On 8/16/2022 10:32 AM, Parav Pandit wrote:
From: Zhu Lingshan <lingshan.zhu@xxxxxxxxx>
Sent: Monday, August 15, 2022 5:27 AM
Some fields of virtio-net device config space are conditional on the feature
bits, the spec says:
"The mac address field always exists
(though is only valid if VIRTIO_NET_F_MAC is set)"
"max_virtqueue_pairs only exists if VIRTIO_NET_F_MQ or
VIRTIO_NET_F_RSS is set"
"mtu only exists if VIRTIO_NET_F_MTU is set"
so we should read MTU, MAC and MQ in the device config space only when
these feature bits are offered.
Yes.
For MQ, if both VIRTIO_NET_F_MQ and VIRTIO_NET_F_RSS are not set, the
virtio device should have one queue pair as default value, so when userspace
querying queue pair numbers, it should return mq=1 than zero.
No.
No need to treat mac and max_qps differently.
It is meaningless to differentiate when field exist/not-exists vs value valid/not valid.
as we discussed before, MQ has a default value 1, to be a functional
virtio-net device,
while MAC has no default value, if no VIRTIO_NET_F_MAC set, the driver
should generate
a random MAC.
For MTU, if VIRTIO_NET_F_MTU is not set, we should not read MTU from
the device config sapce.
RFC894 <A Standard for the Transmission of IP Datagrams over Ethernet
Networks> says:"The minimum length of the data field of a packet sent over
an Ethernet is 1500 octets, thus the maximum length of an IP datagram sent
over an Ethernet is 1500 octets. Implementations are encouraged to support
full-length packets"
This line in the RFC 894 of 1984 is wrong.
Errata already exists for it at [1].
[1] https://www.rfc-editor.org/errata_search.php?rfc=894&rec_status=0
OK, so I think we should return nothing if _F_MTU not set, like handling
the MAC
virtio spec says:"The virtio network device is a virtual ethernet card", so the
default MTU value should be 1500 for virtio-net.
Practically I have seen 1500 and highe mtu.
And this derivation is not good of what should be the default mtu as above errata exists.
And I see the code below why you need to work so hard to define a default value so that _MQ and _MTU can report default values.
There is really no need for this complexity and such a long commit message.
Can we please expose feature bits as-is and report config space field which are valid?
User space will be querying both.
I think MAC and MTU don't have default values, so return nothing if the
feature bits not set,
for MQ, it is still max_vq_paris == 1 by default.
For MAC, the spec says:"If the VIRTIO_NET_F_MAC feature bit is set, the
configuration space mac entry indicates the “physical” address of the
network card, otherwise the driver would typically generate a random local
MAC address." So there is no default MAC address if VIRTIO_NET_F_MAC
not set.
This commits introduces functions vdpa_dev_net_mtu_config_fill() and
vdpa_dev_net_mac_config_fill() to fill MTU and MAC.
It also fixes vdpa_dev_net_mq_config_fill() to report correct MQ when
_F_MQ is not present.
Multiple changes in single patch are not good idea.
Its ok to split to smaller patches.
OK, I can try to split it.
+ if ((features & BIT_ULL(VIRTIO_NET_F_MTU)) == 0)
+ val_u16 = 1500;
+ else
+ val_u16 = __virtio16_to_cpu(true, config->mtu);
+
Need to work hard to find default values and that too turned out had errata.
There are more fields that doesn’t have default values.
There is no point in kernel doing this guess work, that user space can figure out of what is valid/invalid.
It's not guest work, when guest finds no feature bits set, it can decide
what to do. These code only for the
user space, just MST ever suggest, if there is a default value, we can
return it from the kernel, once for all.
Thanks
