
> >> +					duplex));
> >> +	}
> >>  
> >>  	err = register_netdev(dev);
> >>  	if (err) {
> > 
> > How are we going to validate speed values? Imagine host
> > using a new 1000Gbit device and exposing that to guest.
> > 
> > Need to think what do we want guest to do.
> > I think that ideally we'd say it's a 100Gbit device.
> > 
> > For duplex, force to one of 3 valid values?
> 
> So I didn't provide validation here b/c as you point out its not clear
> how we would validate it. I don't believe h/w drivers do any validation
> here either.

Right but hardware tends not to change as quickly as the hypervisors :)
For virtual device drivers, we need some way to handle forward
compatibility since hypervisors do change quite quickly.

> They simply propagate the value from the the underlying
> device. So that seemed reasonable to me.
> 
> Why do you divide by 10 in the above example? Would you propose always
> dividing what the device reports by 10?

No, that was just an example. I was just suggesting rounding down to
next valid known speed.

> > 
> > 
> >> @@ -2796,7 +2804,8 @@ static struct virtio_device_id id_table[] = {
> >>  	VIRTIO_NET_F_CTRL_RX, VIRTIO_NET_F_CTRL_VLAN, \
> >>  	VIRTIO_NET_F_GUEST_ANNOUNCE, VIRTIO_NET_F_MQ, \
> >>  	VIRTIO_NET_F_CTRL_MAC_ADDR, \
> >> -	VIRTIO_NET_F_MTU, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS
> >> +	VIRTIO_NET_F_MTU, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS, \
> >> +	VIRTIO_NET_F_SPEED_DUPLEX
> >>  
> >>  static unsigned int features[] = {
> >>  	VIRTNET_FEATURES,
> >> diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
> >> index fc353b5..acfcf68 100644
> >> --- a/include/uapi/linux/virtio_net.h
> >> +++ b/include/uapi/linux/virtio_net.h
> >> @@ -36,6 +36,7 @@
> >>  #define VIRTIO_NET_F_GUEST_CSUM	1	/* Guest handles pkts w/ partial csum */
> >>  #define VIRTIO_NET_F_CTRL_GUEST_OFFLOADS 2 /* Dynamic offload configuration. */
> >>  #define VIRTIO_NET_F_MTU	3	/* Initial MTU advice */
> >> +#define VIRTIO_NET_F_SPEED_DUPLEX 4	/* Host set linkspeed and duplex */
> >>  #define VIRTIO_NET_F_MAC	5	/* Host has given MAC address. */
> >>  #define VIRTIO_NET_F_GUEST_TSO4	7	/* Guest can handle TSOv4 in. */
> >>  #define VIRTIO_NET_F_GUEST_TSO6	8	/* Guest can handle TSOv6 in. */
> > 
> > I think I'd prefer a high feature bit - low bits are ones that can
> > be backported to legacy interfaces, so I think we should hang on to
> > these for fixing issues that break communication completely (like the
> > mtu).
> > 
> 
> So I went with a low bit here b/c in the virtio spec 'section 2.2
> Feature Bits':
> 
> 
>  0 to 23
>     Feature bits for the specific device type
> 24 to 32
>     Feature bits reserved for extensions to the queue and feature
> negotiation mechanisms
> 33 and above
>     Feature bits reserved for future extensions.
> 
> So virtio_net already goes up to 23 (but omits 4 and 6), and I wasn't
> sure if it was reasonable to use the higher bits. It looks like the code
> would handle the higher bits ok, so I can try that - bit 33 perhaps ?
> 
> Thanks,
> 
> -Jason


Transports started from bit 24 and are growing up.
So I would say devices should start from bit 63 and grow down.

> 
> > 
> >> @@ -76,6 +77,9 @@ struct virtio_net_config {
> >>  	__u16 max_virtqueue_pairs;
> >>  	/* Default maximum transmit unit advice */
> >>  	__u16 mtu;
> >> +	/* Host exported linkspeed and duplex */
> >> +	__u32 speed;
> >> +	__u8 duplex;
> >>  } __attribute__((packed));
> >>  
> >>  /*
> >> -- 
> >> 2.6.1

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