RE: [PATCH 2/2] vDPA: conditionally read fields in virtio-net dev

From: Jason Wang <redacted>
Sent: Thursday, August 18, 2022 12:19 AM

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On 8/16/2022 10:32 AM, Parav Pandit wrote:

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From: Zhu Lingshan <redacted>
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

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when these feature bits are offered.

Yes.

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For MQ, if both VIRTIO_NET_F_MQ and VIRTIO_NET_F_RSS are not

set,

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the

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virtio device should have one queue pair as default value, so
when userspace querying queue pair numbers, it should return

mq=1

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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.

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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
Networks> 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

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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.

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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.

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There is really no need for this complexity and such a long commit

message.

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Can we please expose feature bits as-is and report config space
field which

are valid?

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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.

I have stressed enough to highlight the fact that we don’t want to
start digging default/no default, valid/no-valid part of the spec.
I prefer kernel to reporting fields that _exists_ in the config
space and are valid.
I will let MST to handle the maintenance nightmare that this kind of
patch brings in without any visible gain to user space/orchestration
apps.

A logic that can be easily build in user space, should be written in
user space.
I conclude my thoughts here for this discussion.

I will let MST to decide how he prefers to proceed.

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+    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.

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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.

Above code of doing 1500 was probably an honest attempt to find a
legitimate default value, and we saw that it doesn’t work.
This is second example after _MQ that we both agree should not
return default.

And there are more fields coming in this area.
Hence, I prefer to not avoid returning such defaults for MAC, MTU,
MQ and rest all fields which doesn’t _exists_.

I will let MST to decide how he prefers to proceed for every field
to come next.
Thanks.

If MTU does not return a value without _F_MTU, and MAC does not
return a value without _F_MAC then IMO yes, number of queues should
not return a value without _F_MQ.

sure I can do this, but may I ask whether it is a final decision, I
remember you supported max_queue_paris = 1 without _F_MQ before

I think we just need to be consistent:

Either

1) make field conditional to align with spec

or

2) always return a value even if the feature is not set

It seems to me 1) is easier.

+1