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Date:   Wed, 20 Dec 2017 12:07:55 -0500
From:   Jason Baron <jbaron@...mai.com>
To:     "Michael S. Tsirkin" <mst@...hat.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 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. 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?

> 
> 
>> @@ -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


> 
>> @@ -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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