OK, I read the spec (pasted below for easy of reading), but I'm still
confused over how this will work.
I thought normal net drivers have the hardware provide an rxhash for
each packet, and we map that to CPU to queue the packet on[1]. We hope
that the receiving process migrates to that CPU, so xmit queue
matches.
For virtio this would mean a new per-packet rxhash value, right?
Why are we doing something different? What am I missing?
Thanks,
Rusty.
[1] Everything I Know About Networking I Learned From LWN:
https://lwn.net/Articles/362339/
---
Transmit Packet Steering
When VIRTIO_NET_F_MULTIQUEUE feature bit is negotiated, guest can use any of
multiple configured transmit queues to transmit a given packet. To avoid packet
reordering by device (which generally leads to performance degradation) driver
should attempt to utilize the same transmit virtqueue for all packets of a
given transmit flow. For bi-directional protocols (in practice, TCP), a given
network connection can utilize both transmit and receive queues. For best
performance, packets from a single connection should utilize the paired
transmit and receive queues from the same virtqueue pair; for example both
transmitqN and receiveqN. This rule makes it possible to optimize processing on
the device side, but this is not a hard requirement: devices should function
correctly even when this rule is not followed.
Driver selects an active steering rule using VIRTIO_NET_CTRL_STEERING command
(this controls both which virtqueue is selected for a given packet for receive
and notifies the device which virtqueues are about to be used for transmit).
This command accepts a single out argument in the following format:
#define VIRTIO_NET_CTRL_STEERING 4
The field rule specifies the function used to select transmit virtqueue for a
given packet; the field param makes it possible to pass an extra parameter if
appropriate. When rule is set to VIRTIO_NET_CTRL_STEERING_SINGLE (this is the
default) all packets are steered to the default virtqueue transmitq (1); param
is unused; this is the default. With any other rule, When rule is set to
VIRTIO_NET_CTRL_STEERING_RX_FOLLOWS_TX packets are steered by driver to the
first N=(param+1) multiqueue virtqueues transmitq1...transmitqN; the default
transmitq is unused. Driver must have configured all these (param+1) virtqueues
beforehand.
Supported steering rules can be added and removed in the future. Driver should
check that the request to change the steering rule was successful by checking
ack values of the command. As selecting a specific steering is an optimization
feature, drivers should avoid hard failure and fall back on using a supported
steering rule if this command fails. The default steering rule is
VIRTIO_NET_CTRL_STEERING_SINGLE. It will not be removed.
When the steering rule is modified, some packets can still be outstanding in
one or more of the transmit virtqueues. Since drivers might choose to modify
the current steering rule at a high rate (e.g. adaptively in response to
changes in the workload) to avoid reordering packets, device is recommended to
complete processing of the transmit queue(s) utilized by the original steering
before processing any packets delivered by the modified steering rule.
For debugging, the current steering rule can also be read from the
configuration space.
Receive Packet Steering
When VIRTIO_NET_F_MULTIQUEUE feature bit is negotiated, device can use any of
multiple configured receive queues to pass a given packet to driver. Driver
controls which virtqueue is selected in practice by configuring packet steering
rule using VIRTIO_NET_CTRL_STEERING command, as described
above[sub:Transmit-Packet-Steering].
The field rule specifies the function used to select receive virtqueue for a
given packet; the field param makes it possible to pass an extra parameter if
appropriate. When rule is set to VIRTIO_NET_CTRL_STEERING_SINGLE all packets
are steered to the default virtqueue receiveq (0); param is unused; this is the
default. When rule is set to VIRTIO_NET_CTRL_STEERING_RX_FOLLOWS_TX packets are
steered by host to the first N=(param+1) multiqueue virtqueues
receiveq1...receiveqN; the default receiveq is unused. Driver must have
configured all these (param+1) virtqueues beforehand. For best performance for
bi-directional flows (such as TCP) device should detect the flow to virtqueue
pair mapping on transmit and select the receive virtqueue from the same
virtqueue pair. For uni-directional flows, or when this mapping information is
missing, a device-specific steering function is used.
Supported steering rules can be added and removed in the future. Driver should
probe for supported rules by checking ack values of the command.
When the steering rule is modified, some packets can still be outstanding in
one or more of the virtqueues. Device is not required to wait for these packets
to be consumed before delivering packets using the new streering rule. Drivers
modifying the steering rule at a high rate (e.g. adaptively in response to
changes in the workload) are recommended to complete processing of the receive
queue(s) utilized by the original steering before processing any packets
delivered by the modified steering rule.
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