Receiver ‘packet-splitting’

A look at how a driver can cause the 82573L NIC to separate a packet’s headers from its data

NIC can do packet-parsing

• Intel’s newest gigabit Ethernet controllers offer an enhancement to the ‘extended’ Receive Descriptor, called ‘packet-split’ format, which empowers the hardware to recognize the packet ‘headers’ used with the most common network protocols and to automatically separate those headers from their accompanying packet ‘data’

‘Extended’ RX-Descriptors

Base-address (64-bits)

reserved (=0)

MRQ(multiple receive queues)

Extendedstatus

Packet-length

Packet-checksum

VLANtag

Extendederrors

IPidentification

The device-driver initializes the ‘base-address’ field with the physical address of a packet-buffer, and it initializes the ‘reserved’ field with a zero-value… … the network hardware will later modify both fields

The network controller will ‘write-back’ the values for these fields when it has transferred a received packet’s data into the packet-buffer

CPU writes this, NIC reads it: NIC writes this, CPU reads it:

‘Packet-Split’ RX-Descriptors

Base-address 0 (64-bits) MRQ(multiple receive queues)

Extendedstatus

Packet-Length 0

Packet-checksum

VLANtag

Extendederrors

IPidentification

The device-driver initializes four ‘base-address’ fields (‘even-numbered’ addresses)

The network controller will ‘write-back’ values to these fields when it has transferred a received packet’s data into those packet-buffers

CPU writes this, NIC reads it: NIC writes this, CPU reads it:

Base-address 1 (64-bits)

Base-address 2 (64-bits)

Base-address 3 (64-bits)

PacketLength 3

PacketLength 2

PacketLength 1

HeaderLength

reserved 1

SP

Same ‘Extended’ Status/Errors

0 0 0 0ACK

0 0 0 0UDPV

IPIV

0PIF

IPCS

TCPCS

UDPCS

VP

IXSM

EOP

DD

19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

RXE IPE TCPE 0 0 SEQ SE CE 0 0 0 0

11 10 9 8 7 6 5 4 3 2 1 0

Syntax modifications for ‘fetch’

typedef struct {unsigned long long base_addr0;unsigned long long base_addr1;unsigned long long base_addr2;unsigned long long base_addr3;} RX_DESC_FETCH;

Syntax modifications for ‘store’

typedef struct {unsigned int mrq;unsigned short ip_identification;unsigned short packet_chksum;unsigned int desc_status:20;unsigned int desc_errors:12;unsigned short packet_length0;unsigned short vlan_tag;unsigned short header_length;unsigned short packet_length1;unsigned short packet_length2;unsigned short packet_length3unsigned long long reserved;} RX_DESC_STORE;

Same syntax for the ‘union’

typedef union {

RX_DESC_FETCH rxf;RX_DESC_STORE rxs;} RX_DESCRIPTOR;

NIC Registers involved

RCTL

Reserved (=0)RFCTL

LEN3(1KB)

PSRCTL LEN0(128B)

LEN1(1KB)

LEN2(1KB)

Packet-Split Receive Control register

Receive Filter Control register

EXTEN

31 15 0

31 24 23 16 15 8 7 0

31 10 0

DTYP

Device Control register

base_addr0

Each descriptor has four buffers

buffer0

base_addr1

base_addr2

base_addr3

buffer1 buffer2 buffer3

Packet-Split Rx-descriptor

Four buffers are allocated for receiving one packet

Refresh for ‘reuse’

• As with the ‘extended’ receive-descriptors, it is necessary for a device-driver to setup each ‘packet-split’ receive-descriptor any time it is going to be ‘reused’, since prior buffer-addresses get overwritten during a packet-reception by the network controller

• So driver needs a formula for recalculating buffer-addresses, or use a ‘shadow’ array

Kernel-memory layout

512bytes

65536bytes

Sixteen Rx-descriptors (32-bytes each)

Sixty-four receive-buffers (1024-bytes each)

kmem

KMEM_SIZE (= 66048 bytes)

Caveats

• Short packets are not always ‘split’

• Unrecognized packet-headers may not be separated from accompanying packet-data

• Demonstrating the packet-split capability will require us to devise a way to transmit packets which have the TCP/UDP and IP packet-headers that the NIC recognizes

Our ‘pktsplit.c’ demo

• We created a ‘minimal’ kernel-module for demonstrating the NIC’s ‘packet-splitting’ capabilities

TIMEOUT for an in-class demonstration

In-class exercise

• Can you enhance our ‘pktsplit.c’ module so that its Receive-Descriptor Queue will function automatically as a ring-buffer (as happens in our ‘extended.c’ example)?

• Your best option for this is to install an ISR which will reinitialize some Rx-Descriptors (and advance the RDT index) each time an RXDMT0 interrupt gets triggered