man/man9e/mac_capab_rings.9e

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.Dd July 17, 2023
.Dt MAC_CAPAB_RINGS 9E
.Os
.Sh NAME
.Nm mac_capab_rings
.Nd MAC ring capability
.Sh SYNOPSIS
.In sys/mac_provider.h
.Vt typedef struct mac_capab_rings_s mac_capab_rings_t;
.Sh INTERFACE LEVEL
.Sy Uncommitted -
This interface is still evolving.
API and ABI stability is not guaranteed.
.Sh DESCRIPTION
The
.Sy MAC_CAPAB_RINGS
capability provides a means for device drivers to take advantage of
the additional resources offered by hardware beyond the basic operations
to transmit and receive.
There are two primary concepts that this MAC capability relies on: rings
and groups.
.Pp
The
.Em ring
is a abstract concept which must be mapped to some hardware construct by
the driver.
It typically takes the form of a DMA memory region which is divided
into many smaller units, called descriptors or entries.
Each entry in the ring describes a location in memory of a packet, which the
hardware is to read from
.Pq to transmit it
or write to
.Pq upon reception .
Entries also typically contain metadata and attributes about the packet.
These entries are typically arranged in a fixed-size circular buffer
.Po hence the
.Dq ring
name
.Pc
which is shared between the operating system and the
hardware via the DMA-backed memory.
Most NICs, regardless of their support for this capability, use something
resembling a descriptor ring under the hood.
Some vendors may also refer to rings as
.Em queues .
The ring concept is intentionally general, so that more unusual underlying
hardware constructs can also be used to implement it.
.Pp
A collection of one or more rings is called a
.Em group .
Each group usually has a collection of filters that can be associated
with them.
These filters are usually defined in terms of matching something like a
MAC address, VLAN, or Ethertype, though more complex filters may exist
in hardware.
When a packet matches a filter, it will then be directed to the group
and eventually delivered to one of the rings in the group.
.Pp
In the MAC framework, rings and groups are separated into categories
based on their purpose: transmitting and receiving.
While the MAC framework thinks of transmit and receive rings as
different physical constructs, they may map to the same underlying
resources in the hardware.
The device driver may implement the
.Dv MAC_CAPAB_RINGS
capability for one of transmitting, receiving, or both.
.Ss Mapping Hardware to Rings and Groups
There are many different ways that hardware resources may map to this
capability.
Consider the following examples:
.Bl -enum
.It
Hardware may support a feature commonly known as receive side scaling
.Pq RSS .
With RSS, the hardware has multiple rings and uses a hash function
calculated over packet headers to choose which ring receives a
particular packet.
Rings are associated with different interrupts, allowing multiple rings
to be processed in parallel.
Supporting RSS in isolation would result in a device which has a single
group, and multiple rings within that group.
.It
Some hardware may have a single ring, but still support multiple receive
filters.
This is commonly seen with some 1 GbE devices.
While the hardware only has one ring, it has support for multiple
independent MAC address filters, each of which can be programmed to
receive traffic for a single MAC address.
The driver should map this situation to a single group with a single
ring.
However, it would implement the ability to program several filters.
While this may not seem useful at first, when virtual NICs are created
on top of a physical NIC, the additional hardware filters will be used
to avoid putting the device in promiscuous mode.
.It
Finally, some hardware has many rings, which can be placed in many
different groups.
Each group has its own filtering capabilities.
For such hardware, the device driver would declare support for multiple
groups, each of which has its own independent set of rings.
.El
.Pp
When choosing hardware constructs to implement rings and groups, it is
also important to consider interrupts.
In order to support polling, each receive ring must be able to
independently toggle whether that ring will generate an interrupt on
packet reception, even when many rings share the same hardware level
interrupt
.Pq e.g. the same MSI or MSI-X interrupt number and handler .
.Ss Filters
The
.Xr mac_group_info 9S
structure is used to define several different kinds of filters that the
group might implement.
There are three different classes of filters that exist:
.Bl -tag -width Ds
.It Sy MAC Address
A given frame matches a MAC Address filter if the receive address in
the Ethernet Header matches the specified MAC address.
.It Sy VLAN
A given frame matches a VLAN filter if it both has an 802.1Q VLAN tag
and that tag matches the VALN number specified in the filter.
If the frame's outer ethertype is not 0x8100, then the filter will not
match.
.It Sy MAC Address and VLAN
A given frame matches a MAC Address and VLAN filter if it matches both
the specified MAC address and the specified VLAN.
This is constructed as a logical AND of the previous two filters.
If only one of the two matches, then the frame does not match this
filter.
.Pp
Note: this filter type is still under development and has not been
plumbed through our APIs yet.
.El
.Pp
Devices may support many different filter types.
If the hardware resources required for a combined filter type
.Pq e.g. MAC Address and VLAN
are similar to the resources required for each in isolation, drivers
should prefer to implement just the combined type and should not
implement the individual types.
.Pp
The MAC framework assumes that the following rules hold regarding
filters:
.Bl -enum
.It
When there are multiple filters of the same kind with different
addresses, then the hardware will accept a frame if it matches
.Em ANY
of the specified filters.
In other words, if there are two VLAN filters defined, one for VLAN 23
and one for VLAN 42, then if a frame has either VLAN 23 or VLAN 42,
it will be accepted for the group.
.It
If multiple different classes of filters are defined, then the hardware
should only accept a frame if it passes
.Em ALL
of the filter classes.
For example, if there is a MAC address filter and a separate VLAN
filter, the hardware will only accept the frame if it passes both sets
of filters.
.It
If there are multiple different classes of filters and there are
multiple filters present in each class, then the driver will accept a
packet as long as it matches
.Em ALL
filter classes.
However, within a given filter class, it may match
.Em ANY
of the filters.
See the following boolean logic as an alternative way to phrase this
case:
.Bd -literal -offset indent
match = MAC && VLAN
MAC = 00:11:22:33:44:55 OR 00:66:77:88:99:aa OR ...
VLAN = 11 OR 12 OR ...
.Ed
.El
.Pp
The following pseudocode summarizes the behavior for a device that
supports independent MAC and VLAN filters.
If the hardware only supports a single family of filters, then simply
treat that in the pseudocode as though it is always true:
.Bd -literal -offset indent
for each packet p:
    for each MAC filter m:
        if m matches p's mac:
            for each VLAN filter v:
                if v matches p's vlan:
                    accept p for group
	            proceed to next packet
    reject packet p
    proceed to next packet
.Ed
.Pp
The following pseudocode summarizes the behavior for a device that
supports a combined MAC address and VLAN filter:
.Bd -literal -offset indent
for each packet p:
    for each filter f:
        if f.mac matches p's mac and f.vlan matches p's vlan:
            accept p for group
	    proceed to next packet
    reject packet p
    proceed to next packet
.Ed
.Ss MAC Capability Structure
When the device driver's
.Xr mc_getcapab 9E
function entry point is called with the capability requested set to
.Dv MAC_CAPAB_RINGS ,
then the value of the capability structure is a pointer to a
.Vt mac_capab_rings_t
structure with the following members:
.Bd -literal -offset indent
mac_ring_type_t         mr_type;
mac_group_type_t        mr_group_type;
uint_t                  mr_rnum;
uint_t                  mr_gnum;
mac_get_ring_t          mr_rget;
mac_get_group_t         mr_gget;
.Ed
.Pp
If the driver supports the
.Dv MAC_CAPAB_RINGS
capability, then it should first check the
.Fa mr_type
member of the structure.
This member has the following possible values:
.Bl -tag -width Dv
.It Dv MAC_RING_TYPE_RX
Indicates that this group is for receive rings.
.It Dv MAC_RING_TYPE_TX
Indicates that this group is for transmit rings.
.El
.Pp
The driver will be asked to fill in this capability structure separately
for receive and transmit groups and rings.
This allows a driver to have different entry points for each type.
If neither of these values is specified, then the device driver must
return
.Dv B_FALSE
from its
.Xr mc_getcapab 9E
entry point.
Once it has identified the type, it should fill in the capability
structure based on the following rules:
.Bl -tag -width Fa
.It Fa mr_type
The
.Fa mr_type
member is used to indicate whether this group is for transmit or receive
rings.
The
.Fa mr_type
member should not be modified by the device driver.
It is set by the MAC framework when the driver's
.Xr mc_getcapab 9E
entry point is called.
As indicated above, the driver must check the value to determine which
group this
.Xr mc_getcapab 9E
call is referring to.
.It Fa mr_group_type
This member is used to indicate the group type.
This should be set to
.Dv MAC_GROUP_TYPE_STATIC ,
which indicates that the assignment of rings to groups is fixed, and
each ring can only ever belong to one specific group.
The number of rings per group may vary on the group and can be set by
the driver.
.It Fa mr_rnum
This indicates the total number of rings that are available.
The number exposed may be less than the number supported in hardware.
This is often due to receiving fewer resources such as interrupts.
.It Fa mr_gnum
This indicates the total number of groups that are available from
hardware.
The number exposed may be less than the number supported in hardware.
This is often due to receiving fewer resources such as interrupts.
.Pp
When working with transmit rings, this value may be zero.
In this case, each ring is treated independently and separate groups for
each transmit ring are not required.
.It Fa mr_rget
This member is a function pointer that will be called to provide
information about a ring inside of a specific group.
See
.Xr mr_rget 9E
for information on the function, its signature, and responsibilities.
.It Fa mr_gget
This member is a function pointer that will be called to provide
information about a group.
See
.Xr mr_gget 9E
for information on the function, its signature, and responsibilities.
.El
.Sh DRIVER IMPLICATIONS
.Ss MAC Callback Entry Points
When a driver implements the
.Dv MAC_CAPAB_RINGS
capability, then it must not implement some of the traditional MAC
callbacks.
If the driver supports
.Dv MAC_CAPAB_RINGS
for receiving, then it must not implement the
.Xr mc_unicst 9E
entry point.
This is instead handled through the filters that were described earlier.
The filter entry points are defined as part of the
.Xr mac_group_info 9S
structure.
.Pp
If the driver supports
.Dv MAC_CAPAB_RINGS
for transmitting, then it should not implement the
.Xr mc_tx 9E
entry point, it will not be used.
The MAC framework will instead use the
.Xr mri_tx 9E
entry point that is provided by the driver in the
.Xr mac_ring_info 9S
structure.
.Ss Locking and Concurrency
One of the main points of the
.Dv MAC_CAPAB_RINGS
capability is to increase the parallelism and concurrency that is
actively going on in the driver.
This means that a driver may be asked to transmit, poll, or receive
interrupts on all of its rings in parallel.
This usually calls for fine-grained locking in a driver's own data
structures to ensure that the various rings can be populated and used
without having to block on one another.
In general, most drivers have their own independent set of locks for
each transmit and receive ring.
They also usually have separate locks for each group.
.Pp
Just because one driver performs locking in one way, does not mean that
one has to mimic it.
The design of a driver and its locking is often tightly coupled to how
the underlying hardware works and its complexity.
.Ss Polling on rings
When the
.Dv MAC_CAPAB_RINGS
capability is implemented, then additional functionality for receiving
becomes available.
A receive ring has the ability to be polled.
When the operating system desires to begin polling the ring, it will
make a function call into the driver, asking it to receive packets from
this ring.
When receiving packets while polling, the process is generally identical
to that described in the
.Sy Receiving Data
section of
.Xr mac 9E .
For more details, see
.Xr mri_poll 9E .
.Pp
When the MAC framework wants to enable polling, it will first turn off
interrupts through the
.Xr mi_disable 9E
entry point on the driver.
The driver must ensure that there is proper serialization between the
interrupt enablement, interrupt disablement, the interrupt handler for
that ring, and the
.Xr mri_poll 9E
entry point.
For more information on the locking requirements related to polling, see
the discussions in
.Xr mri_poll 9E
and
.Xr mi_disable 9E .
.Ss Updated callback functions
When using rings, two of the primary functions that were used change.
First, the
.Xr mac_rx 9F
function should be replaced with the
.Xr mac_rx_ring 9F
function.
Secondly,
the
.Xr mac_tx_update 9F
function should be replaced with the
.Xr mac_tx_ring_update 9F
function.
.Ss Interrupt and Ring Mapping
Drivers often vary the number of rings that they expose based on the
number of interrupts that exist.
When a driver only supports a single group, there is often no reason to
have more rings than interrupts.
However, most hardware supports a means of having multiple rings tie to
the same interrupt.
Drivers then tie the rings in different groups to the same interrupts
and therefore when an interrupt is triggered, iterate over all of the
rings.
.Pp
Tying multiple rings together into a single interrupt should only be done
if hardware has the ability to control whether or not each ring
contributes to the interrupt.
For the
.Xr mi_disable 9E
entry point to work, each ring must be able to independently control
whether or not receipt of a packet generates the shared interrupt.
.Ss Filter Management
As part of general operation, the device driver will be asked to add
various filters to groups.
The MAC framework does not keep track of the assigned filters in such a
way that after a device reset that they'll be given to the driver again.
Therefore, it is recommended that the driver keep track of all filters
it has assigned such that they can be reinstated after a driver or
system initiated device reset of some kind.
There is no need to persist anything across a call to
.Xr detach 9E
or similar.
.Pp
For more information, see the
.Sy TX STALL DETECTION, DEVICE RESETS, AND FAULT MANAGEMENT
section of
.Xr mac 9E .
.Ss Broadcast, Multicast, and Promiscuous Mode
Rings and groups are currently designed to emphasize and enhance the
receipt of filtered, unicast frames.
This means that special handling is required when working with broadcast
traffic, multicast traffic, and enabling promiscuous mode.
This only applies to receive groups and rings.
.Pp
By default, only the first group with index zero, sometimes called the
default group, should ever be
programmed to receive broadcast traffic.
This group should always be programmed to receive broadcast traffic, the
same way that the broader device is programmed to always receive
broadcast traffic when the
.Dv MAC_CAPAB_RINGS
capability has not been negotiated.
.Pp
When multicast addresses are assigned to the device through the
.Xr mc_multicst 9E
entry point, those should also be assigned to the first group.
.Pp
Similarly, when enabling promiscuous mode, the driver should only enable
promiscuous traffic to be received by the first group.
.Pp
No other groups or rings should ever receive broadcast, multicast, or
promiscuous mode traffic.
.Sh SEE ALSO
.Xr mac 9E ,
.Xr mc_getcapab 9E ,
.Xr mc_multicst 9E ,
.Xr mc_tx 9E ,
.Xr mc_unicst 9E ,
.Xr mi_disable 9E ,
.Xr mr_gaddring 9E ,
.Xr mr_gget 9E ,
.Xr mr_gremring 9E ,
.Xr mr_rget 9E ,
.Xr mri_poll 9E ,
.Xr mac_rx 9F ,
.Xr mac_rx_ring 9F ,
.Xr mac_tx_ring_update 9F ,
.Xr mac_tx_update 9F ,
.Xr mac_group_info 9S